2018-10-07 Event: OK/KS/TX/NE/CO

[Dan Robinson]

The GFS and Euro agree on a longwave trough becoming established over the western CONUS this weekend, with a negatively-tilted shortwave ejecting out over the Plains on Monday. With plenty of low-level moisture available, this system should bring at least one legitimate fall tornado setup to the Great Plains.

The best day appears to be Sunday, as +50kt southwesterly flow aloft becomes established and a lee surface low spins up on the High Plains. While the main shortwave (negatively tilted +90kt jet streak) will not eject until Monday, some impressive 'classic' thermodynamic profiles are shown for Sunday in western Kansas and Nebraska.

 

[Brett Roberts]

The GFS (and its ensemble, to a lesser extent) is currently an outlier in developing the warm front rapidly northward during the day Sunday, placing the triple point in the NE Sandhills by late afternoon. The ECMWF, Canadian GDPS, and their respective ensembles all suggest the focus would be farther south, with a surface low in the Trans Pecos and the warm/stationary front extending northeastward from there into the Panhandles and OK/KS. Although the latter is not exactly a textbook surface chart, it's worth remembering that the 7 November 2011 event had a similar MSLP configuration, so all is not lost!

Regardless of the northward extent of the threat, I think the bigger concern -- by far -- will be early or ongoing convection. Highly amplified longwave troughs like this are fickle, and in my experience, disappoint more often than not. The GFS shows a slightly broader base to the trough and less of a positive tilt, versus other global guidance, and this would be helpful. The other two deterministic runs I mentioned have large swaths of the warm sector smothered in widespread precip from sunrise to sunset, harshly tempering the likely chase quality.

I'll be watching for subtle trends in the trough's timing and geometry on the guidance over the next couple days, as positive developments could have significant implications. For now, it seems most reasonable to expect a fairly messy setup somewhere on the southern High Plains with the typical autumn drawbacks of poor lapse rates (i.e., skinny CAPE) and an excess of mid-upper level moisture. I recall a fruitless desperation chase out to the TX Panhandle around this time in 2008 (11 October, to be exact) that featured some similarities in the upper air pattern to current forecasts. These same flaws have been present to a degree on the rare big autumn days like 11/7/11 and 10/9/01 (e.g., both featured plenty of early convection), though, so it's worth watching.

 

[James Gustina]

As Brett noted, looks like standard fare from an autumn perspective. The warm sector looks incredibly messy on all 3 long-range solutions, but that's a pretty consistent bet on every setup this time of year. It looks like some of the embedded impulses may have the potential to also create timing issues on their own (i.e. early initiation of any convection further west towards the dryline).

The BL moisture content progged by the GEFS (mid-60s off the Llano) is pretty promising for at least supercells but the pretty high possibility of low-level clouds all day makes this a tricky one on top of any potentially weird shear profiles that seemed to be the feature of this season on the southern Plains.

 

[Taylor Wright]

One thing that we seemed to have going for us with this setup, moisture, even seems questionable depending on the southward extent of the surging cold front on Saturday. If this surges all the way to Midland as the NAM is currently showing, The setup is toast IMO. At the moment, NAM is outlier with taking it this far south, but that doesn't mean it's wrong. If that CF stalls farther north then we can probably get quality moisture into southern CO and TX panhandle. The Euro seems to favor this, which is the solution the GFS is trending towards, but doesn't help us in terms of convection and cloud cover as mentioned in previous posts. My hope for this one is declining, but likely won't know the magnitude of the setup until Sunday morning.

 

[Brett Roberts]

Well, the southward shift in focus is trending even farther than I anticipated a few days ago, largely thanks to the strong cold air push on the High Plains this weekend. Global models destabilize much of the TX Panhandle and eastern NM by Sunday afternoon, while regional models with greater vertical resolution are tending to keep the cold wedge entrenched, limiting the instability axis to the Permian Basin and up into a smaller sliver of NM. With that said, forecast soundings in that area are still intriguing thanks to strong low-level veering with height and very manageable LCLs. I'm mildly interested in the area from around Midland to Hobbs to Roswell, possibly extending northward toward Lubbock and Clovis, depending on trends over the next 48 hours. The biggest concern I have is the timing of the trough progression and associated height falls, which has slowed considerably since earlier this week. If it slows any more, there may not be much opportunity for CI behind the large wave of early-day precip widely depicted in the NWP guidance, and shear would also take a hit. If the system's timing trends slightly faster, though, I can see a solid supercell threat with some tornado risk in the area I mentioned.

 

[John Farley]

I am still undecided on this, a luxury I have since I am only 2 or 3 hours from what would be my target area in eastern NM. The models do show a sufficient combination of shear and instability for supercells and probably for a tornado or two on the eastern plains of NM, which is tempting. And they certainly do break out storms in this area. OTOH, these will be fast-moving storms in an area with a limited road network, and the window for daytime supercells and potential tornadoes is pretty small, maybe only an hour or two right before sunset. The intensity of the storms will likely peak after dark, although by then they will be less discrete and much more numerous. So really the opportunity is only an hour or two, and with fast-moving storms and a challenging road network. For those reasons, I will stay in wait-and-see mode a while longer and watch how things seem to be unfolding.