Stormtalk

Sunday, February 21 Update: – Simon documents Dallas Supercell with Funnel!

5pm CST: At 4:25 p.m. CST Simon Brewer witnessed a supercell thunderstorm with a long rope funnel extending halfway to the ground 10 miles south-southwest of the intersection of highway 148 and I-20, or just just southeast of Dallas, TX.  Jim Bishop immediately called the DFW National Weather Service Office and reported the funnel cloud.  Simon continues to chase this supercell  as it moves southeast over central Texas.

- Jim

—–End Update——–

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Saturday, February 20 Update:

The location and timing of the severe threat still appears to be from near the I-35 corridor in central Texas Sunday morning eastward into east Texas and Louisiana by the afternoon and evening.  But the overall severe threat has diminished somewhat as computer models are not returning as much low level moisture which is limiting CAPE values.  In addition the mid and upper level winds are more veered (westerly) on the latest models, showing less directional shear.  Speed sheer will still be sufficient for severe storms and a possibly supercells – especially late in the morning near the I-35 corridor in central Texas.  But the tornado threat looks quite low.  The main threats will be hail and damaging winds, but even those threats have diminished somewhat since yesterday.

Below:  Latest SPC Outlook for Sunday.  They too have reduced the severe risk for severe storms.  But notice how they have extended there risk area further west to include central Texas/I-35 corridor. I’m happy to see this since it’s more in line with my thinking on the initial location of the severe threat.

- Jim

–End of Saturday update—

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The setup for Sunday, February 21 is looking a little more interesting today and I think the threat for severe thunderstorms has increased across central to eastern Texas into Louisiana.  And may I say it’s nice to have a severe weather setup to talk about in the late winter!  I’ll take what I can get…

Below:  My current severe storms forecast for Sunday, February 21 .  We’ll see how it works out..I just wanted to provide a visual aid.  It’s still early and the details will change.  The tornado threat will diminish/shift east if the storm system speeds up any….

A shortwave trough is expected to eject eastward from the desert Southwest into the southern Plains early on Sunday and move quickly eastward into the lower Mississippi Valley by Sunday night.  The exact timing of this shortwave will be crucial in determining where the severe storms will be most likely.  From looking at all the models it seems the GFS and European models have the best handle on the system, although I wonder if the shortwave will slow down any further.

A surface low should develop ahead of the shortwave early on Sunday over central Texas and move quickly to the northeast, reaching northeast Texas by midday.  Dewpoints in the mid 50s to near 60 degrees are expected to expand northward in the morning through noon across central/eastern Texas.  This combined with cold 500 mb temperatures near -18C will contribute to steep lapse rates.  As a result CAPE values in the 500-1000 j/kg range are expected to develop in the morning to the midday from near Abilene to  the I-35 corridor in Texas spreading further east by the afternoon.

A pronounced dryline is expected to mix east quickly from West Texas to the I-35 corridor by the late morning as the surface low rapidly moves toward the northeast.  This will provide for a good forcing mechanism for thunderstorms to develop late in the morning through midday from just west of the I-35 corridor in central Texas to just east of I-35.  In addition a squall line should develop further north across Oklahoma into north Texas along an arctic front late in the morning.

Wind shear across the whole region will be supportive of severe storms producing large hail and damaging winds.  There will also be a tornado threat as storms develop along the dryline just west of the I-35 corridor near and south of the DFW region during the morning hours.  This is supported by 40 knot 850 mb winds out of the south-southwest on top of backed surface flow, owing to good low to mid level wind shear.  Storms will move rapidly to the northeast, probably around 40 mph or even faster, which will help increase low level wind shear.

We’ll see how the timing of this system changes over the next couple days.  But for now it looks like a decent fast moving supercell threat in the morning through midday across central to eastern Texas with a tornado threat mainly in the morning across central Texas.  In addition a squall line should become dominant by the afternoon and spread across eastern Texas into Louisiana.

Below:  The Storm Prediction Center day 3 convective outlook made this morning (2/19/10)

- Jim