Chase 2011 - Day 21 Discussion

[Nick F]

Quite a wide area for potential severe storms to choose a chase target today from the Hill Country in Texas N and NE across much of Oklahoma, Kansas and eastern Nebraska and further east into western parts of Missouri and Arkansas.

Surface progs at 00z Saturday (18z Friday CDT):

Two areas IMO to target - the first is definately in range of the Netweather chase team who stayed overnight in Great Band, KS and this is central Kansas, north central Oklahoma and up to south central Nebraska - the Pacific cold front arrives this evening across the area from the west - which should create some severe storms, though as often happens it tends to turn linear and outflow dominant fairly quickly ... though still the potential for one or two tornadoes but main threat may be large hail.

The other area of interest is further SE across SE Oklahoma, N-central/NE Texas and on up into Arkansas - where we have increasingly moist air and stronger wind shear later and the likelyhood of outflow boundaries moving east of the current large MCS system over Nern texas, Oklahoma and Kansas. So maybe better chance of seeing a tornado here, SPC have a 5% risk, though yesterday's 15% risk over Kansas didn't really come up trumps.

I'm guessing that Tour 2 team maybe headed south through Oklahoma for the changeover.

[Paul Sherman]

Hi Nick

Going to be heading South and playing the North Central and Central Oklahoma risk later this afternoon and evening, hoping for a few discrete cells before it all lines out. Need to be within 3 hours of DFW For tomorrow morning.

Paul S

[dallas]

Mesoscale Discussion 825

< Previous MD

MESOSCALE DISCUSSION 0825

NWS STORM PREDICTION CENTER NORMAN OK

1030 AM CDT FRI MAY 20 2011

AREAS AFFECTED...AR...ERN OK...NERN TX AND NWRN LA

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE

VALID 201530Z - 201630Z

...AR /LATE MORNING AND AFTERNOON/...

WW MAY BE NEEDED ACROSS AR LATE THIS MORNING INTO THE AFTERNOON.

A LATE MORNING AND AFTERNOON SEVERE WEATHER THREAT APPEARS TO BE

UNFOLDING ACROSS WRN AR PER RECENT TRENDS IN REGIONAL RADAR AND

SATELLITE IMAGERY WITH INCREASING COVERAGE OF TSTM ACTIVITY AND

COOLING CLOUD TOPS...RESPECTIVELY. WATER VAPOR IMAGERY SHOWED A

SHORTWAVE TROUGH MOVING NEWD ACROSS ERN TX...AND FORCING FOR ASCENT

WITH THIS FEATURE WILL AID IN ADDITIONAL TSTM DEVELOPMENT ACROSS AR.

AIR MASS IS ALREADY MODERATELY UNSTABLE GIVEN STEEP MIDLEVEL LAPSE

RATES /7-7.5 C PER KM/ ATOP RICH MOISTURE RETURN /SURFACE DEWPOINTS

UPPER 60S-LOWER 70S SPREADING NWD INTO AR/. INCREASING EFFECTIVE

BULK SHEAR OF 25-30 KT WILL FAVOR MULTICELLS. CURRENT ACTIVITY OVER

WRN AR MAY BE SLIGHTLY ELEVATED WITH HAIL BEING THE PRIMARY THREAT.

CONCERN EXISTS INTO CENTRAL AR WHERE STRONGER SURFACE HEATING

CONTINUES TO WEAKEN SBCINH WITH MODIFIED 12Z LIT SOUNDING FOR

CURRENT SURFACE TEMPERATURE/DEWPOINT IN THE UPPER 70S AND UPPER

60S... RESPECTIVELY...SUPPORTS SURFACE BASED STORMS. WEAKER LOW

LEVEL SHEAR WITH EWD EXTENT SHOULD LIMIT A TORNADO THREAT... THOUGH

DAMAGING WINDS WILL BE A THREAT IN ADDITION TO HAIL.

...ARKLATEX REGION /AFTERNOON AND EVENING/...

RICH BOUNDARY LAYER MOISTURE IS CONTINUING TO SPREAD NWD WITH

SURFACE DEWPOINTS CLIMBING INTO THE LOWER 70S. THIS COMBINED WITH

AN EXPECTED INCREASE IN HODOGRAPH CURVATURE THIS AFTERNOON ACROSS

THE ARKLATEX REGION AS SURFACE WINDS BACK SOME WHILE 850-700 MB

WINDS VEER TO SWLY WILL SUPPORT A GREATER CONCERN FOR TORNADIC

SUPERCELLS.

SURFACE HEATING WITHIN AN ALREADY MODERATELY UNSTABLE WARM SECTOR

EXTENDING E OF A COMPOSITE OUTFLOW BOUNDARY ATTENDANT TO THE SRN

PLAINS MCS WILL FURTHER ENHANCE THE POTENTIAL FOR NEW TSTM

DEVELOPMENT INTO THE AFTERNOON. STORMS SHOULD ALSO CONTINUE TO

DEVELOP ALONG THE OUTFLOW BOUNDARY AS IT MOVES SLOWLY EWD TODAY.

STRENGTHENING EFFECTIVE BULK SHEAR TO 25-40 KT SUGGESTS BOTH

MULTICELLS AND SUPERCELLS WILL BE POSSIBLE WITH ALL FACETS OF SEVERE

WEATHER POSSIBLE.

[nsrobins]

Well quite a shift in conditions in the last four hours or so with a fairly robust OFB pushing east into the Arkalatex, where low level flow is bringing in lovely rich 70+dp moisture. Just had a peek at the 21Z hodo for Broken Bow, OK and a nice looking clockwise curved profile indicative of significant veering directional shear bewteen surface and 500mb. Speed shear not so great but plenty to get supercells spinning I would think. I know the last few days have been pretty naff model v actual but I have a feeling that the old eastward moving OFB will work it's magic. Don't fancy the terrain though. <BR><BR>Edit: A question for the more learned members: At what level do OFB vectors operate? Are they surface or at height? If an eastward propogating OFB (presumable westerly wind) at surface meets the SE flow, what effect does that have on the shear?

[TornadoJo]

Tempted to hang in McAlister SE ok today.