2016-03-13 EVENT: AR/LA/MS

[Ben Holcomb]

The SPC has put out a slight risk with a mention of tornadic supercells over most of Arkansas tomorrow. This 'setup' has not been on my radar, but a little chatter on facebook got me to look into things a bit.

A blocking high pressure in the northeast US seems to keep an incoming shortwave over the moist warm sector which has been sucking moisture from the gulf for multiple days per vis sat loop as this closed-low system has moved from Texas north. The NAM shows a nice little neutral to negatively tilted shortwave currently over the 4 corners to eject over Arkansas tomorrow with an attendant surface low in NW AR. A warm front draped across the south north of I-40 seems to stick around as the warm sector moistens from now until then.

Mixing ratios are nice tomorrow, with observed soundings showing around 10 and forecast soundings showing around 12. Instability in the 1000-1500 J/KG range should be enough to form some supercells.

The degree of directional shear is a question mark in my book, but this might be the first legitimate setup of 2016 I consider chasing.

 

[Quincy Vagell]

A compact, but potent little shortwave is progged to swing from Oklahoma into Arkansas tomorrow. The timing may have slowed down sightly, as storms could initiate in eastern Oklahoma by early to mid afternoon.

Overall, the setup has several things going in its favor, not for a high-end event, but for isolated to scattered supercells, some of which may be intense. Given forecast lapse rates and cold air aloft, large hail seems like the initial and predominant hazard. However, the higher resolution guidance shows nicely backed near-surface winds, SSE to SE, from the Arkansas Valley into south-central and southeastern Arkansas, highlighting the potential for tornadoes. From a local climo perspective, the valley along I-40 from Little Rock to Fort Smith can work to keep the low level flow locally backed via channeling, which can enhance a conditional tornado threat in that region. Moisture return looks modest, but the abundance of saturation across the lower Mississippi Valley from the ongoing flood situation may help give dew-points a slight boost. (This could also prove problematic for chasers if any roads still remain flooded or washed out)

Discrete to semi-discrete cells should fire across western Arkansas by mid-afternoon and spread E to NE across the state. As they mature around the middle portion of Arkansas by late afternoon, this is where I think the tornado threat is maximized. It’s not the best area for chasing, but given the setup (decent LLJ, corridor of moderate instability, locally backed flow and discrete nature of cells), I think it is worth a chase, especially for anyone who is not far from Arkansas.

I would expect at least a couple of tornadoes and if a dominant supercell can remain isolated, I could even see a strong tornado. The setup also favors large to perhaps very large hail with the more robust supercells. It should be a solid chase day for any chaser who can get on a storm, but also have decent visibility.

The storms track east into eastern Arkansas by early evening, likely weakening but the time they reach the Mississippi River. They could merge into a cluster or broken line segments, but most guidance suggests a semi-discrete mode through about 03z. As I mentioned before, the timing is key and if a modest westward trend continues, it’s going to put the threat area into the more rugged terrain from eastern Oklahoma into western Arkansas. Ideally, the eastern third of Arkansas is relatively chaser friendly, but it may not be until close to sunset that any chasable storms move into this region.

I will probably make the trek out there from Atlanta, but will need to leave early in the morning to make it with some breathing room. I want to see how the HRRR handles the setup later tonight, but otherwise, the models in general have been fairly consistent with the handling of this system for several days.

One note about storm prospects in far southern Arkansas to far northern Louisiana… capping and limited forcing may make it difficult for any storms to initiate here. Use caution with the severe parameters (SCP, CAPE, EHI), especially on the GFS, as those are highest across northern Louisiana. (Some have asked me if it’s better to target down there, but I don’t think so) That’s why it’s important to look at the system as a whole and not rely too much upon those colorful maps.

 

[David Williams]

I'm going to agree with Quincy. I'm not buying the GFS placement of a lot of those parameters in Louisiana; however, I do think that the best chance for tornadic supercells will be southeastern and south central AR. Little Rock might get some excitement tomorrow. No doubt the jungle will be a problem tomorrow since it looks like storms will track through or near the Ouachita National Forest. It will be a good opportunity to test my Weboost 4G-X.

Also, I'd like to remind everyone that tomorrow morning will be daylight savings time. We'll be moving 1 hour ahead tomorrow. Is it safe to assume that all models adjust with the time change? Will we be taking -5 from UTC on the HRRR tomorrow morning?

 

[Andy Berrington]

Latest high res guidance (particularly the 00z EMC/SPC WRF and the 00z NSSL WRF) is showing quite a volatile situation across central/S AR tomorrow with robust supercells breaking out by 21z. A couple of them reach well into the upper ranges of the model's UH scales, but that's not really the main point. It has these cells within an environment of 1000-2000 J/kg CAPE and surface winds basically out of the SE, which leads to very strong directional shear in the lowest 3 km since the 700 mb flow is out of the WSW. There would certainly be an elevated tornado threat if this came to pass, perhaps near some highly populated areas (Little Rock metro for example).

 

[Michael Hook]

How did the Weboost 4G X work in Arkansas?


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[David Williams]

Its was great! I only lost reception once for about 5 seconds. I monitored it regularly to see when it would give out, but it stayed faithfully connected through some pretty radical terrain.

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[Michael Hook]

Thanks. I'm thinking about purchasing one to boost signal to my USB modem.


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