The Storm Prediction Center has increased the severe weather risk for parts of South Mississippi and Louisiana for Thursday. While very heavy rain and the prospects of localized flash flooding are still the main concern, the SPC notes that there may be a window of opportunity for some severe weather to occur, too.
I’ll go through the new info, a quick forecast update, then what it means for you…
New Info
The short-range HRRR model is now able to “see” this next round of weather (the HRRR model can only look 36 hours out into the future). And while this new data hasn’t changed much, there are some crinkles that may have pushed the SPC over the hump to issue the Slight Risk.
The first piece of data that looks a bit different is the wind profile through the atmosphere. We talked about this a lot with the last round of severe weather. When the wind speed and direction changes as you go up in the atmosphere in a particular way (for severe weather, we need a south or south-southeast wind at the surface to change back to the west as you move up to about 20,000ft).
For the last few days, it looked like there would be a southwest wind at the surface and also all the way up to 20,000ft. But the latest data, shows there may be a bit more of a southern component to the wind at the surface, and a bit more of a western component to the wind aloft.
Compare the above image, from yesterday morning’s data. That is the Skew-T showing the atmosphere in the form of a graph. This is taken from southern Mississippi. Now look at the Skew-T below from this afternoon’s data. Same spot on the map in Southern Mississippi.
It is a very subtle difference, but I’ve circled it on the Skew-T above. The area I’ve circled highlights the wind speed and direction at the surface and then up at about 20,000ft. That change, means it will be a bit easier for storms to stay organized, and therefore a better chance they produce severe weather conditions (wind gusts up to 70mph, hail up to the size of nickels, and the potential to produce a tornado).
But that doesn’t guarantee severe weather. Looking at the CIPS Analogs, similar setups to this one haven’t historically produced much severe weather.
In fact, the only times were in 1981 and in 2014. The most recent time was the day of the Columbia Tornado two days before Christmas. But even that analog isn’t that analogous.
So the probability that a setup like this produces severe weather isn’t very high. But it isn’t zero.
The Forecast
I’m not going to change the forecast much. I still think that flooding is the main concern. The timeline for the heaviest rain and threat for severe weather is still in the afternoon and evening, Thursday.
The same HRRR model I referenced up top is showing a stripe of more than 6″ of rain across South Mississippi. And a wide stretch of 1″ to 3″ across Louisiana, Mississippi and Alabama.
I am now thinking that multiple reports of localized flash flooding (streets flooding, creeks coming up, water ponding in back yards) will be possible. And getting, at least one report, of flash flooding in my opinion is almost a guarantee.
The change is that now I think that getting a storm that drops heavy rain and produces wind gusts up to 70mph is a bit more likely. And the threat for a tornado is maybe a smidge higher, too. The HRRR model does have a few Updraft Helicity streaks through the region, but (Big but! Large but!) nothing like we saw two weeks ago. So, we will continue to monitor it.
What these changes mean for you
It looks like there is a slighty higher risk for severe weather – including tornadoes. And there is a higher risk for more flooding, too.
It means that instead of just looking at a rainy day on Thursday with a few storms. I think now it may be a day where you need to have your NOAA Weather Radio handy, turned on, and programmed correctly. Have an app on your phone that will ping you when a warning is issued, and know what you would do if a flash flood emergency or tornado warning was issued.
A tornado warning, of course, means getting to an interior room. A Flash Flood Emergency, should be treated like a tornado warning, but instead, you should seek higher ground.
Please keep tabs on the forecast through the evening and into tomorrow morning for updates.
