2013-05-16 FCST: NE, KS

Jeremy Perez

Aug 31, 2008
Flagstaff, Arizona, USA
Like anybody with an itchy chase trigger finger, I’ve been refreshing model forecasts, watching for the trough making its way into the plains toward next weekend. I’m also looking at opportunities on either side of the main event.

in the extended May 12 forecast, SPC makes no mention of earlier severe possibilities Thursday/Friday, so I'm hanging way out on a limb with this thought process:

Prior to arrival of the trough, zonal/northwest flow over the northern plains looks like it may overlap a moist boundary layer. The 00Z May 11 2013 GFS depicts a dryline from Nebraska south through western Kansas and warm front boundary aligned southwest to northeast across central Nebraska. (The last few 00Z/12Z GFS runs have been bringing moisture further north.) I don’t have a feel for how much the GFS overestimates moisture, but it is currently showing 60-65 degree dewpoints and CAPE up to 3000 j/kg into southern Nebraska where better bulk shear through 500 mb on the order of 35-40 kts is available.

700 mb temps in the area have been warming with each run, and currently forecast at 9°C. But GFS and ECMWF are still breaking out precipitation late in the day. Low level shear is not great, but winds are veering with height. If this panned out, supercell structures look like they could be possible along the northern extent of the moisture axis if storms manage to fire along dryline.

As always, overlapping details will shift possible target areas and/or remove chances altogether as the day approaches, but I wanted to get these thoughts out there. I’ll be looking for opportunities to target Nebraska, Kansas, possibly Colorado prior to potential arrival of the trough next weekend.

Although mid and upper flow are not oriented southwesterly: moisture, instability, boundaries and sufficient deep shear are available here. GFS and ECMWF are also breaking out precipitation in the right areas. So I'm very interested in any thoughts on why some degree of limited severe possibility for Day 5 (or Day 6) wouldn't be touched on by the SPC in this morning's extended forecast. And definitely, any other feedback and criticisms of this amateur forecast are welcome.
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Oh, I wouldn't be overly concerned about the lack of an SPC outlook 5 days out. I'm certainly no SPC insider, but I've observed over time that they are very conservative about setting themselves up for having to "back down" from a forecast. In this case, with the general lack of upper air support, no positive outlook would pretty much be expected. Nevertheless, looks like you have brought up some valid points and, by the way, the theta-e looks as strong as it has all season. It might be one of those days to look for subtle clues in the morning and go from there. It looks very conditional, but the climatology is also very favorable.
This morning's 12Z GFS and NAM are in good agreement with position of a shortwave at 500mb moving over SW KS/OK Panhandle around 00Z. Overlap of deep shear and moisture has moved away from Nebraska with these runs. 300mb flow is poor this far south though. Forecast sounding/hodographs from surface to 500mb looks nice though for this day-before-the-day-before in the OK Panhandle.

So, my guess is that flow that shuts down above 500mb/weak anvil level winds, would mean messy inflow and short-lived, undercut storms? As usual, I'll be watching to see how NAM and GFS trend this. It's not close to ideal, but I think it's a good get-the-kinks-out run-up to the hopefully better setups Friday onward.