All eyes in the weather center continue to be on the tropical disturbance in the Bay of Campeche and western Gulf of Mexico. This is actually the same system that we’ve been tracking across the Atlantic and through the Caribbean for over a week now! Now that it has moved over into the southwestern Gulf of Mexico, and it is interacting with other disturbed weather already in the western Gulf, the system is encountering conditions that are becoming more favorable for tropical development. The early morning update from the National Hurricane Center maintains an 80% chance of this system becoming a tropical depression or tropical storm over the next few days, and models are in pretty strong agreement that Tropical Storm Francine will likely form in the western Gulf of Mexico roughly by about Tuesday or so.
Sea surface temperatures across the entire Gulf are well into the mid to upper 80s, and oceanic heat content is favorably high for tropical development, even though the highest heat content is east of where this system will track. With the disturbed weather in the Gulf the past several days, there isn’t a lot of dry air to content with, and for now, wind shear is light to non-existent. All of those factors are favorable for the development and intensification of the tropical system for the next few days. There is even some chance (although not necessarily a big one) that this may try to become a minimal hurricane before landfall. However, just because conditions are favorable for strengthening right now doesn’t mean they stay that way. As potential Francine lifts north toward the upper Texas and Louisiana coastlines, a trough of low pressure embedded within the mid/upper-level jet stream will be approaching from Texas and the southwestern U.S. This is the main catalyst that will pull the system north and northeast through the week. As it approaches, stronger winds aloft will overspread the tropical system and introduce vertical wind shear. There’s a very decent chance that increasing wind shear would put a lid on how strong the system could get before it makes landfall by late Wednesday night or Thursday morning.
There is some small potential that increasing jet stream would act as divergent outflow on the north side of the system for a short time as the tropical system approaches the coast. This has happened several times in the last few years, and has been a major catalyst in us having rapidly intensifying hurricanes upon approach to landfall. HOWEVER, the time window for this to possibly happen would be VERY SHORT. So, while that may give it some window to possibly become a hurricane prior to landfall, that window will close well before the system would have a chance to become a major hurricane. It’s also important to point out that in the vast majority of those cases, the trough of low pressure was north of those tropical systems and pulling away, and because of that, were NOT causing wind shear over the system. With the trough approaching from the west, that is not the case this time, and it is more likely that the system would simply get involved with wind shear instead of having a favorable outflow pattern. Because of all this, it is most likely that the system would make landfall as a tropical storm, with some potential to be a minimal hurricane, versus something much stronger. Simply having a tropical system in the Gulf in September does NOT always mean a major hurricane.
The various computer models are in good general agreement that the system develops and then lifts northward through the western Gulf through midweek before hooking northeast toward the upper Texas or Louisiana coast for a landfall late Wednesday night into Thursday morning, and then continuing to track northeast up into the Deep South and Tennessee Valley through Thursday, Friday, and Saturday. However, until we get the system to actually come together to develop a singular low-level circulation, the models will waffle around a little on the exact track, and it’s that exact specific track that will determine our local impacts here in the Tennessee Valley later in the week.
Having said that, we are getting increasing confidence that there will be some local impacts here in the general Thursday/Friday timeframe because consistency and agreement that continues in the data. On even a very general track such as the one depicted, this system would likely bring a decent soaking of rain to the area in the latter part of the week, even if there are some minor deviations in the exact track. It’s just that the exact specific track will determine specifically how much rain we will get and whether there may be a flooding threat. And yes, we are in a drought, but that could actually help increase the risk of flooding because the ground is harder and less willing to soak in moisture at first versus it running off into streams, roadways, etc. The left of the three maps above shows the current forecast rain totals for the next 7 days from the NWS Weather Prediction Center. They paint an areawide 2-3″ of rain over our entire area with higher totals back south and west. I think that is a good general expectation to start from, but the specific location of those higher totals may shift around as the exact track of the tropical system is figured out. The GFS and Euro models above also show that, while the 2-3″ broad-brush is a good baseline to start from, for whoever gets into the heavier rain (whether it is us or someone else near us instead) there is the potential for totals to actually be in the 4-6″ range with isolated heavier amounts possible. The models have inched a little east tonight from their depictions earlier today. That has caused them to shift those heavier totals into our area. That could end up being the truth since most systems making landfall on the northern Gulf Coast with an approaching trough from the west like this one historically inch to the right with time. However, it is simply too early to know if that will be the case. Because of this, we can’t take these model rain totals at face value, but keep them in the back of our mind as an example of what could happen depending on the track of the system, and then we wait for consistency to figure out the details. IF we get into heavier totals like that though, there would certainly be a flooding and flash flooding threat. It’s just simply too soon to know if that will be the case here locally or if it will be positioned just “near by”.
Anytime a tropical system makes landfall, there is always the question of whether a severe storm and tornado threat will materialize. We will certainly have to watch for that, but it’s way too soon to know if that type of threat may materialize in our area. The first, and biggest, reason is because that is also tied heavily to the exact specific track of the center of circulation. In order for a tornado threat to materialize, you need to be to the right of a tropical system relative to its forward motion, and most favorably, in the outer rain bands on its right side instead of in the steady rain shield closer to the center. This is so that thunderstorms in the outer rain bands can encounter a combination of instability and vertical wind shear and spin. If the center of circulation were to track closer to our area or overhead, that would shunt any potential tornado risk off to our south and east. However, if the low were to track closer to the Mississippi River or just west of it, while that may reduce our flooding potential, it would potentially put us in a more favorable corridor to see a severe storm / tornado threat. It’s simply too soon to try to speculate on those details just yet.
I will say that the secondary reason why I’m not quite bullish on jumping on a tornado threat here locally just yet, even with a favorable low pressure track, is because unlike a lot of tropical systems, this one will be moving into what will initially be a dry and stable air mass in our region. We’ll be warming into the mid to upper 80s the first half of the week, but the leftover dry air from this weekend cool wave will still be lingering over us as the tropical system approaches. That initial period of steady rain falling into a dry low-level air mass will allow for evaporational cooling (similar to what we see in the winter months with a rain shield moving in when there’s a wedge of cooler and drier air here), and that may help to lock in a more stable low-level air mass over the area. Having a low-level stable air mass in place would reduce the tornado threat, regardless of the low track. Should that happen, it would mainly be just heavy rain and breezy conditions.
For those of you that have relatives on the coastline, make sure they are paying attention, but there is no need to panic. There is no reason to believe this will be a violent hurricane at landfall. However, there could be some major flooding in coastal Louisiana, as they have already had big time rain the past several days. For our local area here, confidence in local impacts from rain and breezy conditions is increasing for the general Thursday/Friday timeframe, but it’s simply too far out to know if that will also include a flooding and/or severe storm & tornado threat. We’ll be able to get a better handle on those potential threats (or lack thereof locally) over the next couple of days as the tropical system organizes and the exact specific track becomes more certain.
