10/21/2017: Enhanced Risk for Severe Weather

10/21/2017: Enhanced Risk for Severe Weather

Since Wednesday morning, the Storm Prediction Center has been highlighting a potential for severe weather across much of Oklahoma and parts of North Texas. Since Thursday morning, SPC has had an “Enhanced” risk across these same areas, with today’s Day One Convective Outlook continuing to show the Enhanced risk.

Day One Convective Outlook issued by Storm Prediction Center October 21 at 1300Z (8:00 AM CDT).

Taking a look at morning observations, in the upper levels, a highly amplified trough is noted across the Rocky Mountains. The absolute vorticity maximum associated with this trough is over Northwestern New Mexico, which would mean the greatest forcing for ascent is across the High Plains of Eastern Colorado, Western Kansas, and into the Oklahoma/Texas Panhandles.

October 21 1200Z 500 mb analysis (created using Digital Atmosphere Workstation).

If we go down to the surface, we can see the lee cyclogenesis is already underway across these aforementioned area where the best vertical ascent is noted. A surface low appears to be forming in the vicinity of Garden City, KS. This low is forming along a cold front that runs through the Great Plains, then curves west along a line from roughly Boise City, OK to near Taos, NM. Ahead of this cold front in the warm sector, dewpoints into the 60s are noted as far north as Sioux Falls, SD, with upper 60s dewpoints into Southern Oklahoma. The dewpoints across the enhanced risk area should only increase as the low near Garden City deepens, and warm air advection across the warm sector increases.

October 21 1200Z (7:00 AM CDT) surface analysis (created using Digital Atmosphere Workstation).

The morning Fort Worth sounding shows a strong elevated mixed layer (EML) in place across the region which should limit convective development for a while. The EML is a layer of stable air that acts as a cap, preventing convective development. This is “good” for the storms however, because it will allow the instability in the atmosphere to only increase with time without being disrupted by the cooling effects from thunderstorm development. This cap will certainly be lifted late this afternoon and evening across the region however as the cold front sweeps across the Southern Plains.

Annotated October 21 1200Z (7:00 AM CDT) Fort Worth, TX sounding showing the EML and steep mid-level lapse rates (generated using Digital Atmosphere Workstation).

Model guidance is in good agreement that rapid convective development is likely to begin around 4:00 PM across Northwestern Oklahoma then build southwest into Northwest Texas by 6:00 PM. Given the strong amount of forcing for ascent and deep-layer wind shear vectors oriented nearly parallel to the cold front, storms will likely grow into a squall line quickly. There may be a brief window between 4:00 PM and 7:00 PM for a few thunderstorms to remain as discrete supercells given the strong amounts of wind shear and large instability. During this period, the main severe weather threats will likely be large hail and a few tornadoes. After 7:00 PM, thunderstorms should be almost entirely linear in nature, which will transition the main threat to damaging wind gusts. A rather impressive low-level jet is forecast to develop after sunset, with 850 mb winds likely to exceed 45 knots across parts of the threat area. This makes the threat for damaging wind gusts particularly concerning near and north of the Red River, along and east of Interstate 35.

HRRR forecast 850 mb winds and heights valid October 22 at 0400Z (October 21 at 11:00 PM CDT, image source: Pivotal Weather).

After this cold front passes through, looker for cooler temperatures on Sunday with highs only in the 70s across most of Texas. Another weak cold front looks to come through Tuesday, but a much stronger cold front (perhaps the strongest so far this fall) looks to possibly affect our area late next week, but that’s another post.

Bottomline:

  • Severe weather event likely across much of Oklahoma and far North Texas.
  • Greatest TORNADO threat likely across Northwest Texas and Southwest Oklahoma during a short window from 4:00 PM to 7:00 PM.
  • Greatest WIND threat likely along and north of the Red River near and east of Interstate 35 from 9:00 PM to 12:00 AM.
  • Cooler, but not cold, temperatures expected on Sunday.
  • Another cold front on Tuesday.
  • Strong cold front end of the week?
  • As always, this blog is nothing official, and is my forecast opinions alone! Please refer to products from the National Weather Service and media outlets for official information!

What I will be watching throughout the day:

  • When does the cap break? Current expectation is that it will break across Northwest Oklahoma around 4:00 PM, then “zipper down” the cold front into Northwest Texas by 6:00 PM. I will be watching visible satellite and of course radar to see when this is beginning.
  • How quickly do storms grow into a line? The longer storms persist as discrete supercells (if at all), the greater the likelihood for tornadoes and large hail. Watch the radar trends. Is development spotty at first, or does a line erupt quickly along the cold front?
  • How strong is the low-level jet? The 0000Z soundings this evening may give us some idea, but the low-level jet will likely peak after the 0000Z observations. Keep an eye on VAD wind profiles from Doppler radar.
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