2018-04-20 EVENT: TX/OK

[James Gustina]

I'm way out of practice on writing these so bear with me here. Even though it's just come inside the 144 hour mark as of this morning, this upcoming Friday looks like it might have some oomph behind it in terms of timing and location.

This morning's 12Z run of the GFS still shows somewhat favorable wind profiles ahead of the dryline across the Llano Estacado and down into the Rolling Plains in NW TX/SW OK. A neutral/slightly positively tilted trough is progged to eject out of the Four Corners region around 18Z by both the GFS and Euro, with a 90 knot jet max entering the High Plains. Lee cyclogenesis

While the trough position and timing seem fairly decent for the time of year, it looks like a cold front is set to sweep through the Plains Wednesday night/Thursday morning, which appears to be affecting the quality of the moisture fetch. The GFS has backed off of the low-60s it was spitting out in the previous few days and is painting mid-high 50s ahead of the dryline. That's not a death sentence for a High Plains setup, but considering how absolutely bone dry it is on top of it all, it's something to be mildly concerned about.

The overall shear profile may be another potential issue, with a noticeable weakness around 700 mb. However, the low-level shear has so far looked fantastic on every run and strong anvil level winds over 60 knots/lower dews should hopefully preclude a messy storm mode. The lower moisture content and the possibility of ongoing precipitation through Thursday night and into Friday morning onto the Llano seems to have been the reason for CAPE values plummeting over previous runs to ~1000 j/kg on a narrow corridor at the dryline. But, the volatility of the GFS' solutions and it still being 5 days out makes me think that nothing too meaningful can be gleaned from that until we get into the NAM's range.

Overall, there are a lot of details missing that won't be fully resolved until Wednesday at the earliest. I'll be interested to see what the associated precipitation event does to the target region.

 

[Quincy Vagell]

It looks like the southern High Plains may be setting up to experience a noteworthy early season severe weather event this upcoming Friday. The model signal has been there for several days and with SPC issuing 15% probabilities for the area on back-to-back days, it's time to start a thread.

In the upper levels, a trough with an embedded closed low is progged to move from the Southwest/southern Rockies toward the southern High Plains on Friday. In the lower levels, a robust, south to southeasterly low-level jet should transport at least modest boundary layer moisture northward to portions of West Texas by peak heating.

Mesoscale details will undoubtedly change, so one should not be overly focused on finer details at this stage (still 120+ hours out). Even with the ongoing drought concerns, note that mid to upper 50s dew-points would support regionally sufficient buoyancy over the relatively high terrain for at least isolated severe thunderstorms. Note that mid-50s dew-points at Midland (MAF) moves beyond the 90th percentile with respect to sounding climatology. I am less concerned about the ongoing drought to damper moisture return. If anything, a setup with more isolated activity, as opposed to a mass of convection exploding by early afternoon, would support a more robust severe threat. Wind profiles look favorable in a larger area, unlike this past setup on Friday.

Overall, this does not look like a major or widespread severe weather event, but chasers should be at least mildly intrigued by the synoptic pattern, as it supports chase prospects come this Friday. Even if it winds up being a lower-end severe event, the bright side is that rainfall, which could be locally heavy, falling over the panhandle region should start easing drought concerns. This system will not alone be a drought-buster, but drought-busting has to start somewhere...

Note that Oklahoma was not included in the title, as most of the data seems to suggest the threat centers on West Texas/Texas panhandle.

***Feel free to merge threads, since it looks like another one on this potential event went up shortly before I posted this.

 

[Ben Holcomb]

Hard to get excited over this setup with RH values being nearly saturated in the target area throughout the column most of the day limiting instability. The 12Z GFS today is showing a county wide instability axis 00Z/Sat and only values of maybe 750-800J/KG. I just can't imagine making the trek from Norman for the soupy mess that will happen. Hoping the NAM brings a brighter picture because the dynamics of this system are pretty good, but the details are ugly.

 

[James Gustina]

Looks like just about everything that could have gone wrong for this setup, did. NAM and GFS are painting an incredibly narrow and modest corridor of CAPE around 800 j/kg in the Big Bend region of Texas, along with some ugly shear profiles and capping concerns. The distinct weakness at 700mb has gotten noticeably worse with each run, essentially putting a fork into this as a chase day.

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[Quincy Vagell]

Now SPC has downgraded the event to a MRGL risk with their latest day 2 outlook. This does not seem like a worthwhile chase day (based on virtually all model data, not just the SPC outlook), unless you are local, or, for the lack of a better word, desperate for a chase.

 

[John Farley]

Looks like people may have been a little too quick to write off this day. This morning's models are looking better for far eastern NM into the far western TX Panhandle. Better moisture than earlier predicted, with dewpoints into the 50s per the RAP and at least the 40s per the NAM. Lots of wind shear, including low-level shear. And the HRRR shows a broken line of convection late in the afternoon. And SPC back up to a slight risk for a small area near the NM-TX state line. All this is enough to get me out the door with a relatively easy chase from Santa Fe, with a tentative target of somewhere around Clovis.

 

[Jeff Duda]

Haven't looked too deeply at this, but I see the following:
-lapse rates are okay, but not great. Some EML is present but it's a bit limited in vertical extent.
-moisture is just pathetic. You have to go well into deep south TX to find respectable low-level moisture. Hell, the 2-m Td from the 12Z DRT sounding is only 40 F. Not typically what you will see the morning of a chase day.

 

[Quincy Vagell]

HRRR forecast soundings in the narrow instability plume late this afternoon/early this evening show dew-points <50F and LCL heights up near 2km. (convectively contaminated soundings show dew-points over 50, but even that area is extremely narrow) The wind profiles are supportive for rotating updrafts and the HRRR does suggest a brief window for discrete/semi-discrete storms. However, given such a narrow plume of marginal boundary layer moisture for the elevation and high LCL heights, the threat is fairly conditional. It may yield some brief supercell structures and maybe an LP supercell for good measure, but the chase prospects are not impressive by any stretch of the imagination for April 20th, given the location.

 

[James Gustina]

The West Texas Mesonet has dews sitting around the high-30s/low 40s at the TX/NM state line. Even if the dews bump up another 5-10 degress in the area of the best shear just west of that SLGT, I can't really foresee any sustained supercellular convection in the area. Just echoing what Quincy said above, there may be an LP supercell or two but it's just not enough to get me out the door for a 6 hour drive one way.

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[John Farley]

Dewpoints that never made it higher than the 40s were certainly part of the problem, but I think there was more to it than that. With the relatively high elevation, you can get supercells and/or severe weather on the eastern plains of NM with dews that low, though admittedly that is more true up in the higher elevations around, say, Wagon Mound or Raton than near the TX state line. That said, I think other factors were also at play because the storms did not appear particularly moisture starved, and the failure to get sustained supercell structures seemed more tied to too many storms than to lack of moisture to sustain storms. Every time new storms went up to the southeast of existing storms, lots went up at once and the mode quickly became multi-cellular. I think limited instability was a major limiting factor, not just because of low dews but also relatively low temperatures - even out ahead of the storms, my car thermometer never got out of the 60s. My guess would be that if you added ten or 15 degrees and a little more cap, there would have been fewer and more isolated storms with a better chance of sustained supercells. Certainly the wind profiles were favorable and the way the wind was screaming from the southeast just ahead of the storms, there had to be a lot of surface convergence along the dryline - which may also have contributed to the large number of storms.

The most interesting thing about these storms visually was their brown appearance due to the large amount of dust in the air from many hours of strong gradient winds ahead of the storms. Also, you could not even see any storm features approaching the storms from the east until you were within five miles or so of the storm, due to the amount of dust in the air. Here a couple pictures showing the brown appearance of the storms.





I don't know much about this, but I do also wonder of the dust may have contributed to the large number of storms by providing more than the usual amount of nuclei for condensation.

In any case, these storms were good rain producers and also produced a lot of small hail in some areas and gusty winds in the 40-50 mph range - but nothing severe. Very good news, though, with widespread half-inch to inch rainfall across the eastern plains of NM and the TX Panhandle where moisture has been severely lacking this year. A number of locations in this area, such as Amarillo, got more precipitation yesterday than the whole rest of this year so far. Here is a radar image showing storm total rainfall across eastern NM and the western TX Panhandle: