The Storm Prediction Center continues to monitor a two-punch system on Friday afternoon, evening and into the overnight hours across parts of Texas, Louisiana, Arkansas, Mississippi, and Alabama.
As of now, this two-punch system looks to feature a scattered storms across Texas and Arkansas during the afternoon hours that merge into an MCS in the evening, followed by a few isolated storms ahead of the MCS in Louisiana, Mississippi and Alabama in the evening before the MCS moves through the region.
From the SPC
The SPC is calling for scattered severe thunderstorms on Friday from parts of Oklahoma and Texas that will push to the east through the lower Mississippi Valley. Interestingly, the SPC noted that “Large to giant hail” may be possible across parts of Texas. Then, from eastern Texas through Alabama, there is the potential for damaging wind and “several” tornadoes.
The SPC Discussion:
…Synopsis...
A mid-level low will weaken and meander northward over the western Great Lakes. A more potent, upstream mid-level trough will move from WY southeastward into eastern KS/OK by early Saturday morning. A lower-amplitude disturbance in the southern stream will move eastward from southwest Texas into the lower MS Valley.In the low levels, an area of low pressure over western north-central TX will develop eastward and reach the AR/MS/TN vicinity by daybreak Saturday. A dryline will extend southward near the I-35 corridor in north-central TX. A residual front will advance northward across much of AR and western TN.
…Southern Great Plains…
A seasonably moist/unstable airmass sampled by Wednesday evening raobs along the TX Gulf Coast, and an elevated mixed layer, will overlap and result in a very unstable airmass developing across TX east of the dryline with 3000 J/kg MLCAPE forecast. Rather cold 500 mb temperatures (-15 to -16 deg C) will result in very large CAPE in the -10 to -30 deg C layer. A strong capping inversion will weaken with localized erosion of the cap by the late afternoon via strong heating, convergence near the dryline, and the aforementioned disturbance moving east across TX.Explosive updraft growth is expected with isolated to widely scattered storms developing from southern OK southward into east-central TX. A veering and increasing wind profile with height will strongly favor discrete supercells, before additional storm development occurs during the evening as storms move east into the Arklatex. In addition to hail and a tornado risk, the threat for damaging gusts will probably increase during the evening as this activity moves east into the Arklatex.
…Northern Gulf Coast states…
A complex forecast is seemingly evident for this region and concerning mainly the after-dark severe threat expected to develop from LA eastward into MS/AL. Models indicate a moisture-rich boundary layer beneath steep lapse rates will be present during the day. Despite the presence of a capping inversion, isolated to widely scattered storms are possible during the day with mainly an associated hail hazard.The stronger updrafts may acquire supercell characteristics with some conditional tornado risk over southern AL. However, the primary severe threat will likely develop as the low- to mid-level wind field strengthens during the evening into the overnight. Storms developing within the WAA regime and/or a thunderstorm cluster growing upscale into an MCS are seemingly the favored scenarios for storm development.
Severe gusts (some significant; 65+ kt) and tornadoes will likely become the main hazards as storms move east across northern LA eastward into MS and into western AL by early Saturday morning.
…Southern Appalachian states…
The northeastern periphery of richer low-level moisture (60s F dewpoints) will probably include GA and perhaps extend northward into the western Carolinas during the day. Isolated to widely scattered multicells are forecast to develop during peak heating with an isolated hail/wind risk with the stronger storms. This activity will likely diminish by early evening.—model discussion—
Courtesy: spc.noaa.gov
The 08/00z and 07/18z NAM runs exhibit a too-cool bias with surface temperatures east of the dryline across north-central TX (low to mid 70s near DFW metro), specifically near the I-35 corridor. Moreover, it is likely strong heating will occur across central and north TX during the day with temperatures at least 90 F west of the dryline and into the middle 80s to the east in the warm sector. As a result, ARW-based convection-allowing models (based on the parent ARW model) are likely misrepresenting thunderstorm potential across TX during the 21z-03z period. Conversely, the 08/00z and 07/18z GFS runs are showing excessive boundary-layer mixing and a too-warm bias across north-central TX with the dryline likely displaced too far east compared to its likely position near I-35 by late afternoon. The 07/12z ECMWF appears superior to the NAM/GFS in this regard with adequate boundary-layer mixing implied by the warmer surface temperatures (mid 80s F). This forecast across the southern Great Plains coincides with model output similar to the ECWMF in recent model runs.
Trying to read between the lines, it sounds like the SPC is more concerned about destructive wind and destructive hail more than they are concerned with destructive tornadoes. That is good news, in a sense, but remember that big hail and strong wind can do just as much damage as a tornado.
Model data
As noted above by the SPC the NAM/GFS computer weather models are struggling with the evolution of the overall setup and thus the severe weather potential. The Euro and the shorter-range HRRR and HRW suite of models seems to be doing a better job, though still aren’t really getting a hold of things with the level of accuracy we would hope for. And, of course, none of these models are perfect (as you guys know).
Looking at some of the model data for around midnight Friday night and into Saturday morning, you can see that the HRRR data suggests two areas of interest (image on left). One a line of storms in southern Arkansas, northern Louisiana and northern Mississippi. And a cluster of isolated storms across parts of south Mississippi.
Notice that the amount of spin in the lower atmosphere (imagine in middle) is maxed ahead of the line of storms but also north of the cluster of storms. This is likely due to the cluster of storms interacting with the atmosphere, and not because the atmosphere is lacking any natural turning of wind with height. Then, looking at the mixed level instability (image on right) that the boundary that goes from ‘plenty’ to ‘a lot’ is pretty sharp from east to west splitting Mississippi in half.
That means any storm that can tap into that bonus instability within the environment with sufficient wind turning with height, ahead of the line where it can be isolated away from other storms, would have the best chance to turn severe.
That leaves places like Louisiana, Mississippi and Alabama in a ‘primo’ spot for the development of severe weather on Friday night and into Saturday morning.
At this time, I’m going to forgo showing any more model data as a lot of it is pretty muddled as the models are struggling with the overall evolution. As we move through the day and into this evening I will update things if the models com into better agreement.
For south Mississippi
This looks to be an overnight event for south Mississippi late Friday night. Depending on storm evolution and how things shake out, it may linger into the early morning on Saturday, too.
Showers and storms will begin as early at 9pm for some folks. These storms will be isolated in nature and ahead of the main line of storms that look to arrive closer to 3am.
The main concern for the storms ahead of the main line will be heavy rain, frequent lightning, wind gusts up to 60mph, hail up to the size of quarters, and a tornado (up to EF-3 in strength)
The main concern for the main line of storms will be heavy rain, localized flooding, frequent lightning, wind gusts up to 75mph, hail up to the size of pennies, and brief tornadoes (up to EF-2 in strength).
Storms should clear the area by 7am Saturday morning and move into Alabama
The Bottom Line
Showers, thunderstorms and the potential for severe weather is going to begin in Texas and Oklahoma Friday afternoon and then move toward Arkansas, Louisiana in the evening. Then, into the overnight hours, and into Saturday morning, storms will move through Mississippi and Alabama.
Have a severe weather plan in place. Know what you would do if a Severe Thunderstorm Warning or a Tornado Warning was issued for your area. Where would you take cover? Do you have the necessary items with you? If you live in an area where storms will be moving through overnight, make sure you have a way to get alerts from the National Weather Service that will wake you up.
So far, this doesn’t look like a knock-down drag-out tornado event. But that could change. So please keep up witht he forecast during the next 24 to 48 hours.
