2019-02-23 EVENT: TX/OK/IL/AR/LA/TN/KY/MS

[Jesse Risley]

Though it's still 5-days out, models are advertising one of the first appreciable severe weather events of the late winter season for Saturday. GFS and ECMWF have been fairly consistent in advertising the second of two pieces of energy ejecting across the Red River valley and gyrating eastward by midday Saturday as the main trough becomes more negatively tilted. A fairly potent surface low will move into W MO by Saturday afternoon, with what appears to be a secondary area of low pressure being advertised over the MO/AR border my late Saturday.

Models are advertising seasonable ample Gulf moisture return, juxtaposed with ample low-level shear profiles (some forecast SKEW-T profiles are indicating SRH values AOA 300 m2/s2). The actual position of surface features will likely continue to evolve over the next few days, as it will also become more evident as to whether or not surface instability will be maximized in the corridor of best low-level shear profiles depending on the amount of sunshine realized over parts of the lower Mississippi valley and the Ozark region into early Saturday evening.

It's definitely a system to keep an eye on as we get closer to the actual date.

 

[Ethan Schisler]

I've been watching this system as well. I'm watching the northern end of the setup where the strong surface low is going to track through Northern MO. If we can get some decent moisture and a dry slot to move in, we could see some convection in that area too....given we have instability. Really too far out to determine any of this, but its another area that needs to be concerned about (IA/MO/IL), somewhat reminiscent of December 1st (just a broad glance at the surface pattern).

 

[Jesse Risley]

The system is finally starting to come into range of the NAM. I would definitely continue to keep an eye on the northern target, though unlike the December system, it seems that the actual moisture and instability axis may not overlap the with the colder upper low like we saw on December 1st. Even in the warm sector, lapse rates aren't overly impressive. Right now the big negative factors seem to be the overall narrow corridor of instability and the very strong forcing associated with the main surface front.

Just glancing at some of the forecast skew-T profiles, it appears that the models want to keep the lower levels saturated throughout the day, which might be indicative of the lower level grunge/cloudiness hanging around INVO of convection early and/or throughout the day, limiting destabilization in the open warm sector which, while relatively uninhibited by a lack of formidable CINH, would conventionally tend to host the the best probability for discrete supercells ahead of the main forcing. If more drying can take place than advertised, that will be something to monitor. Some of the better advertised instability, albeit narrow, is shunted further south, just shy of the most magnanimous 0-1 km deep layer vertical shear profiles. Nevertheless, there are some impressive looking hodos in parts of the warm sector, and the NAM is also advertising a secondary area of low pressure in the southeastern flank of the Ozark Plateau region of MO.

 

[James Gustina]

That marginal CAPE/free ascent might be what turns this into a 4 state QLCS. Forcing looks very strong ahead of the dryline and every forecast sounding I've pulled off the latest run of the NAM has a completely uncapped airmass from the early morning onward. As Jesse noted, the profiles also look saturated for the entirety of the day which will likely mean that storms are moving through a grungy stratus deck all day. That said, hodographs on the northern end of the target area look very impressive, on the order of 400-500 m^2/s^2 0-1KM SRH. The unidirectional nature of the winds above 3KM doesn't look like it should really matter in terms of possible tornado production other than maybe muddling the storm mode.

I'd be interested in the southern target around the Mississippi/Arkansas state line for marginally better CAPE but the shear profile also look a bit more janky (though still with impressive 0-1 turning). Regardless, there will almost assuredly be tornadoes on Saturday. Whether or not people can see them is another question.

 

[John Sacrey]

For those of you who may be coming to Arkansas to chase this weekend, please be advised we have flooding currently occurring over the eastern half of the state. It is ongoing and will most likely be worsening. Many state and county roads are under water, with more likely to be that way soon. May I suggest downloading the iDriveArkansas app from your favorite app store. This is kept up to date in real time.

 

[Quincy Vagell]

Details are subject to change a bit, but the most favorable chase focus for Saturday's event looks to be from northern Louisiana into the eastern half of Arkansas/Missouri bootheel vicinity, eastward into much of Mississippi and middle/western Tennessee. It's a bit unclear if there will be much buoyancy realized farther north into southern Illinois and western Kentucky, but don't count those areas out either.

Throughout much of the risk zone, dew-points should have no issue rising well into the 60s, which when coupled with strong wind fields and modest mid-level lapse rates should create an environment favorable for severe thunderstorms. On the other hand, profiles look fairly saturated with tall, skinny CAPE, so expect fast storm motions (40-50kt in the warm sector and 50-60kt with a likely MCS).

What's appealing about this areas is that locations roughly 50-75 miles on either side of the Mississippi River in this area have favorable terrain and decent road networks for chasing. One complicating issue will be ongoing/worsening flooding in the areas.

A few targets I can identify, ranked by preference, based on a quick look at the latest guidance...

Middle target: Eastern Arkansas into western Tennessee and northwestern Mississippi.
The best overlap of shear, instability and forcing is focused in this area, based on the latest guidance and trends. While there may be some morning shower/convective activity here, that is expected to move out by midday, allowing for some modest destabilization. With minimal capping, convection could initiate as early as 17-19z in two regimes, with the first being in the open warm sector near/east of the Mississippi River and farther west along a wind shift/pseudo dryline/moisture gradient. The latter will tend to grow upscale quickly, but could feature at least some supercellular elements.

Southern target: Northern Louisiana into southwestern/west-central Mississippi.
The main differences from areas farther north appear to be slightly less steep 0-3km lapse rates and somewhat less impressive forcing, but richer low-level moisture and a higher likelihood of more discrete/semi-discrete storm modes. This setup could yield supercells as early as late morning in parts of northern Louisiana, although it may not be until stronger forcing for ascent and cooler air aloft moves in that things really get going. Since early day convective activity appears fairly unlikely here, capping will be negligible, allowing quick initiation.

Northern target: Southeastern Missouri into far southern Illinois and western Kentucky.
This area appears to be more of a wildcard, as meager instability should prove to be a limiting factor. Nonetheless, wind fields and mid-level lapse rates will be favorable here and if there is just a bit of instability, watch any convective elements interact with a lifting warm front. If early day convection is less prevalent than currently progged, this could support higher confidence of this being a favorable chase target.

 

[Jacob Parrish]

Alot of flooding here in Mississippi. Be very wary of back roads as there are multiple bridges out here in the Tupelo area. Im assuming the Delta area around the Mississippi river will be very flooded as well.

 

[Ethan Schisler]

I'm growing less impressed with this setup with each model run. Between the flooding along the Delta flood-plain (no pun intended), strong forcing along the front and skinny/saturated CAPE profiles...just not sure there will be much in the way of "visible" tornadoes. If I had to pick a target 2 days out, I'd pick Clarksdale, MS area. Decent overlap of ~1000j/kg of MLCAPE and ~300-400 0-1km SRH. I'm afraid we will see a strongly forced line of convection sweep the target area and be done before nightfall. Secondary target area for me is near the surface low in South Central IL close to a warm front. Instability across this region isn't forecast to be nearly as predominant across this regime versus further south and is highly conditional, but I've seen it work out before where instability is higher than forecast (2/20/14 comes to mind). With this extreme shear systems though, it doesn't take much CAPE to get it done. It also doesn't take much for updrafts to get sheared apart or forced into a QLCS either.

 

[MikeD]

The Memphis area does not have a large amount of flooding. Just don’t head into any back roads/dirt roads and you should be fine. Most of the road network is paved, so getting your car stuck in mud is highly unlikely. I-55 is a great setup highway, but don’t go too far south.

 

[Ethan Schisler]

Well things sure have turned around based on morning data. I'll be making the trek south with a preliminary target of SW TN, NW MS. 1000-1500 CAPE should exist given enough clearing across the target area, juxtaposed by 0-1km SRH of ~300-400m2/s2 aided by 80 knots of flow at 500mb as a perturbation rounds the base of 100+ knot mid-level jet. High resolution models such as the 3km NAM and even the longer range HRRR are starting to indicate the development of a broken line of cells moving into this environment after 2-3pm and racing northeast. This will be more of an "intercept chase" than a typical "chase". I think given a discrete supercell in this environment, there is the potential for a strong tornado in this area.

 

[Jesse Risley]

Today's 18z model runs are still advertising impressive shear. If the models verify, I do think the far northern target into S IL will progenate a few tornado reports, given that seasonably ample instability overlaps impressive shear there tomorrow (though still conditional an amount of instability and actual northward extend of the warm front). I am concerned about how the early round of convection will impact the atmosphere into early afternoon tomorrow. The most impressive low-level lapse rates are confined to NW MS, which happens to be one of the worst areas for late winter flooding in rural areas. 18z runs are overdoing the northward extent of moisture return commensurate with 23z surface obs, but given the strength of the system, I don't think areal moisture return will be overly anemic. The real key in the warm sector will be actual, realized instability given the impressive shear in place. I'm a bit impressed at some of the surface based instability values progged on the 12z ECMWF for 18Z Saturday across the entire risk area, particularly in NC MS.

Right now I think the central and southern targets have the best chance at birthing several significant tor reports, and given the magnanimous directional shear ahead of the main forcing, a genuine QLCS tornado threat emerges with the primary convection associated with the front later on. The lower atmospheric profiles, seasonably high PWATs, and extremely low LCLs will make for less than bucolic supercell scenery, but I still expect pre-frontal supercells to yield at least several significant tornado reports tomorrow, especially across E AR, W TN, and NW MS where the LLJ intensifies INVO the right exit region of the potent H5 jet.

 

[Jeff House]

Columbus MS and Burnsville MS tornadoes were on a pre-frontal trough ahead of the quasi dry line coming out of Arkansas. Sometimes goes that way in the Plains (Gerard KS 2003) but it is a little more common in Dixie. In addition to the north-south troughs, an east-west outflow boundary arced back into Mississippi and adjacent Tennessee from Alabama. OFB was from all the morning rain.

I'm a little surprised how fast the atmosphere destabilized as the OFB lifted north, though models had it well day ahead. ARW correctly favored the pre-frontal trough over the dry line. HRRR lit up both, which was too aggressive. NAM seemed to favor the DL. ARW won that battle.

Had I believed the instability forecast I would have made the easy drive to Florence AL to then look at data over lunch. Psychology played a role waking to cool moderate rain in Chattanooga. Just doesn't get me fired up. Should I have chased this close to home? Probably.

If the pattern continues I believe more opportunities will arise. Bottom line: Especially in Dixie, consider forecasts that favor pre-frontal troughs over quasi dry lines. As usual, go with boundary intersections.