Sunday, April 3, 2011

4/3 - 5:40pm - Severe Weather Tomorrow Across the South

The Bottom Line

Central/North Mississippi: Severe weather will be possible starting in the late morning hours tomorrow and extend until likely just after 7pm. Supercell thunderstorms could form in front of a main squall line in the early afternoon and produce some tornadoes, but this will be highly dependent on some "iffy" factors like the cap and surface winds. Straight line wind damage, large hail, and some embedded brief tornadoes will be possible with an approaching squall line of storms as we get closer to sunset.

West Tennessee: A squall line of storms will travel from west to east tomorrow between the late morning and middle afternoon hours. This line will strengthen due to daytime heating as it travels eastward, so be especially on the lookout if you live east of Jackson, TN. There's also a possibility for the development of a supercell or two in the southern portions of West Tennessee before the passage of the main line of storms. Straight line winds, hail, a few brief tornadoes, and flash flooding will be possible as the squall line passes through.

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Severe weather will be an issue tomorrow around the South as a sharp trough and accompanying cold front advance eastward. The Storm Prediction Center has already put a large portion of the Southeast US under a Slight Risk for severe weather tomorrow, putting particular emphasis on an enhanced severe threat in Mississippi, Alabama, and Tennessee with a 30% outlined area. The SPC's outlook calls for the chance of discrete superecells in front of a main squall line, or QLCS, of storms that could bring damaging winds and some embedded tornadoes.

Here in Mississippi and West Tennessee we'll be watching for a particular area where the most favorable wind shear and instability will come together to create the greatest threat for severe weather. Wind profiles will be most favorable for severe weather as you go northward into Tennessee and Kentucky, but instability will be strongest over Central and North Mississippi as we head into the afternoon hours tomorrow. The meeting of these two "peak" areas of winds and instability will likely be North Mississippi into North Alabama and extreme Southwest Tennessee. The SREF computer model's Significant Tornado Ingredients parameter on the image to the right shows a pretty good estimate of where this area will be during the afternoon tomorrow in the 20 and 30 outlined areas.

To the north of this area straight line winds and brief embedded tornadoes will be the greatest threat as the squall line pushes through during the afternoon. Inside and slightly south of this area is where the best chance for front-running discrete supercells could be. These supercells, should they form, will have a risk of producing tornadoes and large hail. Surface winds from the south or southeast will be needed to produce these tornadoes, so that's certainly something to watch for. Sunlight during the late morning and early afternoon will be the fuel for these supercell storms and the "cap" (warm layer of air above the surface that blocks air parcels from rising and creating thunderstorms) will be crucial. The NAM model indicates that the cap will break and allow for storm development sometime in Central and North Mississippi between 10am and 1pm tomorrow as you'll see on the left. Forecasting when and if a cap will break is extremely tricky, so don't be surprised if we get lucky and supercells don't form due to a stronger than anticipated cap.

Instability in general will be quite adequate tomorrow across Central and North Mississippi as dry air aloft will allow rising moist air parcels from the surface to continue upward, which is the main mechanism for building thunderstorms. The NAM model sounding at 7am tomorrow on the right indicates where this dry air is in relation to the moist air at the surface.

The timing of when the main line of storms will come through has been an issue on some of the computer models. The SREF and NAM models seem to be in agreement that the severe weather threat will begin sometime after 10am in North Mississippi with the potential for front-running supercells and then having the main line of storms come through Northeast Mississippi near the 7pm hour. The GFS has been a little quicker with this, but it seems the last couple of runs have put it more in agreement with the NAM/SREF and the ~7pm timing of the main line coming through Northeast Mississippi.

With the threat of supercells in the early afternoon and the squall line near nightfall, this will be a fairly long event. There isn't one particular parameter that sticks out as being "historic" or much above average for any severe weather event. The significance here is that each of these parameters (instability, shear, moisture, etc) will meet the required criteria for severe weather and converge over a good chunk of Mississippi, extreme Southwest Tennessee, and Alabama. I don't think there will be a large tornado outbreak of any sort, but tornadoes could be possible in both the front-running supercells and main squall line.

Be sure to keep those NOAA weather alert radios in "alert" mode tomorrow if you live within the SPC's 15% or 30% risk areas. Even though this severe weather event will not break any records, it has the potential to affect a large population and cause damage.

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