The big story in the tropics this morning is still Hurricane Sally which is crawling ever so slowly towards the Gulf Coast.
Imagery via CIRA SLIDER
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.
Imagery via TropicalTidbits
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.
Model maps via TropicalTidbits
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.
Imagery via CIRA SLIDER
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.
Trend graphic via Brian Tang’s TC page
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.
Model graphic via TropicalTidbits
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.
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.
Imagery via NESDIS STAR
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.
Imagery via NESDIS STAR
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.
Image via TropicalTidbits
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.
Imagery via TropicalTidbits
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.
Image via TropicalTidbits
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.
Image via TropicalTidbits
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.
Image via TropicalTidbits
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
Jack Sillin's analysis of the latest atmospheric happenings
