2018-02-24 EVENT: AR/MO/MS/TN

[Dan Robinson]

The GFS shows Gulf moisture in the form of mid-60s dewpoints advecting northward into the lower Mississippi valley region ahead of the final shortwave within a broader western US trough that will exit stage right over the weekend. The shortwave features a 90-knot speed max making an attempt at a negative tilt as it ejects over AR/TN/MS if the GFS is to be believed. The NAM's picture of the parent trough at the end of its range (00z Friday as of this post) is not too dissimilar from the GFS.

As is typical with systems this time of year, clouds and precip threaten to be widespread over the warm sector as a lot of the upper forcing is shown as being well overhead fairly early in the day.

I'm not impressed enough as it stands to consider this as a chase prospect, though if the shortwave's timing and compactness can improve, this may become more interesting as future runs come in.

 

[Jonathan Beeson]

One thing I'm not really a huge fan of is the warm temps above 700mb. The system has a few things going for it such as an unsually potent warm sector for this time of year, but mid and upper level temps need to cool some for me to be fully on board.

Also, I'm worried about the trough shearing itself out with the strength of the SE ridge. That will need to be watched as well.

 

[Jeff House]

The most chasable area in the outlook (our thread, not necessarily SPC) is the Delta in eastern Arkansas and/or Bootheel of Missouri. One can work with North Mississippi and West Tenn. but the AR/MO Delta is as good as the Plains.

Looking for evidence that the warm sector will not be disturbed. As Dan writes, low clouds are going to create and uphill climb either way. Some of the recent NWP has precip lifting north of I-40 which would open up the warm sector - iff low clouds do not hamper. Mid-level warmth noted above will make it even harder to mix/burn out low clouds.

Kinematics should be there. Just in time JIT backing is forecast at the low levels. Normally I'm not a fan of JIT anything, esp this early in the year, but the warm sector will have rich moisture already for backing winds to leverage.

Main question is the instability. No chase is planned attm. Greater instability, on this weekend day, would make it more interesting.

 

[Jonathan Beeson]

There have been some marked improvements over the last 48 to 72 hours as the Euro has fallen more in line with the GFS. Seeing a little bit of improvement in the mid-upper level temperatures, and the Euro also wants to burn off some of the low level clouds as the day progresses, at least across MS. Also, seeing a much more amplified trough as opposed to previous runs which is allowing for deeper and more organized surface cyclogensis. Still not fully on board yet, but improvements are being made as we get closer.

 

[Dan Robinson]

Models have been trending toward a stronger, slightly slower and more negatively-tilted shortwave with the surface low cyclogenesis in western MO and the warm sector pushing another 150 miles farther north. Aside from some VBV (not as much of an issue with these types of setups IMO), the wind fields are quite nice in the warm sector owing to the deeper and closer surface low in MO. The major negative is still the thermodynamic situation, with poor lapse rates and solid unbroken clouds/copious precip early in the day resulting in negligible MLCAPE for deep convection. If the clouds can break up anywhere between I-44 and I-40, the low-level instability situation will improve dramatically and the tornado potential would be certainly chaseworthy, at least for anyone within a few hours' drive. A large clearing on the first few visible satellite images of the day might get me out the door on this one.

 

[Jesse Risley]

Based on the 12z/22 runs, there certainly is a nice veered wind profile and plenty of shear. The RH profiles at H7 and H85 are still concerning regarding potential cloud cover, and associated mediocre lapse rates, though if more ample MLCAPE does materialize, I certainly think the threat will be enhanced. It doesn't appear that adequate CAPE and shear profiles look to support as high of a supercell threat closer to the main surface low, hence everything further south will be associated with more frontal forcing. This seems to be why the models are trying to sustain a fair quick evolution to a more linear complex on Saturday. An ancillary issue regarding picking a target would be ongoing flooding across the southern portion of best risk area, being exacerbated by ongoing rainfall. I still think that, right now, the best area for isolated daytime convection in more favorable terrain would be east central Arkansas after 19z, probably ahead of the main line proximal to the surface front. This is certainly a setup that, given the time of year, has more caveats related to instability than not.

 

[Quincy Vagell]

QLCS tornadoes are probable, if not likely on Saturday, but the storm mode appears to be largely linear. Some brief supercellular structures cannot be ruled out within a line, but forecast soundings and hodographs show several limiting factors.

Low level instability does not look as impressive as some of the more severe analogs and issues in the mid-level wind fields suggest the window for any discrete supercells will be minimal at best. The quality of the deep layer directional shear is lacking, at least compared to historic winter events. Plus, it's not like we have a warm front draped across AR/MS, so most of the convection should verify immediately ahead of a cold front. There is little to no evidence to suggest robust open warm sector convection this go around. Even the agressive 3km NAM shows a linear storm mode, with some cellular elements within the larger scale line. Essentially it shows no open warm sector activity, where the parameter space and backing of near-surface winds are maximized.

A typical forecast sounding ahead of the line (late afternoon in southern/eastern Arkansas) looks like this:


The 0-2km wind fields are quite favorable, but then above that it gets junky looking.

With all of that said, such considerable low level shear, combined with seasonably impressive low-level moisture and <1km LCLs suggest an elevated potential for tornadoes, although they'll probably be short-lived and embedded within a line.

Unless wind fields improve, and/or CAMs show a greater potential for warm sector convection, I don't find this to be a particularly exciting chase prospect. (Unless you're local)

 

[wfrogge]

This is a typical Mid South winter setup ("good enough" CAPE, high shear) and everything that can be said about the system has been said. NAM 3km, GFS, and NAM for the most part all agree with the 3km being the most robust. For this time of year the wind fields look good, MLCAPE looks good, and seeing a glimmer of hope on the NAM that we may get some breaks in the clouds giving me a glimmer of hope for one or two quality pre-frontal supercells.

Right now the best dynamics look to be over an area that is normally "chaseable" with flat terrain and few trees. For this event keep in mid there are a few roads closed in this area due to flooding. This is literally IMBY and on a weekend so I will be going after this one. If this setup was 400+ miles away I would pass.....

Target location will be Pine Bluff Arkansas around noon with a focus on areas south of interstate I-40 and east of highway 65 and 125 all the way to the MS river and chase (or in this case let the storms run over me) back home to Memphis.

 

[Dan Robinson]

The CAMs are coming in pretty much as expected for tomorrow - at least we've had good model consistency with this event. Thus, not much has changed. Nothing in this setup looks worthy of more than an hour or so drive in my personal chase criteria. If I was a chase-everything type, I'd probably head down to Hayti to keep a bridge across the river in play and hope for a daytime storm ahead of the main frontal squall line. As it stands now, given it's a Saturday (day off) I'll just be watching for a nice dry slot to work north after morning storms. In that case, southern IL may have a brief shot if a broken line can fire, stay discrete and ahead of the undercutting front. I think we have less than a 10 percent chance of that happening. I see the most likely outcome is some close-to-home lightning, which I'll always take in February.

 

[Tim Paitz]

I was hoping to chase in southern illinois this evening but it's just been raining much of the day. Not even any thunder or lightning here!

Last night the NAM 3km had a couple discrete cells south of St. Louis. I'm getting less and less impressed with today's setup throughout the day. It's just been a couple rounds of rain and that's it.


I can only imagine what spring will be like....