Wednesday, May 18, 2011

2011-05-18: Chase Forecast (Updated 0900 WDT)

Forecast time: 0100 WDT...From DEN

Looking at the 00z NAM, the 990mb sfc low continues to impress in SE CO by late day with some intense easterly progged winds AOA 20kts near Lamar to Campo. Cloud cover is going to play a big role in limiting sfc heating, and I think the NAM is right about forecast Ts in the 70s near the sfc low. Today in DEN it took a long time even to crest the 60 deg mark, and I'm worried cloud cover currently seen over the CO/NM border (presently 08z as I write this) may keep it cooler in SE CO. I'll have to verify after the morning soundings.

My target area remains firmly close to the sfc low (E CO, along 287) where hodographs favor broad 90 deg clockwise 0-3km shear profiles.

The dryline looks messy due to very poor sfc Tds, but should drape NW to SE, possibly with the hint of a NE bulge somewhere east of Pueblo by 00z. This still puts the best "pooled" moisture close to KS border, and in that respect, I'm hoping for some High Plains Hijinks to aid things. The backing winds should force both upslope flow and some upslope theta-e into the region, but T/Td spreads will still be pronounced.

Things get a bit trickier in the upper levels with the nose of the H5 jet just barely arriving before sunset, but still providing 30-45kt WSW winds over my tentative target area. The 850mb winds look great (ESE AOA 20kt at the CO/KS border). Steep lapse rates and only modest capping should allow convection.

FWIW, yesterday's 00z 4km WRF breaks out a nice, seemingly isolated precip blob in SE CO around 21z, likely reflecting arrival of the nose of the H5 jet with superimposed intensification of the LLJ at around that time. Things will become linear and multicellular after sunset, but sticking to the southerly cells should allow a pretty good opportunity for small tornadoes and some terrific lightning along the dryline into the night.

My chase target is now almost certainly going to be in an area between Lamar and Springfield along Hwy 287 pending am soundings and SAT.


Forecast update time: 0900 WDT...From DEN

SAT data shows a thick stratocirrus lid on things over almost all of CO, but this should move north and dissipate as the day goes on. W OK alread has cleared considerably, so I think the two targets will both find their daytime highs. I'm not optimistic about W OK hitting its Tc (AOA 95 deg) with H7 temps already in the 8-9 deg region and less forcing from the dryline. That said, my confidence in the CO target has diminished a bit too.

There are two sfc mesolows on obs this am. One is over Raton and should be the one that evolves in SE CO as the day goes on. But to my surprise, there appears to be a second mesolow in the region of the TX PH (mostly noted by wind shifts). If this were to move in a similar trajectory as the western low, it would really have a detrimental affect on both sfc winds and returning moisture to E CO by bringing more northerly winds into what I was hoping would be a strictly easterly upslope situation. That warrants hourly watching for now.

So my chase strategy for now is to get to Pueblo (which I'd be doing anyway), and then make the painful choice of either continuing S on I-25 to Raton to try to hit the Boise City area, or more likely hold up in Pueblo to begin a move eastward on US 50.

Normally most days with targets just a couple of hundred miles away from each other stress me out, but I think keeping my expectations on sculpted supercells today should more than help with indecision I'm feeling.

Packing up. En route to Pueblo.

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