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.
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.
Jack Sillin's analysis of the latest atmospheric happenings