The latest threat for severe weather across the United States looks to take place over two days. Day 1 will feature storms in East Texas, extreme eastern Oklahoma, northern Louisiana, most of Arkansas and southern Missouri.
Day Two will put the threat for severe weather across parts of Louisiana, Mississippi Alabama, central Tennessee, and the Florida panhandle.
This discussion will be focusing on the threat on April 14.
We know more!
If you read the last discussion about the upcoming threat for severe weather, I promised to know more when I wrote the net discussion. Well, here we are!
The Storm Prediction Center has outlined sections of the area with an Enhanced Risk for severe weather – a 3-out-of-5 on their 1-to-5 scale where 5 is the most severe – while other areas are just a Slight Risk or a Marginal Risk.
The Karrie Meter was at a 5.08 yesterday afternoon – that lines up with what the SPC is saying, too. So we have corroborating evidence that there is a decent threat for severe storms.
The shorter-range models are starting to get a better handle on the system, too.
The above is a look at the NAM computer weather model data for 500mb for Saturday through Monday from 4/12/18 at 12z. In other words, it is the morning model data looking at the wind speeds and pressure at 18,000 feet up for Saturday and Sunday from the model that was run today, Thursday, April 12th.
It shows the area of low pressure going ‘negative tilt’ as it passes through Louisiana, Mississippi and into Alabama. If that holds, that may increase the severe weather threat for parts of Alabama, Tennessee and perhaps even Georgia.
That brings me to my favorite section the “what we know” / “What we don’t know” / “What we don’t know, we don’t know”
Known knowns
1. Rain is likely
One thing hasn’t changed for the Gulf Coast states: It is going to rain Saturday. Perhaps a lot.
The WPC has highlighted the region with a Slight Risk for excessive rainfall. Model estimates show totals between 1″ and 3″ probable in most areas, 3″ to 5″ possible for some places, and up to 7″ perhaps in localized areas that end up stuck under the heaviest rain for the longest period of time.
2. Severe weather will happen
The ingredients are all there to guarantee at least one severe storm between Louisiana, Mississippi and Alabama on Saturday.
I know that may seem underwhelming, but keep in mind that means that you could be the one under the promise of at least one severe storm.
As a thought experiment, think of it this way, if we can guarantee one severe storm, it is probably a pretty good likelihood there are two severe storms. And a really good chance there are three or four. Perhaps even five.
And when you think of it that way, it starts to make sense why the SPC put out an Enhanced Risk for severe weather: There is a very good chance the area sees multiple reports of severe storms.
3. Tornadoes are a concern
Unfortunately, with this go-around, there is a concern that the threat for tornadoes is a bit higher than just “a tornado or two is possible” for the region.
I want to pause for a second and reiterate that this discussion is for Louisiana, Mississippi, Alabama and the Florida panhandle.
The CIPS Analogs out of Saint Louis University shows the Tornado Risk for the region as high as 15 percent for southeastern Louisiana, most of Mississippi and western Alabama.
That means, according to history, and in the area highlighted, you are 15 times more likely to see a tornado on Saturday than any other day.
How strong will they be?
That is still a “known unknown” and we won’t know until 12 hours out how strong they may be. But as of now, there is some concern that within the line of storms – and out ahead of the line – there could be a few tornadoes.
What we know that we don’t know
1. The specific timeline of the storms
A lot like yesterday, we are still a bit too far out to talk specifics for the timeline. But we do know that the threat for storms will be between 3am Saturday and 9pm Saturday – perhaps lingering a bit longer.
When they get to a specific point? That is still up in the air, so to speak. Above is the latest data from the NAM computer weather model showing storms moving through the area between 3am Saturday morning and 3am Sunday morning.
But, before you take that and run, that is just one model. And just one set of data from just one model. We continue to get new data in (every three to six hours) and will continue to modify the forecast as new data arrives.
2. How strong will the strongest storms / tornadoes be?
We still don’t know this yet, either. And we probably won’t until Friday night – if then. Generally with events like this EF-0 and EF-1 tornadoes are always possible (not probable, but possible). In this case, we will need to watch how the models handle the low-level Helicity, Mixed-Layer CAPE, and the Effective Inflow Layer.
But again, we probably won’t have a good idea about specifics like this until Friday night or Saturday morning.
Figuring out how strong individual storms will be or areas where the strongest storms will be takes some of our highest-resolution models. And those are only available about 15 hours before storms begin for a certain area.
3. The rainfall totals for specific areas
The WPC has put out estimates and forecasts, but how much you may see at your specific spot is still unknown.
Like I said above, generally, there will be a lot of rain. Probably some localized flooding, too.
What we don’t know we don’t know
We don’t know. Because weather is always changing sometimes it is difficult to pin down specific things. Those things – at this distance in time – can make big changes to the forecast 72 hours from now.
