There’s a whole lot of mayhem continuing to plague the Tropical Atlantic and some portions of the land surrounding it this afternoon.
We’ve had two more storms, Wilfred and Alpha, named this morning which brings us into the Greek Alphabet for the first time since 2005 and the second time since tropical storm naming began back in 1950. Wilfred is located southeast of Teddy and poses no threat to any landmass. Alpha is barely visible on this image near Portugal where heavy rain and strong winds are ongoing. Meanwhile in the Gulf of Mexico, TD 22 is spinning between Texas and the Yucatan Peninsula. TD 22 does pose a serious threat to Texas in the form of heavy rain, but it will not be the subject of this post. Instead, we’ll focus on Teddy here.
Teddy is one of those hurricanes you see on the front cover of meteorology textbooks. It has a circular and nearly symmetrical eye, a ring of extremely intense eyewall thunderstorms, and expansive upper-level outflow in all directions away from the center. As of the 11 AM EDT update from the NHC, Teddy had maximum sustained winds of 130 mph making it a Category Four storm.
Microwave imagery from polar-orbiting satellites lets us peer under the upper-level clouds into the storm’s structure.
The storm’s eyewall is clearly visible using this tool as the ring of red/green surrounding the eye which is visible in bright purple. The northeastern eyewall is clearly much more intense than the southwestern eyewall. That’s due to a bit of southwesterly wind shear the storm is dealing with on the southeastern side of a weak upper-level low. Teddy’s outflow is working to blast a hole in that upper-level low so the storm can continue moving northwest with minimal disruption.
The only thing slowing Teddy down prior to its arrival in Bermuda will be interaction with a pool of cold water left by Paulette. This should be enough to knock the storm’s maximum winds down a little bit, but all most of that angular momentum will stick around in the form of an expanding wind field and the balance will be transferred to the ocean in the form of massive waves.
Thankfully, most model guidance has the storm sliding just east of Bermuda as it makes its closest approach on Sunday night. That will put the island on the “weaker” side of the storm, and perhaps just outside the eye. However, because the storm’s windfield is so massive, strong winds are basically a lock for the island at this point. It’s just a matter of figuring out whether that’s 50 mph, 75 mph, or 100+ mph.
After Bermuda, Teddy is still expected to make a turn to the north or north-northwest as an upper-level low cuts off to the west/northwest of the storm. The exact details of this interaction between the hurricane and the trough are still yet to be determined. That said, guidance has shifted east a bit in the last day or so. That means a direct hit for eastern New England now looks unlikely. Nova Scotia is much more likely to get the brunt of Teddy as it moves onshore as a hurricane rapidly undergoing extratropical transition.
This is the forecast shown by the majority of EPS ensemble members, though it should be noted that outlier solutions exist both to the west (farther into the Gulf of Maine) and to the east (closer to Sable Island).
This is one model depiction of what the storm could look like by Tuesday morning. The shading on the map depicts winds at 10m above the ground. If you don’t have your ruler handy, by this point in the cyclone’s evolution, the storm could have hurricane-force winds extending well over 100 miles from the storm center. So even if the hurricane’s maximum winds weaken dramatically before landfall (that’s expected to happen due to the cool waters in this area), strong winds will have no problem making it onshore.
With this in mind, Nova Scotia needs to be ready for serious impacts from Teddy including heavy rain, strong winds, and storm surge.
The large windfield will also produce massive waves.
Significant wave height forecasts for Tuesday morning are approaching 50 feet near Teddy’s center off Nova Scotia while waves over 10 feet roll onto the East Coast from Maine to Florida and the Bahamas. Near-shore breaking waves will vary in height by exact location, but this shows that the threat for huge waves and all the dangers they pose (beach erosion, rip currents, washing people off the rocks, etc.) is quite high.
The big story in the tropics this morning is still Hurricane Sally which is crawling ever so slowly towards the Gulf Coast.
The storm has been struggling to tighten up its inner core in the face of mid-level dry air and some southwesterly wind shear. Note the lack of cloud cover southwest of the storm’s center compared to the wide expanse of clouds to the east of the center. While the shear is good news for folks in the path of Sally’s inner core, it is bad news for the Florida Panhandle which is seeing torrential rains this morning despite being relatively far from Sally’s center.
Here’s a look at early morning (6:15 CDT) radar imagery of Sally showing a ragged eyewall wrapping about three quarters of the way around the center. The eyewall is open to the southwest which is what we’d expect given the southwesterly wind shear and dry air located west of the system. Heavy rain and strong winds are now moving into southern Alabama and far southeastern Mississippi and the window of time to prepare safely for the storm has closed accordingly.
Farther east, an intense feeder band has set up from Mobile Alabama to Pensacola Florida and southeast into the Gulf of Mexico. This band will likely linger in this area for the rest of the day today into tonight, and will produce extreme rainfall totals of 1-2 feet along with embedded tornadoes.
Most model guidance is now in agreement that Sally’s center moves onshore late tonight near Mobile Bay in Alabama as a Category One or Two storm. This would put the system’s strongest winds and highest surge right over the city of Mobile. It’s important to note that the worst of the surge in Mobile Bay will hit after the eye has passed which is when winds shift from cross-shore to onshore.
After moving onshore, Sally will continue northeast tomorrow and Thursday, bringing heavy rain along with it. Folks in N GA and parts of NC/SC should be ready for flooding even though Sally will be rapidly moving towards dissipation at that point.
I’ll be covering this storm in much more detail on twitter today.
The main focus of my coverage in recent days has been on Hurricane Sally which poses a serious threat to the Gulf Coast, but there are six other tropical entities in the Atlantic that bear watching today. Thankfully, none of them pose the level of risk to land that Sally does. This post will briefly examine each of those other systems. Another post will focus on taking an updated look at Sally.
Satellite imagery of the Northwestern Hemisphere clearly highlights each of our disturbances to watch today. Our four named storms in various states of organization plus three disturbances that we’ll have to watch for potential development in the next few days.
Off the East Coast, Paulette continues to move northeast away from any and all landmasses.
The storm was expected to intensify during this part of its journey due to lower shear and favorable interaction with a mid-latitude jet streak over far eastern Canada. However, the storm’s inner core structure is a bit sloppy (a la Sally) and it hasn’t really been able to pull itself together. Regardless, the storm remains quite dangerous for mariners in this part of the North Atlantic.
Paulette’s remaining threat to land will come in the form of large swells along the East Coast today.
This buoy about 50 miles southeast of Nantucket shows swell heights around 8-10 feet this morning with a period (not shown) of 14-15 seconds. That means these waves are packing tons of energy and will be quite dangerous for those that get close to them as they break. If you’re an experienced surfer, have fun riding these waves. For the rest of us, keep a safe distance.
Moving farther out into the Atlantic, we’ll find Tropical Storm Teddy several hundred miles east of Barbados.
Teddy looks really good on satellite imagery at sunrise this morning, it just needs to focus its intense thunderstorm activity near its center. Once that happens, it should have no trouble intensifying into a major hurricane. The storm is currently in a moist, low-shear atmospheric environment above very warm ocean waters. Its intensity ceiling is very high, and we may see it make a run at Category Four status by this weekend.
Teddy is forecast to move basically due northwest for the next five days, during which time it will not impact land. There’s some uncertainty once we get to the day 4-5 timeframe when some guidance suggests a potential shift in the storm’s track off towards the west-northwest. As of right now, I’m not inclined to see Teddy as a major threat to land but it does bear watching. We’ll revisit this steering pattern after we’re done with Sally.
Northeast of Teddy is tropical storm Vicky.
Sunrise over Vicky reveals a cyclone being acutely impacted by strong southwesterly wind shear. Notice how the storm’s low-level swirl is displaced significantly from its deep thunderstorm activity. This shear is being delivered by a strong upper-level low to the northwest of Vicky and will bring about the system’s rapid demise later today.
Rene became a remnant low yesterday, so this concludes our tour of the active tropical cyclones in the Atlantic not named Sally.
Closer to the African coast we’ll find our next tropical wave to watch for development.
This wave is producing relatively deep convection this morning and it’s starting to coalesce around what might be an incipient center of circulation. The environment surrounding the system is fairly supportive of intensification, and I’d expect to see yet another tropical depression or storm develop from this system in the coming days.
Most model guidance isn’t too bullish on this system’s future intensity, though the GFS parallel model (new version in beta testing) takes it up to hurricane strength briefly while recurving into the open ocean. No impacts are expected from this system. If named, this will be called Wilfred and will use up the last name on the list for the Atlantic this season. Future storms will then be referred to by the various letters of the Greek Alphabet.
One of those future storms may very well be this feature west-northwest of Portugal.
Satellite imagery shows the system’s frontal features becoming detached from its center where convection is beginning to build. If convection persists for long enough, it will release enough latent heat into the storm’s core for it to be considered a tropical or subtropical storm.
This graphic shows the cyclone’s expected evolution from “asymmetric warm core” or subtropical (which is where it is right now) towards “symmetric warm core” or tropical by later today/tomorrow. Even if the storm does become tropical, it will weaken substantially before impacting Portugal in four or five days. But perhaps the storm could snatch the name “Wilfred”, leaving the wave in the eastern Atlantic to become “Alpha”.
Way back in the southwestern Gulf of Mexico, we’re still dutifully watching this system which is producing some shower and thunderstorm activity east of northeastern Mexico.
The system hasn’t yet been able to focus its thunderstorm activity around a single center, but if this current burst is able to hold through the day today, it could. NHC penciled in a recon flight for this afternoon, but unless it looks like a low-level center might be present, they’ll probably cancel it.
Either way, it doesn’t pose much threat to land in the short-term. We’ll revisit its future in more detail after Sally.
Finally, it appears as though the Atlantic Basin’s tropical cyclone activity is mildly contagious.
The Mediterranean Sea may join in on the action this week as a low near Libya moves northeast towards Greece. The Mediterranean gets systems like this every once in a while, so this development (if it were to occur) would be unusual but not unprecedented.
More coverage of these systems in the coming days, especially after we’re done with Sally.
Yesterday, we talked a lot about how Sally’s intensity forecast was quite uncertain but that the storm was likely to intensify into a hurricane if it was able to wrap deep convection around its low-level center. Unfortunately, that has been happening over the past twelve hours or so, though in fits and starts.
Last night’s convective burst waned a little bit as it ran into shear and dry air to the west of the storm’s circulation, but another burst has taken its place this morning and is once again attempting to wrap around the NW side of the storm. Whether or not it will succeed, only time will tell.
In the meantime, there is a bit of a shift in track guidance that’s worth talking about.
Yesterday, most guidance took Sally into SE LA before moving NW very near New Orleans then into SW MS or NE LA. Newer model runs are now suggesting the system may turn towards the north before reaching SE LA and instead make landfall in S MS. The HWRF does a particularly good job highlighting this trend, though it would work with any model of your choosing.
Why is this happening? Guidance is making some subtle adjustments to the steering pattern and is also expecting Sally to strengthen a bit faster today.
Remember that the stronger the storm gets, the more it will “feel” the southerly winds aloft that want to push the storm north. Because the storm appears to be strengthening this morning, I have confidence that this eastward trend in guidance is more signal than model noise.
What does that mean for impacts?
Here’s a look at the latest overview map from the NHC. Note the eastward shift in the forecast track from near or just west of New Orleans to near Biloxi Mississippi. The good news is for folks in New Orleans who are now very unlikely to see the worst of Sally’s rain, wind, or surge. Gusts to 40-50 mph will still cause power outage issues, and surge off Lake Pontchartrain (4-6+ feet) will still cause flooding. But given the potential Sally had to push hurricane or major hurricane-force winds through downtown NOLA, this shift in track is very welcome news.
When it comes to shifting tracks for landfalling hurricanes, good news for one area is bad news for another. The bad news is for southern MS, southern AL, and far western FL which will now be closer to the core of Sally as it moves onshore. Rainfall totals here will soar past 20″ as the system slowly crawls onshore. Meanwhile, surge will push into the various bays and inlets and could produce inundation of up to six feet as far west as the AL/FL border. Residents in these areas should be prepared to evacuate today if told to do so by local officials. Remember that storm surge risk is highly localized and depends on very small shifts in track/intensity. The values blanket outlined for large portions of the coastline may not be representative of how much water you see in your backyard.
While the water-related threats from Sally are far more serious than the storm’s winds, portions of the SE LA and S MS coastlines will see hurricane-force winds as the core moves onshore tomorrow. Tropical storm-force winds currently extend up to 125 miles from the storm’s center and will be capable of causing power outages even in areas that do not experience the storm’s core.
Here are a few more maps that show just the surge/rain/wind forecasts individually and are thus a bit easier to read.
Note that the highest surge, heaviest rain, and strongest winds are all focused near and just east of the center’s expected track. If the storm continues to strengthen today and ends up tracking a bit farther east, the worst of the storm would slide from near Biloxi MS closer to Mobile AL. Folks in this area should be preparing as though this were expected to happen. It’s also important to note that even on the storm’s current heading, one to two FEET of rain is expected for much of the MS/AL/W FL panhandles. Even if you don’t get Sally’s worst surge or winds, the rain is going to cause some serious flooding issues.
As the storm moves north on Wednesday, it will rapidly weaken though heavy rains will continue to push inland over MS/AL. The storm should dissipate by Thursday as it moves closer to Georgia.
I’ll have many more updates on Sally on twitter throughout the day. I also plan to do another post later this morning exploring some of the other tropical systems in the Atlantic basin.
While there are plenty (seven!) of systems to watch across the tropical Atlantic this morning, our focus remains on Tropical Storm Sally which is expected to intensify into a hurricane before impacting the New Orleans area and adjacent parts of the northern Gulf Coast beginning tomorrow and continuing through Wednesday.
In recent days, I’ve spent considerable time on the blog explaining the meteorological processes working both for and against intensification, as well as pushing the storm in different directions. If you want to dig into the nerdy details of this system’s forecast, feel free to read over last night’s post if you haven’t already. Pretty much all the info there is still current even if the satellite loops are a little stale. This update will focus much more on the system’s expected impacts as it moves onshore somewhere in southeastern Louisiana and/or southern Mississippi.
As a quick reminder, you can sign up via the box on the left sidebar to receive my updates via email the moment they are posted if that’s something you’re interested in.
Our primary guide for this morning’s updates will be the info NHC sent out in their 5 AM EDT advisory on the storm.
This map shows the storm’s expected track, intensity, and rain/wind/surge impacts as well as the watches and warnings in effect as of 5 AM EDT. Click the map for a larger version that’s a bit easier to read. The key idea to take away from this graphic is that serious impacts, especially heavy rain, will extend relatively far from the center of Sally, especially to the east. So folks in the Florida Panhandle and southern Alabama shouldn’t let their guard down despite not being near the center of the cone (or in the case of most of FL, outside the cone entirely).
This map is zoomed in near where Sally is expected to make landfall in southeastern Louisiana and shows the storm’s forecast track, intensity, and storm surge. Note that areas outside the New Orleans levee system could see over nine feet of storm surge inundation as Sally’s circulation pushes water from the Gulf of Mexico west and northwest towards Lake Pontchartrain. As someone unfamiliar with the New Orleans levee system, I do not know which levees might or might not be in danger of failure from a surge of this magnitude. Local officials in this area do have that information, and if they tell you to leave, please make every effort to do so. If you don’t have the resources to evacuate, please get in contact with your local officials who will likely be able to assist you with transportation, accommodations, etc.
Surge will also be an issue well east (and somewhat west) of where Sally moves onshore. Southern Mississippi and parts of southern Alabama could see over six feet of surge if Sally ends up making landfall in SE Louisiana as a hurricane (which is the current NHC forecast). Areas as far east as the Big Bend region of Florida will see surge from Sally in the range of 1-3 feet. This will cause problems in the usual low-lying spots prone to flooding from even “minor” events.
Heavy rainfall will pose an extremely serious threat to parts of SE LA, southern MS, southern AL, and western FL. Light westerly wind shear has pushed most of Sally’s thunderstorm activity to the east of the center which means that heavy rain will extend well east of wherever the center makes landfall. As the map above highlights, 9-12″ of rain is likely as far east as Panama City Florida. This amount of rain, with locally higher amounts (to 20″ or more) will cause serious and widespread flooding. If your area is vulnerable to flooding from heavy rain, consult with information from your local officials and be prepared to evacuate if told to do so.
Heavy rain will also be an issue near the center landfall location in SE Louisiana, but totals will drop off fairly quickly as you head west.
I’ve discussed wind last here because even for those areas expecting hurricane-force winds, what’s happening with the air will be the least of your worries. What’s happening with the water (both rain and surge) is a much bigger threat to your life. That said, strong winds will cause problems along and especially east of Sally’s center track. As of 5 AM EDT, NHC forecasts call for landfall as a 100 mph category two storm. That means that most places near the center of the storm will see sustained winds of 75-85 mph. Remember that the “maximum sustained wind” forecast number really only applies to areas over open water and perhaps some immediately adjacent beachfront locations. It’s also important to keep in mind that you don’t need 100 mph winds to cause serious issues. Even winds of “only” 50 mph will knock down trees and power lines. Winds of 50-75 mph will cause increasingly widespread tree/power line issues and may start to cause some minor structural damage. Winds above 75 mph begin to pose a more serious threat to siding/roofs.
All the above information is based on the best available forecast info we have this morning. But that’s not to say we’re 100% confident about how strong Sally will be at landfall or where exactly it will move onshore.
Recall from yesterday’s post that the environment Sally will occupy starting tonight is extremely favorable for rapid intensification ifthe storm can wrap convection around its inner core.
With that in mind, there remains the potential for Sally to strengthen more than currently forecast. I’ve advised my friends in the New Orleans area to prepare as if Sally were making landfall as a Major Hurricane (Category Three) because that outcome is well within the range of reasonable possibilities given the environment. If the northwesterly shear is just light enough to keep the low- and mid-level circulations displaced, and Sally ends up coming onshore as a “weaker” (but still quite dangerous!) hurricane, we can all be thankful it wasn’t worse. Prepare for the reasonable worst-case outcome (Category Three landfall) and hope for the reasonable best-case outcome (low-end Category One or high-end Tropical Storm landfall).
It’s also worth noting again that the category system only takes into account a storm’s maximum sustained wind speed which, as mentioned above, is not representative of the winds most folks (even near the center) will experience, nor does it convey any information about the significantly more dangerous rain/surge threats.
Looks Will Be Deceiving
Even in the scenario where Sally moves onshore as a Category Three hurricane (depicted by the 00z HWRF model as an example), it is not likely to achieve its final form until it’s right at the coastline.
This image is from that HWRF model run that represents the “reasonable worst-case outcome” for Sally. The forecast map is valid tonight at 8 PM EDT. Note that the system is still a mid-grade tropical storm with the low-level center significantly displaced from the strongest convection. Even if we were to see little organization of the system today, it still has the potential to rapidly intensify tomorrow. You should not wait for the storm to take on the appearance of a “classic” hurricane with a well-defined eye before you take it seriously. By then, it will be too late to do anything but hunker down and hope for the best.
This animation shows the HWRF’s prediction of what satellite imagery might look like between now and the storm’s landfall. Note that even in this bullish model depiction of Sally’s intensity, the eye only shows up clearly once the system is directly over New Orleans!
After making landfall, Sally will move slowly north until it dissipates over Mississippi. The slow movement is welcome news from a wind perspective (widespread wind damage will not continue well inland like it did with Laura) but is very bad news from a surge/rainfall perspective. Heavy rain and strong onshore winds will linger into Wednesday even as the storm weakens just inland.
I’ll have updates throughout the day on twitter and will most likely be back here on the blog this evening with an updated look at expected impacts.
It’s a very busy evening in the tropics as we watch four active tropical cyclones and two disturbances that might develop in the next five days.
Each of our systems is visible on this loop of the tropical Atlantic. From west to east, we have a disturbance in the Gulf (30% chance of development), Sally (near S FL), Paulette (SE of Bermuda), Rene (SE of Paulette), TD 20 (embedded within the Inter-tropical Convergence Zone about half way between Barbados and Africa), and Invest 97L (southwest of the Cabo Verde Islands). If you look at the far eastern edge of the loop, you’ll see the first hints of our next wave set to emerge off Africa in a couple days. Unfortunately, we’ll have to keep an eye on that one too.
This update will focus mostly on Sally because it poses the most serious threat to US interests. If you’re in Bermuda and curious about my analysis of Paulette with a focus on Bermuda impacts, please check out my update on Mark Sudduth’s page earlier this afternoon.
Sally has changed little in organization today as it meandered near southern Florida. The system’s low-level center is located a bit west of Naples Florida. Note that this is a bit northwest of the storm’s deep convective activity which is located near the Florida Keys. This indicates that the system’s low- and mid-level centers are still misaligned with the former being northwest of the latter.
For most of last night, that was due to northwesterly shear imparted on the system by an upper-level low to its northwest. This evening, that low is weakening and moving away from the system so I don’t anticipate we’ll see shear continue to be an issue for Sally as we move into tonight.
That said, area-averaged soundings suggest that low-level southeasterly flow will be a bit stronger near the surface than farther aloft. This actually is a bit of a northwesterly shear signal even though there aren’t really any northwesterly winds (excluding the 5kts at 150 mb) in the atmosphere. So while the low-level center will continue to “run ahead” of most of the convection, without a strong force pushing storms to the east of the center, I’d expect to see thunderstorm activity develop and persist near the center starting later tonight or tomorrow morning.
Why wait that long? The primary factor holding back convection on the north/northwest side of Sally isn’t any upper-level wind issue or mid-level moisture issue, it’s actually a low-level problem.
I made this graphic earlier today so the radar data isn’t “fresh” but it highlights the process well. Low-level air parcels (think right near the surface) flowing into the northwestern side of Sally have spent considerable time over the Florida Peninsula instead of over the warm waters of the Gulf of Mexico. As a result, they’re a bit drier than they need to be to support convection. Parcels flowing into the southern side of the system have spent considerable time over the Gulf of Mexico picking up moisture and heat from the warm water beneath them. As a result, these parcels are jam packed with energy to support convection in a way that those parcels originating over Florida are not.
As Sally continues moving west-northwest into the Gulf of Mexico, I suspect we’ll see convection start to develop a bit more on the northern side especially once the system passes Tampa Bay’s latitude. At that point. there will be much more open water to the storm’s northeast for parcels flowing into the northern side of the system to pass over and acquire heat+moisture.
The HWRF model, which is a high-resolution model specifically designed to predict hurricanes, shows this idea well. It depicts Sally struggling to organize until it passes north of 28N which is about where Tampa is. After that, it has a bit more “room to breathe” on the northern side and as a result is able to strengthen quite a bit more.
A look at the upper-level environment shows a setup extremely conducive for intensification come tomorrow evening. The storm will be located under an upper-level anticyclone well-positioned to vent the system in the upper levels of the atmosphere. That venting will be assisted by an upper-level trough northeast of the storm and an upper-level low southwest of the storm (the same one that has been causing shear today). This upper-level pattern is a very strong indicator of potential for rapid intensification if the storm’s structure is organized enough to take advantage.
As the storm approaches the Louisiana coastline, it will start to encounter another obstacle: mid-level dry air and increasing westerly shear ahead of an upper-level trough.
Storm-centered cross sections on the HWRF model show the potential for some dry air intrusion as the storm nears the SE Louisiana coastline. It’s really hard to tell ahead of time how exactly the storm will interact with this type of dry air/shear. If its inner core is intense enough, it will barely notice this level of disruption (a la Laura). If the inner core is still struggling to get together, it could weaken quite a bit (Marco was an extreme example of this, though I wouldn’t expect that much weakening from Sally). I include this in here not to downplay the threat from Sally but to offer a little bit of hope that the door is still open to a lower-impact scenario. Prepare for the worst and hope for the best.
So what should residents of the northern Gulf Coast expect from Sally?
This map shows various key information from the NHC’s 5 PM update including the cone of uncertainty, projected wind radii, projected maximum wind speeds, rainfall, and watches/warnings. Note that dangerous impacts in the form of heavy rain extend well east of the center. So while landfall is expected in Louisiana, heavy rain (6-12+”) capable of major flooding issues will fall as far east as the Florida Panhandle. Storm surge of 1-3 feet is projected as far east as the Big Bend area of Florida. So while the landfall point and cone of uncertainty thereof get lots of attention, remember that even if you’re outside the cone and perhaps outside the worst of the winds, this may very well still be a dangerous storm for you.
Also note that Hurricane Watches are now posted for parts of southeastern Louisiana including New Orleans as well as southern Mississippi and southern Alabama. The time to start preparing for hurricane conditions in these areas is now.
As Sally approaches the northern Gulf Coast, it will slow down in response to a building ridge over the Plains. It may only be moving 3-5 mph, about as fast as most people can walk/run, when it makes landfall. This means that the system’s winds will have additional time to pile storm surge into the coast and heavy rain will linger over the same areas for quite a while. The net result, particularly if the storm does end up moving quite slowly, is an amplification of water-related threats.
The system will move inland sometime during the middle of next week. Heavy rains will continue for a while inland, but because the storm is moving so slowly, intense winds are not expected to continue inland like they did with Laura.
If you’re currently under a Hurricane Watch, begin preparations for Sally now. Always heed the advice of local officials and consult with official NHC/NWS forecast information when making any decisions that involve the protection of life and/or property.
This is your morning briefing on Tropical Depression Nineteen which is the system in the Atlantic most likely to bring significant direct impacts to the US over the next several days. Of course there are several other systems worth watching (Paulette if you’re in Bermuda, 95L if you’re in the NE Antilles) but there’s not much new to say about those systems this morning, so they will be the subject of future updates.
One of the GOES-East instruments capable of capturing satellite imagery every 60 seconds is pointed towards TD-19 this morning which gives us a helpful look at the system’s organization. There have been several bursts of impressive deep convection associated with the system this morning, though they’ve all been south or southeast of the storm’s low-level center. How can we find the low-level center? Once the sun comes up you can use visible satellite imagery, but radar imagery and surface observations are a bit more reliable so let’s use those.
In white, I’ve highlighted some surface wind direction observations as well as the low-level swirl you’d see if you looked at a radar loop. Note that all the heavy rain (>40 dBZ) is south and east of the low-level center. While last night there was one cluster of thunderstorms making a convincing case to be the low-level center, that’s no longer the case this morning with a few different swirls drifting through the Florida Straits.
So what happens next? I see three general possibilities. The first is that deep convection redevelops over the low-level center once it moves offshore later this morning. This is pretty much the going NHC forecast, and it’s definitely possible. This would lead to TD-19 strengthening and becoming TS Sally by this afternoon. The second possibility is that the initial low-level center dies off and a new low-level center forms under one of those convective bursts to the south. This would also support strengthening later today, though it might take a little bit longer. The third possibility is that the initial low-level center remains dominant but doesn’t get any deep convection. That would lead to a much weaker system, at least for the next 12-24 hours.
Unfortunately, it’s darn near impossible to say which of these possibilities will occur. I’d put the odds somewhere near 45%-45%-10% for scenarios 1-2-3. There really isn’t much shear over the storm, and the shear that is around will be fading later today so I don’t see this vortex misalignment persisting for much more than the next 6-12 hours.
By tonight/tomorrow morning, the system will be over the Gulf of Mexico and gaining a bit of separation from Florida. It should then begin intensifying thanks to a favorable environment including very warm water, relatively low wind shear, and high mid-level moisture. If you want a more complete breakdown of TD-19’s environment, please refer to last night’s post.
This graphic shows some key information from the NHC’s 5 AM advisory along with a couple notes from me. The system did indeed job farther southwest than was originally expected, though note that this morning’s center fix is still within the cones of uncertainty from yesterday. So it’s not that big of a surprise. This has nudged the storm’s expected track a hair west over the next 24-48 hours. As a result, the system might have a little more time over water and “room to breathe” as it moves NW towards the SE LA/S MS/S AL area early next week.
The intensity forecast for the system remains mostly unchanged from last night. TD-19 should approach the coastline as a strong TS or low-end hurricane, though there’s some uncertainty regarding its exact intensity because we don’t know how quickly it will organize in the next 24 hours. I wouldn’t be surprised to see future updates show an increase in forecast intensity if the storm can consolidate today around a single low/mid level center under some persistent deep convection.
Tropical Storm watches are now posted for parts of the Florida Panhandle and will almost certainly be expanded westward towards SE LA later today. Residents in this area should be paying close attention to forecasts and should have their hurricane plans ready to implement if/when watches go up and local officials begin issuing guidance about how to prepare for the storm.
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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Jack Sillin's analysis of the latest atmospheric happenings