After becoming a hurricane early last evening, Francine has continued to intensify overnight with maximum sustained winds of 90 mph as of the 4am advisory this morning. A little additional strengthening is expected before wind shear and dry air entrainment increase this afternoon before the system makes landfall, and Francine is officially forecast to become a Category 2 hurricane today prior to making landfall on the Louisiana coast. Francine is then forecast to track up through Mississippi into western Tennessee or far eastern Arkansas as we head from Thursday into Friday, becoming a remnant low going into Friday. With Hurricane Francine tracking just off to our west, that puts us on the “dirty” east side of the circulation center, and that will mean significant impacts to our local weather here from Thursday into Friday. Let’s talk about what to expect…
Overall Timeline of Events
Predawn Thursday morning through early/mid afternoon: Even as early as late this afternoon and into tonight, a few spotty showers are probable to lift northward into the area, but the initial dry low-level air mass will keep rain totals thru about sunrise Thursday morning general 0.1″ or below. As we head into the predawn hours of Thursday morning, the more steady rain from Hurricane Francine’s remnants will spread from south to north across the area, along with gusty winds of 30-45 mph. As we head deeper into the morning, this rain will get heavier with time, and there could be some localized instances of street flooding or minor flash flooding in poor drainage areas as early as the morning hours of Thursday. As we head into the midday and early/mid afternoon, the steady stratiform rain starts to lift to the north with time, but additional off and on heavy downpours will be situated in banding across the area.
Thursday late afternoon through overnight hours: As we head deeper into the afternoon and the steadier rain shield shifts north and west of our area, a bit of mid-level dry air works into the region as it gets ingested into Francine’s circulation. This and the steadier rain coming to a stop allows the southeasterly winds to bring warmer air, higher dewpoints, and more instability into northern Alabama and eventually southern Tennessee. The drier air aloft may even allow for an afternoon quick sun break or two, which would further allow that destabilization as temperatures warm back into the low/mid 70s and dewpoints climb into the upper 60s and lower 70s. As this happens, bands of more cellular type storms will start to set up across north-central to northwest Alabama up into southern Tennessee and maybe as far west as far northeast Mississippi. These banded storms will have higher rainfall rates and will bring a better chance of flash flooding over time as they begin to train over the same areas as we head into the evening and overnight. As we get the instability to work into the area during the late afternoon and carrying into the overnight, it’s these feeder bands of cellular storms that will also have to be watched for a threat of a few isolated spin-up tornadoes. In addition, since we are working in a little dry air aloft and we’re getting a more surface-based instability profile during this time, those higher wind gusts will be easier to mix down to the surface, and its from the late afternoon into the overnight when the potential for 40-45 mph wind gusts will increase, and we can’t rule out a gust as high as 50 mph here and there.
Thursday late overnight/Friday predawn through Friday midday/afternoon: Banding of heavy rain and storms with a continued flash flooding, gusty wind, and spin-up tornado threat will be ongoing by midnight over north to northwest Alabama into southern middle Tennessee. Unfortunately, because of the strong wind shear in the atmosphere and higher dewpoints moving north during the evening and overnight, this is not going to be a case where the threat decreases with the loss of sunlight. If anything, the spin-up tornado threat will be higher during the evening and overnight than it will be during the afternoon hours of Thursday. As we get closer to 2-4am early Friday morning, the Baron high-resolution Futurecast model starts to shift that banding of heavy storms eastward across our area, and by sunrise, it is shifting out of our viewing area and over into eastern Tennessee and northeast Alabama. This is just one computer model we are showing, but other high-resolution models are agreeing with this general progression of things. As we head deeper into the morning and afternoon, while we get some sun breaks, the Baron Futurecast also shows drier air aloft mixing down to the surface and lowering instability despite temperatures climbing into the upper 70s to lower 80s. Some of the global level models like the Euro still have the instability farther west, but the higher-resolution models usually handle this better once we get within time range of them, and if this is indeed correct, this may shift the heavy rain and tornado threat completely east of our viewing area for Friday.
Rainfall Totals and Flash Flooding Threat
With higher confidence in where the training band of heavier rain and storms is likely to set up across our area, we are now able to highlight a zone from southern middle Tennessee into north Alabama where 3-5″ of rain is projected to occur through Friday evening. There are several models that suggest that isolated totals could even be higher than that in this zone. In addition, a 3-5″ rainfall with isolated higher totals is likely just to our west over north Mississippi, closer to the track of the center of circulation. Between those two zones over western parts of southern Tennessee down into western parts of northwest Alabama, we are still projecting a general 2-4″ of rain through Friday evening, but keep in mind that there could also be isolated heavier totals in these areas as well. In addition this does not include off and on weekend rain showers and rumbles of thunder that may add another half inch to one inch of rain to the overall totals going through Monday.
Where these heavier totals happen, there will be an increasing threat of flash flooding of the street and poor drainage variety as the training storm bands will put down the rain in a short amount of time, and with the drought conditions hardening the ground, the soil won’t soak in the rain as fast as it needs to, leading to runoff. River/lake/stream levels are fairly low because of the drought; so, we don’t expect big flooding issues in that regard, but small creeks and streams may have minor flooding issues in the heaviest rainfall zones.
High Wind Gusts
As the steadier rain shield starts to spread into the area Thursday morning, it will bring with it increasing winds and gusts at times of 30 to as high as 40 or 45 mph spreading from south to north. Attached at the left is a time of arrival map for the projected start times of the 30-45 mph wind gusts. For our northwest Alabama and northeast Mississippi zones, that still looks to be between the 3am-7am timeframe Thursday. In southern Tennessee along the U.S Highway 64 corridor, our thinking of 6am-9am for the arrival and start time of those higher wind gusts has not changed. Our thinking of a start time of 8am-11am for the Highway 412 corridor in Tennessee has not changed either.
Keep in mind, these are projected start times for these higher wind gusts. Once they start, they will be with us off and on through the day Thursday, through Thursday night, into the morning of Friday before the remnant wind fields from Francine finally weaken with time through the day on Friday. Also, as we get more surface-based instability and drier air aloft into the area from the late afternoon into the overnight of Thursday, this will actually increase the potential for off and on 40-45 mph wind gusts, and we can’t rule out an isolated 50 mph gust here and there. That means wind gusts may be high enough to cause a few trees to come down and possibly cause some isolated power outages. We do not expect widespread long-duration type power outages, however.
Tornado Threat
With Francine tracking up through Mississippi Thursday into Friday and our area being on the eastern side of the circulation, that means we will have to watch for the potential for a few isolated spin-up type tornadoes across our area. The threat of tornadoes on the right side (relative to forward motion) of a landfalling tropical storm or hurricane is one of the most common things ever. It’s actually extremely rare for a hurricane to make landfall and there not be at least a few tornadoes involved.
Since you and I last talked yesterday, things have trended a little differently for Thursday afternoon and overnight (and also possibly Friday as well). With drier air in the mid-levels wrapping into the eastern side of Francine by Thursday afternoon and night now instead of waiting until Friday, that allows for surface-based instability (storm fuel) to be able to spread farther north than model data had previously indicated. High-resolution models often have a better handle on things like this, and now that we are getting within their time range, they are consistently showing that. That surface-based instability spreading into our are Thursday afternoon and night means that the tornado threat for our area of northwest Alabama and southern Tennessee has increased a little. The NWS Storm Prediction Center has maintained a low-end Level 1 of 5 risk of severe storms for that spin-up tornado threat across north Alabama, and they have now expanded it a little deeper into southern Tennessee. They have also added a Level 2 of 5 risk just to our south that comes as far north as central Alabama. If the trends of better instability working into our area continue, it’s possible that Level 2 risk may be pulled northward a little more, possibly into north and/or northwest Alabama.
Below all this text explanation area, you will see a set of images showing the timeline of the instability using the Baron Futurecast model. Other high-resolution models are showing similar trends. Those images show a parameter called CAPE (Convective Available Potential Energy) which shows the “storm fuel” that storms have available to them. In this circumstance, we are looking for values roughly around 250-400 or higher for this threat. You can see that starts to spread into our area as early as 3-4pm Thursday afternoon and then it actually continues north and trends higher as we head after dark and into the overnight hours, before the instability shifts east of our area by sunrise Friday morning. As mentioned earlier, this will not be a case where the tornado threat goes away when the sun goes down. If anything, it will actually increase near sunset and into the overnight. For now, we think the threat of a few isolated spin-up type tornadoes begins around 3-4pm Thursday afternoon and carries all the way through sunrise on Friday morning. That means there is a long window of time where you are going to have to be alert to changing weather conditions and be ready to act if a tornado warning is issued. As we head into Friday, we are still maintaining a morning to midday/early afternoon threat as far west as I-65 because one or two of the global level models are slower with the instability moving east. However, not only are things trending toward the instability shifting out of our area by sunrise Friday morning, but the wind shear (spin in the atmosphere) will also be drastically weakening on Friday, and this would lower the tornado threat with time, even if instability were to hypothetically be there. We’ll hold that I-65 area threat for Friday for now, but if these trends continue, we may be removing that Friday threat from our forecast entirely!
This is not a higher-end tornado threat, and we are not talking about long-track violent tornadoes. However, a tornado is a tornado, whether it is an EF0, an EF5, or anything in between. They are all dangerous, and you have to take them all seriously. Tornadoes from tropical systems happen from very shallow thunderstorms and sometimes even rain showers without thunder and lightning, and their shallow nature makes them sometimes tough to be detected by Doppler Radar before they develop. That means they can sometimes form before a tornado warning can be issued by the National Weather Service. They tend to be fast-moving and don’t stay down on the ground for long in most cases. They are usually of the lower-end variety (EF0-EF1), but occasionally, they can sneak into the EF2-EF3 range. It usually takes more favorable conditions than what we will have this go around for to happen though, but you can never entirely rule out a tornado sneaking above EF1 intensity in these situations. For reference, there have only ever been two documented (E)F4 intensity tornadoes in the United States from tropical systems (both of them in the 1960s in Texas and Louisiana), and there has never been a documented (E)F5 intensity tornado in the United States from a tropical system. Those type of high-end violent tornadoes usually require a really deep, intense storm updraft and the kind of deep-layer instability that tropical systems just do not allow. But, as we mentioned, it doesn’t matter if it’s an EF0, an EF5, or anything in between. All tornadoes are dangerous. Unlike tornadoes ahead of cold fronts and associated with jet stream disturbances, these will not be coming from the west or southwest. These storms will be rotating around the outer circulation of Francine, and that means they will be moving on the map in a counterclockwise direction. Any tornadic storm on radar in our area Thursday afternoon and night would actually come from either the south or maybe even southeast. That means you’re going to have to pay attention to counties upstream from you in a different direction than you’re used to. Plan accordingly! Drag out those county maps if you are unfamiliar with which counties and towns are to your south and southeast.
Summary of Impacts
- Flooding threat: Baseline 2-4″ of rainfall through Friday evening, with a band of 3-5″ totals over south middle Tennessee into parts of northwest to north Alabama. Both zones could see isolated higher totals. Flash flooding of the street and poor drainage variety is most likely, with river/stream flooding less likely because of low water levels. Still be alert, however.
- High Wind Gusts: 30-45 mph wind gusts begin Thursday morning and continue off and on through Thursday, with the potential for 40-45 mph gusts (isolated up to 50 mph?) increasing late afternoon through the overnight as surface-based instability becomes present. There may be a few tree limbs and trees come down and cause isolated power issues, especially any unhealthy or dead trees.
- Tornado Threat: A few spin-up fast-moving tornadoes will be possible in an expanded period of time starting around roughly 3-4pm Thursday afternoon all the way through around sunrise Friday morning. We’re not totally removing the Friday threat from the board just yet, but data is trending toward the Friday threat being completely east of our area.
- Power Outages: A few isolated short-lived type power outages will be possible from tree limbs and unhealthy trees that may come down, but widespread long-duration power outages are unlikely.
We will continue to look over the latest data as it comes in today and make adjustments to the forecast as needed. Kelli Rosson will be LIVE at Noon today with more details and updated information, and I will be LIVE at 5:00 this afternoon. We will be staffed through the storm to provide live updates or live non-stop coverage as conditions warrant!
Thank you so much for this detailed post. I appreciate all the information.