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andyhb

April 13th-14th Severe Threat

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First time starting a thread here in awhile, but this deserves attention. Models have converged on a volatile scenario for Saturday (4/13), with a potent mid level trough shifting into the S US, yielding a strong surface low lifting into the Arklatex and ample moisture advecting north with a 50 kt LLJ towards the I-20 corridor in E TX/LA/MS. Forecast soundings across guidance indicate potential for significant severe/tornadoes with very strong low level shear and large hodographs/amounts of low level moisture both near the warm front and the open warm sector, with also hints of an EML in the early day to limit junk convection. Number one concern right now would be storm mode, but I have pretty good suspicion there will be at least some discrete convection in the open warm sector.

Synoptically there are some similarities to the 4/2/2017 tornado event (high risk in a similar location) and also the 11/21-23/1992 event (three day tornado outbreak across the southern US with a similar looking synoptic trough), with the possibility of after dark problems in MS/AL. SPC currently has a 30% risk for D4 and 15% risk for D5.

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This setup is definitely something to watch closely. All the signals I've seen so far point to a significant severe weather threat, and I hope those in the affected area are monitoring this.

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Overnight in MS/AL is a troubling forecast I hope changes. Otherwise, I'm of course interested in the daylight hours.

Latest guidance does not have a big ol' MCS over in Mississippi Saturday; therefore, it lifts the warm front more. Morning showers are forecast though. Should be at least the WF and plus some outflow boundaries Saturday afternoon. Pre-frontal trough is forecast head of the synoptic front. Pattern recognition this time of year in Dixie says discrete cells ahead.

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12Z NAM depicts a high-end risk on Saturday. Everyone else has discussed the various details. But if you look at the NAM forecast verbatim (not exactly recommended), we **could** be seeing tornadic supercells for quite a long period of time on Saturday across TX and LA, and maybe MS. Forecast environment by as early as 15-18z is already extraordinarily impressive. 60-80kt bulk shear atop low-level helicity values in excess of 400-600m^2/s^2, not to mention 0-3KM CAPE values of 150-200+ J/Kg and ground scraping LCLs (<750m on NAM forecast soundings). Going to need at least some CINh to keep things discrete, otherwise could be pretty grungy in the warm sector.

GFS and Euro are both a bit more lax than the NAM (surprise surprise) showing a lot more precip in the warm sector.

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I'm in DFW and I'm glad to see that the NAM and GFS models are not putting me in the warm sector here.  That said, the warm front will likely be fairly close to DFW (especially if the GFS verifies), though the best helicity will be off to the east by the time that happens (the helicity seems to be in place with the NAM though, and today's 18z NAM run seems to be moving the warm front ominously close to if not on top of parts of DFW).  If this storm system winds up northwest of where the models are depicting it for some reason, then I might be a bit more concerned where I am at.

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00z NAM continues the trend of a significant tornado outbreak across LA, Southern AR), East TX, and West-Central MS. SREF doubles down and pops a 75 contour which is bigly 4 days out. 

SREF_prob_combined_sigtor__f072.gif

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34 minutes ago, MattPetrulli said:

SREF is popping a 90 on Day 3.......

SREF_prob_combined_sigtor__f051.gif

Haven't seen a 90 in a long while... that's not good at all to see

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That's quite a volatile warm sector as depicted on the 00Z NAM, especially around 21Z. Anything out in the open warm sector could have a field day should that come close to verifying.

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4 minutes ago, 1900hurricane said:

That's quite a volatile warm sector as depicted on the 00Z NAM, especially around 21Z. Anything out in the open warm sector could have a field day should that come close to verifying.

The fact NAM is still uptrending is quite telling and nerve racking for something potentially dangerous in LA Saturday.

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*If* NAM were to verify, this would atleast qualify for a MOD risk day IMO. Low LCLs, 2000+ CAPE,  sub 1000 mb Low, upper 60s/low 70s DP, and not to mention pretty significant 0-1km and 0-3km helecities all scream for a significant event in central/northern Louisiana. Only inhibiting factor I could see is for possible cell interference if things get too numerous/grungy. Seems that is an issue quite often this close to the gulf.  

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The combo of low level moisture and hellacious low level shear is at a level seldom seen here. The possibility of multiple bands of convection interacting with this environment assuming things stay at least semi discrete certainly has me thinking that there could be a) an expansion of the ENH/MDT east to account for sig severe storms moving across MS after dark into AL and b) potentially an upgrade to the current MDT in the D1.

My concern would go even higher if something like the 06z NAM were to verify with more SW 500 mb winds atop a raging southerly/even slightly backed LLJ. The amount of veering with height in the lowest 3 km and a rapid increase to near 60 kts at 850 mb is sure to send SRH values skyrocketing perhaps even higher than current model progs. I'll say right now that 4/2/17 didn't look as favorable as this on a synoptic scale, but as always, the mesoscale will make or break it.

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6Z HRRR at the extended range has the warm front farther north by 15Z Saturday than the NAM. The result would be a significant severe weather threat for the D/FW Metroplex as a QLCS moves in from the west (hopefully just a big hail/wind threat with brief tornadoes). It fires up discrete storms *ahead* of that QLCS in eastern North Texas south into the Brazos Valley by 17-18Z in a volatile and dangerous environment. That solution would result in the threat of significant tornadoes beginning just east of Interstate 45 versus the 100-125 miles east that SPC has highlighted. The 0Z TTU WRF (Texas Tech WRF) does show a similar solution. Just throwing that out there as I hit the sack. 

 

HRRR-3-km-Texas-Simulated-Radar-5.gif 

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43 minutes ago, David Reimer said:

6Z HRRR at the extended range has the warm front farther north by 15Z Saturday than the NAM. The result would be a significant severe weather threat for the D/FW Metroplex as a QLCS moves in from the west (hopefully just a big hail/wind threat with brief tornadoes). It fires up discrete storms *ahead* of that QLCS in eastern North Texas south into the Brazos Valley by 17-18Z in a volatile and dangerous environment. That solution would result in the threat of significant tornadoes beginning just east of Interstate 45 versus the 100-125 miles east that SPC has highlighted. The 0Z TTU WRF (Texas Tech WRF) does show a similar solution. Just throwing that out there as I hit the sack.  

 

HRRR-3-km-Texas-Simulated-Radar-5.gif 

Definitely an interesting solution.  I was wondering if this is a bit of an outlier solution though (however the 09z RAP is bringing mid-60s dewpoints into DFW), but the NAM also seems further south than the other models by comparison... 6z GFS (and 6z GFS-FV3) is somewhat between that HRRR run and and the 6z NAM with respect to warm front placement.

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Earlier runs of the NAMs expanded the threat back to the west a bit too, but have come more in line with the globals/SPC recently. It'll be interesting to see if some of the CAMs like the TTU WRF do the same.

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Saturday the prefrontal trough should light up with supercells in the Delta (flat few trees) before dark. This is just one in what could be multiple strings of pearls. Prefrontal trough will have backed low level winds, very much so 925 mb down to surface. Overcomes any 500 mb direction issue. 700 mb is fine with that 850/925 mb. EML should hold off midday junk. And for once, robust instability down there. Wow!

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6 minutes ago, cheese007 said:

Anyone think SPC may go for a high risk?

It is certainly possible, given the general agreement between models on the environmental parameters and discrete convective mode.  I would probably go high risk if I were in the forecast seat...

Probably the best looking day 1 setup I've seen since 2012.

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35 minutes ago, cheese007 said:

Anyone think SPC may go for a high risk?

I think they would wait till Day 1 OTLKs for high risk... I don't remember last time SPC issued a high risk on a day 2 OTLK

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3 minutes ago, yoda said:

I think they would wait till Day 1 OTLKs for high risk... I don't remember last time SPC issued a high risk on a day 2 OTLK

It has been a while, but it does happen.  Either way, if the current model projections pan out, there WILL be a day 1 HIGH.

EDIT: I just checked and it has only happened twice, on 7-Apr 2006 and 14-Apr 2012, so it's probabilistically unlikely.

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Just now, jpeters3 said:

It has been a while, but it does happen.  Either way, if the current model projections pan out, there WILL be a day 1 HIGH.

I expect there to be one on the 0600 DAY 1 OTLK... just saying that I would be a lil suprised to see SPC go HIGH on the 1730 Day 2.

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2 minutes ago, yoda said:

I expect there to be one on the 0600 DAY 1 OTLK... just saying that I would be a lil suprised to see SPC go HIGH on the 1730 Day 2.

Yeah, I just checked and they have only done it twice.  So you're right, it's probably unlikely.

For some reason I thought both 4-27-11 and 5-24-11 were day-2 HIGH, but not the case.

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