As we head into next week, the next system slated to push across the Gulf Coast will bring the potential for severe weather. Because of that, the Storm Prediction Center is now highlighting the Gulf Coast, as well as parts of the Southeast and Mid-South with a chance for storms and severe weather.
Heavy rain, gusty wind, hail and tornadoes – the full portfolio of severe weather threats – will be possible next Wednesday from eastern Texas to southern Kentucky and as far east as a sliver of western Georgia. The specifics and timeline of events is still a bit out of reach, but generally speaking, any outdoor plans for St. Patrick’s Day in eastern Texas, Louisiana, Arkansas, Mississippi, Tennessee, Alabama, the Florida panhandle, and southern Kentucky need to also come with a back-up plan.
From the SPC
The SPC introduced a Slight Risk for severe weather overnight and it covers a broad area of the South.
Here is a look at their discussion…
…DISCUSSION…
Plenty of spread/differences continue between the GFS and ECMWF for the medium-range period — even as early as Days 4-5. While the broad/overall picture remains fairly clear that a notable increase in severe threat will occur perhaps as early as late Tuesday afternoon (Day 4) near a southern Plains dryline, and then continuing Day 5/Wednesday (lower Mississippi and possibly the lower Ohio Valleys east across the central Gulf Coast states/Mid South), and potentially Day 6/Thursday (southern Appalachians/Carolinas).
The differences amongst the models are centered around evolution of the next upper system moving out of the western U.S. at the start of the period. The ECMWF depicts a closed low — strengthening with time as it ejects, and thus the feature advances more slowly than the GFS, which depicts a more progressive, open wave.
This has substantial implications for timing/location of surface features through the first few days of the period — with these differences becoming great enough by Day 6 that no meaningful assessment of areas of possible risk can be confidently highlighted.
With the ECMWF’s pattern evolution very similar to the EC’s ensemble mean, and also similar to the UKMET solution, will lean a bit toward a slower/less progressive evolution and thus a slightly more westward forecast.
Day 4 (Tuesday), some conditional risk for mainly hail may exist as far west as a northwestern Texas/western Oklahoma dryline during the early evening, with some hail risk spreading across parts of Oklahoma and North Texas overnight. However, with this risk more conditional, will opt not to include a risk area for Tuesday at this time.
By Day 5/Wednesday, a more substantially unstable environment, encompassing a large area, is likely, as a moist Gulf airmass streams northward, beneath rather steep lapse rates advecting eastward.
With an eastward moving surface cyclone and attendant warm and cold fronts focusing ascent, and the likelihood for broad warm advection/QG ascent within the expanding warm sector, scattered severe storms appear likely even given timing/location differences of surface features amongst the models. While a more concentrated area or areas of severe weather may evolve Wednesday, those details — and any associated outlook highlights — will require discernment in subsequent outlooks nearer the event.
For now, a large/broad 15% risk area will be introduced, representing a large envelope of all-hazards severe potential.
Day 6, model differences become substantial enough that — while severe weather risk may continue, no areas will be highlighted due to the predictability concerns.
Courtesy: spc.noaa.gov
Some Data
There have been some subtle, and important, changes to the data from yesterday morning to this morning. Mainly with the movement of the area of low pressure within the data available from the ECMWF.
Here is a look at yesterday’s data:
And today’s data:
The shift with the central area of low pressure to the south and southwest in the upper-levels (300mb) and lower levels (850mb) would increase the risk for severe weather across the region slightly.
The things that haven’t changed are the SW flow aloft and the pull of moisture in the lower levels from deep in the Gulf and NW Caribbean. In fact, the guidance is now showing dewpoint values at ground level in the 70s for some parts of the southeast on Wednesday.
The purple areas are dewpoint values in the 70s. The dark blues show dewpoint values in the upper 60s. So this will be a rather juicy airmass. And with all of that moisture in place, and some daytime heating, with an area of low pressure nearby… you get instability.
If you’re wondering, ‘Uhm… why are you talking about unstable capes, Nick? You’ve been watching too many superhero movies!” I promise CAPE is an actual weather term.
In this case the “most unstable CAPE” is looking at the ‘worst case scenario’ within the atmosphere where the ‘most instability exists’ within a particular column of air. This would not be a great indicator for forecasting anything specific, but it does have utility when looking at potential.
And any time you get into the blues there is a potential. Yellows and oranges? There is even more potential. Red? Well, you get the idea.
Again, this isn’t great for specifics. So looking at the map and seeing red in Pearl River County in Mississippi doesn’t mean that Pearl River County is going to get the worst storms. That is not the case. Because this is better for potential, this is a good indicator that certain spots may be able to achieve CAPE values in the 3000 range. Where those spots will be are ‘tobe determined’ but it is a good indicator that this type of environment will be possible across the region.
More Data
I ran the Karrie Meter map and it shows a very broad area across multiple timeframes – Wednesday afternoon, evening and overnight into Thursday – with the potential for severe weather.
Here is a look at the Karrie Meter map for 1pm on Wednesday:
This data is pulled from the GFS computer weather model, so it won’t be perfectly lined up with the ECMWF I was showing earlier. However, it does highlight that both models see an increased risk for severe weather closer to the Gulf Coast and another across parts of northern Mississippi and Alabama.
I posted on twitter that the green for St. Patrick’s Day is festive, but not welcome.
There is a lot of green and yellow on the map. Recall, the greens and yellows are numbers between 2.01 and 4.99 which converts back to a Marginal and Slight Risk from the SPC.
And that is where the SPC has the area, too. So the data line up! Giving us increased confidence in the potential for severe weather.
Severe weather threats
The data still support the potential for heavy rain, frequent lightning, wind gusts exceeding 55mph, hail up to the size of tennis balls (perhaps larger), and tornadoes cannot be ruled out at this point. On top of that, a strong tornado (EF2 or EF3) cannot be ruled out across the region, either.
Another note that just because something cannot be “ruled out” doesn’t mean that it is “ruled in” either. It is important to remember that we are still working with early and incomplete data. Once we get closer we will be able to hone in on the potentials and specifics much better.
The Bottom Line
If you are making plans next week, make sure those plans include keeping tabs on the forecast. Make sure you have a NOAA Weather Radio handy and an app on your phone next Wednesday so you can be alerted to any potential severe weather. And if you are going to be outdoors (Spring Break camping) it is very, very important that you keep tabs on the weather and have a place to seek shelter.
We will know more as we move through the weekend and into next week. Please check back for updates on the forecast.
