Winter is here and there is plenty of interesting stuff to talk about from a synoptic meteorology perspective. This post will take a more in-depth look at the first of two storms expected to bring at least some snow to parts of the East Coast this week.
Here’s a glimpse at what I’m seeing on satellite imagery this morning. We have a storm moving through Illinois bringing snow to parts of the Great Lakes and rain to parts of the Northeast. While not directly bringing any wintry weather along the East Coast outside far northern Maine, this system will help set the stage for the next two systems, waiting in the wings over Oregon and Alberta. It will help chip away at the subtropical ridge over the western Atlantic, and it will shove the polar front towards the coast.
This sets up the possibility of cyclogenesis (the formation of a new storm) in the Southeast tomorrow evening as the Oregon disturbance meets up with the lower-level front.
By tomorrow evening, a classic cyclogenesis event will be underway as the upper-level disturbance from Oregon supports the development of a surface wave along the polar front.
North of this surface low, a jet streak (region of extra-fast winds embedded within the larger jet stream) is noted over the Ohio Valley. The jet streak doesn’t move forward as fast as the winds within it, so there are regions near the entrance and exit of the jet where winds aloft are accelerating and decelerating. The acceleration into the jet entrance region, particularly the right entrance region in situations like this, draws near-surface air north and upward into the jet. This circulation will contribute to the northward movement (advection) of moisture from the Gulf of Mexico and the subsequent condensation of much of that moisture into rain/snow north of the low’s track.
The critical question is how this developing storm interacts with the Alberta disturbance as it dives east-southeast towards New England.
Current forecasts show the Oregon disturbance east of the Alberta disturbance on Monday evening as it approaches the East Coast, but not by much. Perhaps more critically, the Alberta disturbance is sliding ESE rather than digging SE. That means that the prevailing flow responsible for steering the Oregon disturbance (and our storm) is more west-southwesterly than southwesterly. The result is a track towards the ENE rather than NE. So this won’t turn into a blockbuster blizzard until it gets up to Newfoundland. That doesn’t mean we don’t still have some unanswered questions about just how far northwest precipitation might be able to reach.
This loop shows the last four forecasts from the NAM model for precipitation and sea level pressure all valid Monday evening. Note that the original forecasts showed low pressure slipping offshore without much if any precipitation over the Northeast. The latest forecast has a solid moderate snow event for much of the coastal Northeast as the low tracks a bit closer to the coast.
A look at upper-level wind forecasts shows a favorable setup for northward moisture advection into the right entrance region of a jet streak that curves right as you move east along its axis (meteorologists call this “anticyclonically curved”).
If only that Alberta disturbance were digging southeast, we could have a nice blizzard on our hands but instead all our relevant features will slide quickly ENE and so too will the precipitation shield. This should produce a solid light-moderate snow event for a swath of Southern New England and adjacent parts of PA/MD.
Here’s a first guess on possible accumulations from this system.
I’m a little more bullish on this system because it’s a setup that often supports a bit of last-minute NW trending (like we saw in the NAM forecast comparison above) thanks to poorly-resolved convective processes off the East Coast. That said, I think I’d probably bet on the lower ends of these ranges at least to start.
The storm quickly departs Tuesday morning leaving very cold air poised to surge into the region.
This is true Arctic air, at least for those of us in Northern New England. This bitter cold airmass will help support heavy snow as our next system approaches on Wednesday.
After re-emerging into the Caribbean Thursday night and re-organizing into a coherent system yesterday, Eta is now re-strengthening south of Cuba.
The storm has assembled some very impressive convection this morning which has pulled the center northeast of forecast expectations. This relocation will have some impacts for the storm’s track but it’s probably more important for Eta’s intensity. By moving the center northeast, the storm has put a bit more distance between itself and an upper-level low over the Gulf of Mexico. That low will be a source of dry air and wind shear in the days to come, both of which are detrimental to a storm’s organization and intensification.
Hurricane Hunters have been exploring the storm this morning and found a small region of strong winds right near the storm’s new center NW of Grand Cayman Island. Winds at the surface are estimated to be around 50 mph as I write this at 11:15 AM EST.
What’s next for Eta? I suspect our current period of intensification will continue until the storm makes landfall in Cuba tonight. Could it briefly reach hurricane strength? It’s definitely within the realm of possibility. Interaction with Cuba should weaken the core a bit. Most likely though, it’ll stay a moderate/strong tropical storm.
By tomorrow morning, the storm will be getting closer to the upper-level low over Florida. This means that shear and dry air will start ramping up a bit. After running over Cuba, Eta’s core will likely be disrupted which will make it easier for the shear to induce further weakening. That said, a strong outflow channel will be located northeast of the storm and waters between FL and Cuba are still quite warm so we’ll probably see the storm remain more or less at the same intensity (60mph TS to 80 mph hurricane) as it pivots north and northwest around the eastern side of the upper-level low/trough.
Because Eta could very well be a hurricane as it moves near or over southern Florida on Monday, hurricane watches are now up for the Keys and parts of the southern peninsula even though the official NHC forecast shows a maximum intensity near 70mph.
Regardless of Eta’s max winds, rain will be the biggest threat to southern Florida.
The storm’s slow forward motion and large pocket of moisture (despite wrapping some dry air in near the core) mean that most of southern Florida will see at least 5″ of rain over the next few days. Some spots near Miami are expected to see over 10″ of rain! Be prepared for freshwater flooding issues starting tonight and continuing through Thursday.
Another issue will be storm surge flooding as gusty east winds push waves and water onshore not just in southern Florida but all the way up the coast to South Carolina. Watch for beach erosion and minor/moderate coastal flooding in these areas as well as the threat of rip currents.
What happens after the storm passes south Florida?
The upper-level low mentioned above will keep pulling Eta west until it’s in the Gulf of Mexico, at which point it will probably slow down considerably. Just how far into the Gulf it can move is an open question, but the storm is likely to still retain its large impact footprint so rain and breezy conditions will continue in Florida.
Because the upper-level low and the storm will eventually be co-located, Eta will have plenty of dry air to deal with. This probably means the storm will slowly weaken. However, we’ll have to watch for another round of potential intensification if the storm can end up creating a small pocket of moisture-rich air once the shear settles down mid/late week. Either way, Eta will eventually move northeast once another trough enters the central US so folks in central/northern FL are likely to at least see some breezy rain, and potentially some more significant impacts, before we can finally say goodbye to Eta.
In yesterday’s update, I outlined how Zeta was expected to modestly strengthen as it moved through the Gulf of Mexico supported by rich moisture and somewhat warm water before getting slowed down by cooler ocean temperatures and increasing shear as it neared landfall. Unfortunately, the storm has organized and intensified significantly overnight and it appears as though any weakening that occurs right before landfall will be minor and inconsequential.
Satellite imagery shows a marked increase in convective intensity and organization overnight. What were once scattered storms are now powerful “hot towers” becoming increasingly symmetric around the center. The once-ragged eye is increasingly well-defined as the updrafts surrounding the center are balanced out by strong descent in the center itself. Additionally, we can see expansive outflow aloft unimpeded (so far) by the trough to the west of the storm. All of these signals point towards a healthy and strengthening hurricane.
Peering underneath the upper-level cirrus shield using microwave satellite imagery, we can see Zeta’s well-defined eye and eyewall flanked by a few robust bands. This means that Zeta’s structure is conducive to continued intensification today. It has plenty of room to continue adding convection so long as ocean temperatures permit.
Speaking of ocean temperatures, I have thought for a few days now that 25N is roughly the northern extent of really supportive warm water. The storm is just now crossing that threshold so I’ll be watching to see if convective activity begins to struggle from here on out.
However, the water doesn’t get cold (<26C) until the last 50 miles or so before Louisiana. So it’s possible that favorable jet stream dynamics (upper-level divergence) could help Zeta continue strengthening so long as ocean temperatures aren’t unsupportive of convection. This is the scenario consistent with NHC forecasts and its the scenario folks in the path of the storm should be ready for. At this point, with the storm structure appearing as robust as it is and the storm’s forward motion being so fast, I don’t think we’ll be able to sneak much weakening in during the hours just before landfall.
Zeta will move onshore in southeastern Louisiana this evening around 5-6 PM CDT. The storm will then continue quickly inland towards southern MS/western AL.
High resolution forecast model guidance provides a good look at how Zeta’s wind field will look as it moves ashore even though the numbers on this map will be an underestimation of actual winds. Note that the strongest winds will be focused east of the storm center’s track. This means that places along the Mississippi coastline 30-50 miles east of the center will see significantly stronger winds than parts of Louisiana 10-15 miles west of the center.
Another key item to note is the wind direction. Winds will be out of the southeast across the Gulf of Mexico immediately south of Mississippi. This will funnel water inland and produce significant storm surge. The shape and topography of the coastline in this area is favorable for enhancing storm surge and folks along the MS coast should be prepared for water 6-10 feet above normally dry ground.
Because Zeta is moving so fast, its swath of damaging winds will extend farther inland than most hurricanes.
Even as the storm races through central Alabama and begins to lose its tropical characteristics early tomorrow morning, it will still be producing a region of 40-50 mph winds capable of producing damage to trees and power lines. These winds will move into northern Georgia later tomorrow morning. Folks in these areas should be ready for power outages even after Zeta weakens below hurricane strength.
Zeta’s remnants are still expected to bring snow to interior parts of the Northeast on Friday though the trend in recent model guidance is towards a weaker and less-amplified storm bringing lighter snow to a smaller area. Oh well.
Now that we’re facing another tropical threat, it’s time to revisit this blog. Perhaps I should’ve started these updates earlier but I’ve been trying to clear the to-do list so I can focus on Zeta for the next several days but here we are.
Zeta formed this past weekend in the NW Caribbean after a cluster of shower/thunderstorm activity finally coalesced underneath a sprawling area of upper-level high pressure. The storm almost met the criteria for rapid intensification when it became a hurricane yesterday before making landfall in the Yucatan last night.
A look at satellite imagery this afternoon shows a very broad system with a dry slot separating the inner core from the primary outer band. This dry slot exists because air originating over southern Mexico and moving over the Yucatan Peninsula can’t pick up much if any moisture as it travels over land rather than water.
As Zeta continues moving northwest away from the Yucatan, air parcels moving into the storm’s southeastern quadrant will originate over the Bay of Campeche rather than southern Mexico. These parcels will be rich in moisture having traveled over warm water for a considerable time. This will help the storm’s convection redevelop tonight/tomorrow.
Speaking of redevelopment, the process is already underway as seen in this loop. Cloud tops have rapidly been cooling as convection redevelops at the edge of the storm’s center. This ring of thunderstorms will eventually become the storm’s eyewall.
With generally favorable conditions and ongoing convective development, expect Zeta to reintensify into a hurricane tonight as it moves NW then begins to turn off to the north. That turn will take place as the storm begins moving between a strong ridge of high pressure over the Atlantic and a strong upper-level low over Texas as seen in the graphic below.
Both these features will be pulling Zeta northeast so the storm will begin to accelerate tomorrow. By the time it makes landfall, Zeta will be racing at around 30 mph. This is great news from the rainfall perspective as we don’t have to worry about a stalling system dropping inches of rain for days. However, it means that the wind threat will increase because the system won’t have as much time to weaken over the cooler waters immediately south of Louisiana. Additionally, dry air likely won’t have enough time to wrap into the storm’s core before it moves onshore. So while the storm will likely not make landfall at peak intensity, it probably won’t lose a lot of steam between tomorrow morning and landfall tomorrow afternoon/evening.
The current NHC forecast, shown above, expects Zeta to make landfall in SE Louisiana as an 80 mph hurricane tomorrow afternoon/evening. The storm’s biggest threats will be high winds that will continue well inland into MS/AL as well as storm surge near the coast. Surge may exceed 5-8 feet in parts of SE Louisiana that are not protected by the levee system as well as parts of southern Mississippi and the southwestern coast of Alabama.
Another threat to keep an eye on is tornadoes along and east of the storm center track tomorrow afternoon/evening. Remember this is a very large storm so rain/wind/tornadoes in the outer bands will impact regions many miles away from where the center actually makes landfall.
After moving onshore, Zeta will zip through the southeastern US bringing gusty winds and heavy rain to a swath extending from near New Orleans to Birmingham Alabama and eventually into the mountains of TN/NC.
During this time, the storm will lose its tropical characteristics as it merges with a non-tropical storm moving northeast from Texas. The non-tropical system that will emerge once these two entities join forces is expected to bring heavy snow to parts of the Northeast on Friday. Though it’s noteworthy to have the remnants of a hurricane bring snow to parts of the northern US, snow at this point in the season isn’t super unusual in New England and interior New York.
Hurricane Delta is moving through the Gulf of Mexico this morning and intensifying slowly as it does so.
The storm is still struggling to become a well-organized hurricane this morning. Wind shear is lighter now than it was yesterday, so it’s not immediately clear to me why we haven’t seen convection take off on the storm’s eastern/northeastern side. Perhaps that will change later today. Either way, the big takeaway from current observational data is that Delta’s footprint has expanded significantly over the past couple days. While this makes extremely rapid intensification harder to pull off, it means that a wider swath of the coastline will experience strong winds and storm surge will be considerably larger.
The modest intensification and significant expansion of Delta’s wind field is best visualized with hourly forecast model data. I made this animation yesterday afternoon but the forecast hasn’t changed meaningfully in the intervening hours.
The same thought applies to the animation below which uses data from yesterday’s model guidance but remains relevant because forecasts haven’t changed much in the time since.
As Delta turns north today, the biggest question will be how quickly it’s moving. Ensemble members are in excellent agreement about the general track of the storm. Landfall will occur somewhere in SW Louisiana (exact town dependent on wobbles etc.) it’s just a matter of figuring out when. The current NHC forecast calls for landfall sometime late tomorrow afternoon. The range of possible outcomes in my mind is somewhere between midday tomorrow and early Saturday morning. Either way, today’s the last full day of relatively quiet weather during which you can safely prepare for the storm.
While I don’t think this matters all that much in the end, a slower Delta might slide a smidge farther west due to the weakening trough over Texas. But this would probably be on the order of 15-25 miles rather than some big lurch into Texas. If you’re in Houston, I think you can rest easy knowing that the worst of Delta is headed well to your east (though minor impacts are possible).
Now let’s take a look at Delta’s forecast from an impacts-based perspective.
This map is an overview of the National Hurricane Center’s forecast for Delta’s track, intensity, and rain/wind/surge impacts. The areas most seriously impacted by the storm will be along the coast between Lake Charles and Lafayette Louisiana, as well as points inland. Unfortunately, many of these areas are still recovering from the devastation of Hurricane Laura just six weeks ago.
As per usual, storm surge is the biggest concern as Delta moves ashore. This map provides a clearer view of the NHC’s inundation forecast which calls for parts of coastal Louisiana southeast of Lafayette to be under 9+ feet of water sometime Friday evening. If you’re in this area, please heed the instructions of local officials and evacuate if you are told to do so. Surge will cause some issues as far west as Beaumont/Port Arthur and as far east as coastal Mississippi.
High winds will also be of concern near where Delta makes landfall. Our current best guess at Delta’s maximum sustained winds during landfall is somewhere around 100-110 mph. Perhaps if the storm can intensify a bit more today, those numbers might be a little higher. But I’m less concerned by the maximum wind number than I am about the size of the wind field and how fast the storm will be moving. The larger wind field means that more people will experience tropical storm-force and hurricane-force winds even if there aren’t any major-hurricane-force winds right near the core. The storm’s fast forward motion means that strong winds will be able to move farther inland than we’d otherwise expect. Even if you’re 100-200 miles north/northeast of the point of landfall, you should be prepared for strong winds and power outages.
Delta’s fast movement isn’t all bad news. Because the storm won’t be lingering in any given location for very long, the rain threat should be manageable. Our best rainfall forecasts currently indicate the potential for 4-8″ of rain along and just west of Delta’s track. That’ll undoubtedly cause flash flooding issues in spots but we shouldn’t see much if any major/widespread flooding like we did with Sally or other slow-moving storms.
Delta will quickly dissipate over the Tennessee Valley this weekend.
Now that we have yet another active tropical cyclone and I’ve cleared the decks with my other projects a bit, it’s time to revisit this blog. I’ll be providing updates on Hurricane Delta here each day until the storm has made landfall along the northern Gulf Coast sometime Friday night or Saturday morning. As a reminder, you can also find my tropical weather discussions on twitter (@JackSillin), on the Hurricane Tracker App, and on Mark Sudduth’s HurricaneTrack Patreon page.
Delta formed in the Caribbean just 24 hours ago and is already a powerful Category Two hurricane with maximum winds of 100 mph. Satellite imagery shows very intense convective activity occurring in the storm’s small core west of Jamaica.
Though the storm’s eye is not yet readily apparent on satellite imagery, I suspect it will be soon. The reason we can’t see the eye on satellite imagery yet is that the thunderstorms in the eyewall are too intense relative to the strength of the circulation. Basically, the thunderstorms are pushing cirrus into the eye faster than the sinking motion in the eye can clear them out. As the circulation ramps up to match pace with the thunderstorms, sinking in the eye will become stronger and we’ll see an eye clear out. So at this point in a storm’s lifecycle, not being able to see the eye on satellite imagery is a sign of strengthening not weakening.
Taking a quick look at the storm’s inner core as observed by radar imagery from the Cayman Islands as well as GOES-East satellite imagery, we can see that while the storm has a small and circular eye, there’s a bit of a “hole” in the core on the northeastern side. This is likely due to some very subtle northeasterly shear imparted on the storm by the upper-level high off to its north. The convection we’re seeing in Delta’s eyewall is so intense that I don’t think this shear will slow the storm down much, but perhaps this is why we don’t have an even-stronger hurricane yet.
By this afternoon/evening, that shear should weaken and the environment will be picture-perfect for continued rapid intensification.
Area-averaged soundings from the HWRF show every environmental box for tropical cyclone intensification checked this evening. Winds will be uniformly around 15kts out of the east-southeast so shear will be very low. The spread between the temperature and the dew point is very low throughout the atmospheric column which means that dry air won’t be of any concern. Strong heat and moisture fluxes from the incredibly warm ocean mean that rising air parcels will be considerably warmer than their surroundings and thus will rise rapidly due to buoyancy. Outside of inner-core fluctuations that are hard to anticipate, there’s nothing stopping Delta from continued RI today.
This is most immediately concerning for Cancun and adjacent parts of the northeastern Yucatan Peninsula, where Delta will arrive early tomorrow morning.
Current forecasts from our suite of high-resolution hurricane models indicate that Delta will make landfall near Cancun as a powerful Category Four hurricane. This is what the official NHC forecast is calling for too. Impacts in this area will include destructive winds of 120-140 mph, storm surge of several feet, and torrential rains capable of causing flash flooding and mudslides. Residents in these areas need to be preparing now for life-threatening conditions beginning later tonight.
While some weakening is expected as Delta interacts with land, the storm’s brisk motion means that it won’t spend much time away from water. By tomorrow afternoon, the storm will be back out over the Gulf of Mexico ready for another round of intensification. While waters won’t be quite as hot as in the Caribbean, the environment over the southern Gulf tomorrow night looks pretty darn similar to that which we’re seeing in the Caribbean today.
On Thursday, the storm will begin turning towards the north in response to southwesterly flow developing aloft ahead of an upper-level trough over Texas.
While the confidence in some type of northerly->northeasterly curve is quite high, we don’t yet know exactly where that turn will happen. If it occurs sooner and/or is a bit sharper, the storm’s track could shift a little bit east. If the turn occurs a bit later and/or isn’t quite as dramatic, the storm’s track could shift a bit west. That’s one reason why even though Louisiana has the highest chance of seeing a landfall from Delta, folks anywhere from the Upper Texas coastline to the western Florida Panhandle need to have a very close eye on this storm. The other reason of course is that hurricanes are not point features, and dangerous impacts will extend well away from the center.
Delta will approach the Gulf Coast Friday evening before making landfall sometime Friday night or Saturday morning. During the last 6-12 hours before landfall, the storm will face a few hurdles that are likely to knock a bit off its maximum intensity (whatever that ends up being).
The first hurdle will come in the form of some increased wind shear due to that upper-level trough over Texas. While winds turn southwesterly aloft, they will remain southeasterly closer to the ground. That change in wind direction with height is wind shear, and will put some stress on the hurricane’s vortex. If Delta were a small and newly-formed system (like Marco earlier this year), I’d say that the odds of this shear weakening the storm dramatically before landfall were relatively good. However, when the shear kicks in over Delta, it will be a powerful Category Four hurricane that already has a well-established vortex. Persistent shear can weaken strong hurricanes like this, but it takes some time. These storms don’t just go poof like little Marco did. They take a while to “spin down”. Unfortunately, Delta will be moving quickly at this point in its lifecycle. Even 6-12 hours of wind shear won’t be enough to avoid a powerful hurricane landfall with significant impacts.
The second speedbump for Delta will be the decrease in ocean heat (hurricane fuel) over the northern Gulf. This is due to the cooling effect of several cold fronts that have swept through the region in the past few weeks. While the storm is currently enjoying off-the-charts heat over the NW Caribbean, cooler water lurks over the Gulf. There’s enough heat in the southern Gulf (south of 25N) to support a major hurricane, but Delta won’t be able to spend much time north of that without running low on gas.
That said, the forward motion caveat applies here too. Much like a car running out of gas going down the highway, hurricanes don’t stop immediately after moving into cooler waters. It takes time for all that momentum to dissipate. Delta will only have 6-12 hours over this cooler water before making landfall. While that is welcome news in a season where several hurricanes have rapidly intensified right up until landfall, it will not be enough of an impediment to prevent major impacts.
The third speedbump will be dry continental air moving into the storm’s southwestern flank. This dry air will attempt to wrap into and weaken Delta’s core, but I’m skeptical it will have enough time to make a substantial difference. That said, given the southwesterly shear, the dry air may have an easier path into Delta than it did into Laura earlier this year in the same part of the Gulf.
So all that to say that Delta will likely be slowly coming off its peak intensity as it moves towards the Louisiana coastline. Its maximum winds at landfall are likely to be somewhat lower than its maximum winds over the Caribbean or southern Gulf of Mexico. But that does not mean this won’t be an extremely dangerous hurricane for parts of the Louisiana coastline (and perhaps parts of far eastern TX and/or MS/AL/W FL). It’s crucial to remember that the process of hurricanes weakening involves all the momentum they’ve built up slowly becoming less concentrated. So while winds near the center might be decreasing, the overall wind field usually expands to satisfy the conservation of angular momentum. This process is helped along by eyewall replacement cycles, of which Delta is likely to complete at least one.
Residents in Louisiana need to have their hurricane plans ready to go for when watches and warnings are issued in the next couple days.
I’ll leave you with a look at the NHC’s official forecast for Delta which neatly summarizes many of the ideas I’ve discussed here. I cannot stress enough how little you should focus on the icon over Louisiana at 2 AM Saturday being an “H” for Hurricane instead of “M” for Major Hurricane. I can assure you that you will not be able to notice the difference between a 110 mph Category Two and a 115 mph Category Three, and this storm will be extremely dangerous regardless of its maximum sustained winds at landfall.
I’ll have more updates throughout the day on twitter (@JackSillin).
It’s an extremely busy day in the tropics as we have six systems to watch from the Gulf of Mexico to the shores of western Africa.
Each of these systems can be seen on this satellite image of the Atlantic a little earlier this afternoon. In order from west to east: the trough over the NW Gulf with a 30% chance of development within five days, TD-19, Paulette, Rene, 95L, and the next wave just emerging off Africa.
This post will start with a quick look at Paulette before taking a deep dive into TD-19’s forecast. None of the other systems will be mentioned due to their relatively low threat to land (zero for Rene, unclear long-term threat for 95L, minor longer term threat from the GOM trough, and potential for showers in Cabo Verde from the next wave) and the presence of more pressing systems.
Before we really dive into this evening’s update, a couple notes. If images are too small for you to get a good look at, you can click them for a larger version. If you’d like to get these updates in your inbox, you can sign up with the box on the left sidebar near the top of the page. If you’d like to see more tropical weather updates from me while supporting a super cool hurricane research project, head on over to Mark Sudduth’s Patreon page. If you prefer to support me directly, I also have a personal Patreon page. It’s like Mark’s except without all the cool extra posts and the money goes to buying my textbooks instead of collecting data from inside hurricanes.
The strongest system in the Atlantic this evening is Paulette with winds of 65 mph. The storm is expected to become a hurricane tonight or tomorrow as it continues tracking northwest towards Bermuda. Significant impacts from Paulette are likely in Bermuda as the storm passes nearby on Monday morning. Just how close to the island the storm gets will determine the extent to which rain/wind/surge will cause problems there. If you’re looking for more of my thoughts on Paulette and Bermuda, I took a bit of a closer look in my post this afternoon for Mark Sudduth’s page.
For those of us along the East Coast, Paulette is nothing to worry about outside of concerns relating to large swells and rip currents. If you’re an experienced surfer, by all means enjoy the powerful swells. For the rest of us, stay out of exposed ocean waters where intense rip currents pose a serious threat to your life.
TD-19 was just designated east of Florida from the system we’ve been calling 96L up until today.
Satellite imagery shows an increasingly healthy system this evening with fairly expansive upper-level outflow (still needs to work on the northwestern quadrant and intense thunderstorm activity bubbling up near the system’s center (especially towards the end of the loop). Those persistent bursts of thunderstorm activity over the center will be key for the system’s development over the next few days. If thunderstorm activity continues tonight the way it has been this afternoon, I honestly wouldn’t be shocked to see TD 19 upgraded to TS Sally by the 8 PM or 11 PM EDT updates.
TD-19 is close enough to the Miami radar that we can actually get some pretty good information about the system’s structure by peering under the cirrus canopy and into the lower-level parts of the storm. The mid-level circulation is located in the cluster of intense thunderstorms just off Andros Island while the lower-level center (lowest surface pressures inferred by observations in Florida and buoys/ships offshore) is a bit northwest over the Florida Straits. This infers the presence of some light northwesterly wind shear which is preventing the system from becoming fully aligned.
That wind shear is being caused by a weak upper-level low drifting southwest across central Florida. As that low slides farther southwest, continues weakens, and the deep convection associated with TD 19 begins to produce a response in the lower-level mass fields (i.e. enough air rises in the big thunderstorms that more has to move horizontally towards the storm’s center to take its place), I’d expect to see this misalignment resolved. It also helps that the low-level center is somewhat poorly defined and doesn’t have much angular momentum built up yet so a center relocation underneath that deep convection is pretty easy for a storm like this to pull off.
Microwave imagery from around 5 PM shows a system well-positioned for intensification with a curved band of convection developing around the mid-level center. Note that the convection hasn’t yet wrapped around to the northwest of the mid-level center, due in part to the wind shear mentioned above. If that band can close off, TD-19 could make a run towards mid-grade TS status before landfall tonight. That’s far from a guarantee given the NW shear, and it would only impact a few towns right along the beach where the very small core comes ashore, but it’s something to be mindful of if you live in the Miami area. Don’t be shocked if your power flicks off for a few hours tonight if TD-19 happens to tighten up a bit before landfall and you happen to end up in the small portion of the circulation that contains 40-50 mph winds.
After moving onshore tonight, TD-19 will quickly move across the southern part of the peninsula before emerging offshore tomorrow afternoon. The state of the system at this point is a bit of an unknown because we don’t know how robust the circulation will be when it moves onshore tonight and we don’t know to what extent the circulation will fall apart over land. We also don’t know how long the storm will spend over land because its exact track is a bit uncertain owing to near-term wobbles induced by the strong convection discussed earlier.
Either way, by tomorrow afternoon TD 19 will be offshore between Naples and Key West. At this point, it will begin to strengthen in an environment favorable for tropical cyclone development.
For tropical cyclones to develop and intensify, three main ingredients are needed: warm water, low vertical wind shear, and relatively high mid-level moisture. Sea surface temperatures are over 30C in the Gulf of Mexico at the moment (need ~26C for tropical storms, ~28C for hurricanes) so there’s nothing holding the storm back on that front. What about shear and dry air?
This area-averaged sounding over TD-19 tomorrow evening shows no dry air or wind shear in sight as the system sits within a very moist environment characterized by nearly uniform east-southeasterly winds between 5 and 10 kts. This basically means that there’s very little if anything holding TD-19 back as it moves into the eastern Gulf on Sunday.
As a result, it’s not surprising to see the models that are designed to offer more accurate intensity forecasts for tropical cyclones bring TD-19 quickly up to a strong TS by Monday. The animation above shows the HWRF which shows TD-19 coming very close to landfall in AL as a category one hurricane by Monday night. Much like other storms this season (Hanna, Isaias, arguably Laura), the biggest cap on TD-19’s intensity will be its limited time over water.
With that in mind, generally speaking if TD-19 wobbles south tonight/tomorrow or winds up tracking farther SW on Sunday, it will have more time over water and will thus have more time to intensify.
Now that we understand the rough contours of TD-19’s forecast including what factors will influence its intensity, its a good time to present the NHC’s forecast for not just the storm’s track (cone of uncertainty + “most likely” centerline forecast) but also its size. Note that the system starts out relatively small but quickly expands as it approaches the northern Gulf Coast. This means that, as per usual with tropical cyclones, impacts will extend well beyond where the actual center makes landfall. So even though we still don’t know where exactly the center will move onshore (SE LA or FL Panhandle or anywhere in between), folks along this part of the Gulf Coast should start getting their hurricane plan ready tonight/tomorrow in anticipation of needing to take actions Sunday or Monday.
As the storm approaches the northern Gulf Coast on Monday, it will slow down and turn towards the west in response to a bit of a collapse in steering currents.
This area-averaged sounding from the GFS valid Monday morning shows very little environmental steering flow over the system, aside from some very weak (~5kt) east-southeasterlies in the lower levels of the atmosphere. What impacts might this have on TD-19 and the weather for W FL/S AL/S MS/SE LA?
The first noteworthy impact will be an increase in the heavy rain threat. Slow-moving tropical cyclones, as this looks to be, are notorious for producing very heavy rainfall. Even if the storm doesn’t become a hurricane, it will produce very heavy rain that will cause flooding problems even in places well away from the center. So if your location is prone to flooding and you’re anywhere between Tallahassee Florida and Lafayette Louisiana, you should start thinking about what actions you might need to take should high water become an issue. Hopefully you already have a plan in place so that if/when watches and warnings are issued by your local NWS office, you’re ready to act in a timely manner.
The other impact of the storm’s slower forward motion near landfall mightbe a bit of a reduction in the wind threat. Tropical cyclones, especially if they’re strong, churn up the waters beneath them. When a cyclone is over shallow water, it’s able to mix up cooler water from below the surface a bit easier than if the cyclone is moving over a deep reserve of warm water. So I could see TD-19 intensifying into a hurricane on Sunday night before fading a little bit in the last few hours drifting into landfall. Note that this isn’t a guarantee, and you should not plan on this last-minute weakening, but it’s something to keep an eye out for as we watch the storm’s track forecast shift around a bit in the coming days.
Eastern Atlantic Systems
Out in the eastern Atlantic, we’re still keeping a half an eye on Rene, 95L, and the next wave sliding off Africa. Rene poses absolutely zero threat to any landmass, the next wave might bring some breezy showers to the Cabo Verde Islands, and 95L is the one to watch for potential development into a long-track hurricane next week.
That said, ensemble guidance is still so inconclusive regarding the future of 95L past three or four days that there’s no point speculating much about what potential impacts might look like in the Caribbean or elsewhere. If you’re in Puerto Rico or the northeastern Lesser Antilles, keep a close eye on this system in case it becomes more of a threat this weekend/early next week. For the rest of us, we have plenty to focus on with TD-19 for the next four or five days until guidance gets a better handle on 95L.
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Today, September 10th, marks the official climatological peak of hurricane season, though activity doesn’t usually start subsiding until later in October. Given the time of year and all the conditions both seasonal and subseasonal that are favorable for enhanced tropical cyclone activity, it’s no surprise we have so much to keep an eye on in the Atlantic.
This post will take a fairly deep dive into the forecasts for Paulette and the two disturbances in the Gulf of Mexico. Rene poses no threat to any landmass and thus won’t be covered. I’ll also mention the next waves coming off Africa, though there’s so much near-term uncertainty regarding how those entities interact that I’m not going to spend much time trying to explore the medium-term range of possible outcomes which spans from a hurricane in the Caribbean to a tropical storm dissipating over the far eastern Atlantic. I’ll revisit these waves on Saturday once we have a better idea of how they’re developing offshore.
Satellite imagery today shows Paulette struggling significantly with dry air and wind shear attributable to an upper-level low located west of the system. Note on this “sandwich” satellite imagery that the low-level center visible as the swirl of low clouds in white is located south/southwest of the mid-level center and its associated deep convection which is visible as the red blob indicating very cold cloud temperatures. So long as the storm remains misaligned like this, it will not be able to strengthen, and in fact will continue to weaken.
This area-averaged sounding over Paulette valid tomorrow evening shows dry air and southwesterly shear continuing to impact the system as it continues moving NW. The benefit of looking at an area-averaged sounding like this is that the system’s circulation disappears (average the winds across a circular circulation and you get net zero) which allows us to peer into the system’s environmental wind field. From this, we can clearly see the southwesterly shear vector arising from a difference between low-level easterlies and mid/upper-level southerlies/south-southwesterlies.
Aside from attempting to separate Paulette’s low-level center form its mid-level center, this change in wind direction with height also has implications for the system’s track forecast. If the storm is able to retain really robust convection and maintain a deeper (albeit tilted) vortex, it will “feel” the steering influence of those upper-level southerlies. That means the storm will track farther north. If the system weakens and has a shallower vortex, it will be steered mostly by the low-level easterlies. As a result, a stronger Paulette tonight/tomorrow will wind up farther north by Saturday while a weaker Paulette will stay farther south.
By Sunday, Paulette will begin restrengthening and will make a turn to the west as upper-level flow shifts from southwesterly to easterly. This shift will be driven by the southwestward movement of the ULL currently shearing the system as well as the development of an upper-level ridge off the New England coastline. Note that on the area-averaged sounding over the circulation Sunday morning, winds throughout the atmosphere are out of the east at around 10-15 kts. This lack of change in wind speed or direction with height is the lack of shear that tropical storms need to strengthen.
So how long will the storm move west and how close to the US East Coast will it track? By Monday, the storm will turn towards the north, away from the East Coast. Whether this happens 500 miles offshore, as currently forecast, or 300 miles, or 800 miles, it’s not exactly clear. But I’m quite confident that this is not a system headed close enough to the East Coast to bring substantial direct impacts. Why?
This forecast map shows the upper-level pattern on Monday afternoon. There are three main features contributing to Paulette’s steering flow at this point: the upper-level low that previously had been steering the storm west, a new mid-latitude trough digging southeast through the Canadian Maritimes, and an upper-level low developing over eastern Florida. All of these features will be pushing Paulette north-northeast. If there were a big area of high pressure where that mid-latitude trough is located, I’d say watch out along the East Coast. But the pattern simply isn’t set up to allow those big ridges to hang out off the New England coast. Why not? Typhoon Haishen basically “undid” the jet stream amplification that Typhoon Maysak caused last week. So instead of a very wavy jet stream with slow-moving troughs and ridges, we have a flatter jet stream with features moving relatively quickly from west to east.
With that in mind, despite considerable uncertainty regarding how far west Paulette goes in the next 72 hours, not a single EPS ensemble member brings the system close enough to bring substantial direct impacts to the US East Coast. If you’re in Atlantic Canada, especially Newfoundland, you should keep a close eye on Paulette. Bermuda is the most likely landmass to see substantial direct impacts from Paulette as the storm turns north on Monday.
While rain/wind/storm surge is not expected along the East Coast from Paulette, the shoreline will still take a beating from the system in the form of powerful long-period swells. This animation shows forecasts for “wave power” over the next week as Paulette restrengthens and recurves off the East Coast. These swells will cause dangerous conditions for mariners as well as large breaking waves at exposed locations along the shoreline. If you’re an experienced surfer, enjoy the sweet waves. For everyone else, be mindful of the high rip current risk these waves will pose at beaches exposed to the ocean.
Gulf of Mexico Disturbances
Elsewhere in the Atlantic, we have two disturbances to keep an eye on for possible development in the Gulf of Mexico late this weekend into early next week.
This satellite loop shows one of those disturbances which is currently located just east of the Bahamas. The system is producing a lot of convection this afternoon which is impressive because this is the time of day least favorable for convection over tropical waters. That’s one sign that this disturbance is healthy. Another sign is that cirrus clouds produced by these thunderstorms are fanning out away from the system in an anticyclonic pattern, especially over the eastern half of the disturbance. Note the cirrus moving north to the north of the system, to the west on the southern side of the system, and east on the eastern side of the system. This indicates an upper-level environment favorable for further development.
The other disturbance is producing scattered thunderstorm activity just off the west coast of Florida this evening, but doesn’t look like much on satellite imagery.
By Sunday morning, both systems will be in the Gulf of Mexico. Looking at forecast guidance for this timeframe, it’s clear why model guidance and NHC forecasters are significantly more bullish on the eastern system. There’s a whole lot more moisture and instability available for that system to work with compared to the western system.
That said, any time you get a disturbance spinning over the very warm waters of the Gulf of Mexico, it doesn’t take much for deep convection to develop and mix out any mid-level dry air, especially without any wind shear (there won’t be much if any shear in the Gulf this weekend). So we’ll keep an eye on both systems, even though I think the eastern one has a much better shot at becoming a tropical cyclone.
Here’s a look at afternoon ECMWF ensemble guidance for these systems. Note that pretty much no ensemble members develop the western disturbance beyond a brief TD. The range of possible outcomes for the eastern wave as depicted by the EPS ranges from no development at all to a mid-grade tropical storm. Given the favorable conditions expected in this area, my personal “upper limit” of what seems plausible from this system is a bit higher (perhaps in the strong tropical storm range).
If you’re in southern Florida, you should expect periods of breezy showers as this system passes by tomorrow. If you’re in Louisiana, Mississippi, Alabama, or the Florida Panhandle, you should be paying attention to forecasts in case this system turns out to be something worth thinking more about.
Eastern Atlantic Systems
Out in the eastern Atlantic, there are two additional systems to keep an eye on for tropical development over the next few days. Both are clearly visible on satellite imagery this afternoon.
The central question regarding these systems is the extent to which they will interact over the next 24-48 hours. One possibility is that the western wave will move quickly west as its center develops under that northern lobe of convection south of the Cabo Verde islands. If that ends up happening, and the eastern wave moves offshore a little slower, the two systems will remain separate and we could see two named storms develop this weekend. Another possibility is that the western wave ends up consolidating its center farther west, moves more slowly, and interacts more significantly with the eastern wave. If that ends up happening, we may only get one storm to develop in this area.
A look at ensemble member forecasts for the next few days show two distinct “camps” representing each of the possibilities outlined above. Generally speaking, the ensemble members in the “fast and separate” camp are more threatening for interests in the Lesser Antilles, and eventually the Greater Antilles/US much later down the line. Ensemble members that show more interaction and an overall slower storm motion are substantially more likely to show the system recurving off into the open Atlantic like Rene and Paulette.
Which scenario is more likely? I’m not quite sure at the moment because the problem of “which vortmax will become dominant” is really hard to solve due to its dependence on nearly-random convective interactions. Once both waves are offshore late tomorrow/Saturday, I suspect we’ll have a much better idea of what will happen.
Until then, monitor forecast info closely and make sure your hurricane plan is ready to go if you’re reading from the Lesser Antilles. If you’re reading from anywhere northwest of Puerto Rico, check back in on Saturday to see what’s up and until then, don’t worry too much about these systems. There’s still plenty of time to watch and wait, painful as that may be.
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This week has been full of exciting new developments for me as I move into the next chapter of life as a weather forecaster and aspiring meteorologist. After moving on from a very fulfilling several years at weather.us/weathermodels.com, I’m now working on a few different projects to keep folks informed as hurricane season ticks on.
You can find my updates on your phone with the Hurricane Tracker App, and on Patreon with Mark Sudduth’s HurricaneTrack project.
So why am I opening up another platform here? One reason is that I’ve gotten some questions inquiring about signing up for my updates via email. The option to have updates emailed to you is one that folks following my ME/NH weather blog seem to have appreciated over the years, and this platform will allow for that capability.
Additionally, my other projects each have a specific audience/ purpose/topic area focus. Info posted on the Hurricane Tracker App is aimed at a more general audience interested in quick updates. Info posted on Mark Sudduth’s page is intended for deeper dives “behind the scenes” to support Mark’s work chasing down and documenting landfalling storms. This page will be a catch-all for commentary and updates about weather not just in the tropics but wherever interesting stuff is happening.
So if you’re interested in a “classic” blog interface with the option to sign up for updates via email, this is the spot. If you’re interested in a slick mobile interface with updates that are more concise, Hurricane Tracker App is the place to go. If you’re interested in some more behind-the-scenes stuff in support of a really cool hurricane science project, head on over to Mark Sudduth’s page.
Hopefully you will find my updates interesting and informative during periods of active tropical weather and when things are quieter. As always, feel free to reach out with any questions you may have about the weather. I’ll do my very best to get back to you!
Jack Sillin's analysis of the latest atmospheric happenings