Archive for April, 2009

Favorable setup for well structured supercells across OK today

Another marginal setup is expected over OK today, as somewhat weak NWern flow is present in upper levels while at low levels moderate SSWesterlies are in place across central Plains, overlaping good with extreme instability. At surface, cold front is slowly racing SEwards across central Plains during th day, while dryline is slowly mixing Ewards from Eern TX Panh into SW OK and over W TX. Several complex systems have been on going in the last 12 hours across central and southern Plains leaving several outflow boundaries behind. In addition, those convergence zones will likely be focus points today as well as Ewards mixing dryline interacting will a weak outflow boundary in SE TX Panh/SW OK later in the afternoon. Storms are also expected along SEwards racing cold front further north.

500mb winds

850mb winds

Shortly said, veering wind profiles, even the speed shear is only low to moderate, will create very favorable directional shear for supercells development all across warm sector between dryline on the western edge and cold front to the north. Given the pretty high surface dewpoints, which are currenty in upper 60s to low 70s in parts on N TX and in mid to upper 60s in much of OK, along with strong heating expected as clearing skies are evident on the latest satellites behind the early activity and steep lapse rates in low levels, high CAPE release and rapid destabilization of the boundary layer is expected to accur as cap weakens later in the afternoon.

SFC dews

MLCAPE

Storms will likely initiate along mixing dryline and racing cold front first, while some isolated supercells can be expected in between across south-central OK as well, depends on local convergences along the outflow boundaries. Very high CAPE values along with directional but weak speed shear, will still become favorable for rotating updrafts while a classic setup for structured supercells could take place as such wind profiles, with NW flow aloft and SW flow beneath it, usually supports well structured supercells. Actually, similar happened yesterday over western KS with the monster classic supercell that has produced impressive stucture as well as numerous gustndoes, very large hail and also a couple of tornadoes near Garden City. So today the main threat can be supercells with very large hail and damaging winds, though weak tornadoes are also possible, especially along the boundary interactions which will lead into increased effective SR helicity.

Lawton OK skew-t

I like the area ahead of the mixing dryline in the belt of very high instability near Lawton, OK. That would actually be my target for today. As can be seen from the skew-T diagram above, very favorable directional shear is present as well as several thousands of SBCAPE. However, as earlier said, only marginal to moderate speed shear will be present, but given the mid/upper-level WNWern flow, storms will not have much problems with rapid organization with severe weather threat and photogenic look.

Explosive convection with very large hail from W TX into W KS likely today

Marginal, but very interesting and explosive setup is expected today across W TX Nwards into Wern KS. A large upper-level low is located over northern parts of west US Conus while several disturbances are moving towards the Plains. A new wave from the Sern Rockies with rather strong speed-max embeded in the southern jet stream will eject into south-central Plains in the afternoon, with its left-exit region and upper-level divergence placed over TX Panhandle, creating favorable environment for severe storms development.

500mb heights

300mb jet

Storm Prediction Center has issued a large SLGT risk for portions of western TX, OK, KS and NE with 10% probabillities for tornadoes, while likely higher threat will be very large hail and it was hatched with 30%.

HAIL probs

At low levels, surface warm front is slowly pushed NWwards across the Plains as LLJ kicks in from south and healthy boundary layer moisture recovery can be expected with SFC dewpoints in mid to upper 60s locally over parts of OK and NW TX into Panhandles. A weak surface pressure is expected to develop from NM/TX border Nwards into SE CO/SW KS which will start to mix the dryline, currenty located in eastern NM, Ewards and models suggest a dryline bulge over southern TX Panh.

2m thetaE

Currently a large MCS is on going near southern OK/northern TX border with a massive squall-line moving ESEwards. Visible satellites show a well-defined outflow boundary from this complex racing WSWwards into NW TX which will likely be another focus for afternoon’s storm initiation. Behind the racing MCS complex, strong surface heating will take place and along with an approaching speed-max from the west and accompanied steeper lapse rates, which will come exactly in the mid afternoon hours, rapid destabilization will occur. Models are in good agreement of high values of CAPE, RUC is again overestimating quite a lot due to expected too high SFC dewpoints for a couple of degrees. According to that, RUC has more than 5000 J/kg of SBCAPE placed along/ahead of dryline bulge in SEern TX Panhnadle in area streching between Amarillo-Childress-Lubbock. More reasonably, SBCAPE around 3500-4000 J/kg with MLCAPE values around 2500 J/kg can be expected, which is still pretty high and explosive convection will be easily possible.

SBCAPE

Slowly increasing LL jet ahead of approaching speed-max will increase LL shear while upper-level flow will be increased as well, supporting around 40kts of DL shear. According to Serly LLJ, backed surface winds and SW flow aloft, good directional shear is expected, while speed shear will be marginal to moderate. This will support rapid storm organization into multicells and isolated supercells capable of producing very large hail and damaging winds.

850mb thetaE

A threat for tornadoes would be somehow limited according to model simulations of low SR helicity values, but helicity will likely be enhanced along the boundaries e.g. outflow boundaries, dryline and their intersections. Locally enhanced effective SR helicity will support higher probability of isolated tornadoes as well.

SRH3

Areas more to the north over Wern KS will not be in such a favorable wind shear, but marginal shear and extreme instability can still be expected and isolated severe storms including weak supercells with threat for large hail, damaging winds and isolated tornado are expected along the dryline mixing Ewards during the mid to late afternoon.

So today a place to be would be a trinagle stretching between Amarillo-Childress-Lubbock, my virtual target is Floydada, TX slightly NE of Lubbock where dryline bulge will be located in the mid afternoon. This area can see a couple of tornadoes along with very large to giant hail according to favorable vertical wind shear and extreme instability.

Parts of W OK and S KS upgraded to HIGH risk!

Numerous severe storms already on going as Q-linear MCS is extending from extreme NEern TX Panh across NW OK into E-CNTRL KS with the main threat for very large hail and damaging winds in the most severe cells. Given the favorable environment with high directional shear and favored curved hodographs also strong tornadoes are possible, extensive rainfalls are also included as linear segments are visible within this linear MCS and storm maintain themselves over the same areas.

ICT VIS satellite

Here is the SPC mesoscale discussion No. 0579 which mentions an upgrade for parts of W OK and S KS to HIGH risk:

MCD 0579

MESOSCALE DISCUSSION 0579
NWS STORM PREDICTION CENTER NORMAN OK
1054 AM CDT SUN APR 26 2009

AREAS AFFECTED…KS…OK…NW TX

CONCERNING…OUTLOOK UPGRADE

VALID 261554Z – 261700Z

SEE GRAPHIC FOR MDT-HIGH RISK LINES EXPECTED FOR 1630Z SWODY1 UPDATE.

HIGH RISK UPGRADE WILL INCLUDE SCNTRL KS AND NWRN/NCNTRL-WCNTRL OK WITH A MDT RISK SPANNING FROM NWRN TX/ERN TX PNHDL ENEWD INTO CNTRL OK AND CNTRL/ERN KS.

IMPRESSIVE INGREDIENTS COMING TOGETHER LATER THIS AFTN FOR A HIGH POTENTIAL FOR WAVES OF LONG-LIVED SUPERCELLS WITH STRONG TORNADOES…DESTRUCTIVE HAIL AND DAMAGING WIND GUSTS.

BRIEF THOUGHTS /DETAILS FORTHCOMING IN SWODY1 UPDATE/…

MORNING ACTIVITY DEVELOPING OUT OF ACCAS FIELD WILL TRANSLATE NWD INTO CNTRL/ERN KS AND NWRN OK THROUGH EARLY AFTN. PORTIONS OF THIS ACTIVITY COULD BECOME SFC-BASED AS POCKETS OF HEATING LOCALLY ERODE CINH. ANOTHER WAVE OF STORMS IS LIKELY TO FORM UPSTREAM ALONG THE DRYLINE/FRONT IN ERN TX PNHDL/NWRN TX LATER THIS AFTN AND EVE…TRANSLATING ENE ACROSS THE MDT/HIGH RISK AREAS. PLENTIFUL BOUNDARY LAYER MOISTURE ALREADY IN PLACE BELOW VERY STEEP LAPSE RATES WILL CONTRIBUTE TO VERY UNSTABLE CONDITIONS…MLCAPES 3000-4000 J/KG. BULK SHEAR OF 45-50 KTS ORIENTED NORMAL TO THE INITIATING BOUNDARIES AND PERSISTENT RICH MOISTURE INFLUX WILL FAVOR LONG-LIVED DISCRETE SUPERCELLS. LOW-LEVEL FLOW WILL REMAIN BACKED IN A CORRIDOR FROM NWRN TX INTO SCNTRL KS…IMMEDIATELY DOWNSTREAM FROM AN INTENSIFYING SFC LOW OVER OK PNHDL/SWRN KS…BOOSTING SRH AND RESULTING IN POTENTIAL FOR STRONG TORNADOES.

..RACY/HART/HALES.. 04/26/2009

Storm Prediction Center has then issued an upgrade for parts of W OK and S KS due to very explosive environment that has formed in the last couple of hours, with MLCAPEs in excess of 3000 J/kg with favorable directional 0-6km wind shear above 50kts and enhanced 0-3km SR helicity in excess of 500 m^2/s^2. Clearing skies ahed of dryline in WSWern OK are well visible on latest satellite images. At low levels, backed LL jet continues to advect rich BL moisture northwards into the risk area. Severe storms are expected to continue developing along/ahead of the Eward mixing dryline from E TX Panh Swards into W TX during late afternoon and evening hours, so an outbreak of severe storms with embedded tornadic supercells capable of producing strong and violent tornadoes, giant hail and damaging winds is becoming increasingly likely across the risk area.

HISH risk

TORN probs

Here are several ‘rubust’ lines from their outlook text:

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1125 AM CDT SUN APR 26 2009

VALID 261630Z – 271200Z

…THERE IS A HIGH RISK OF SVR TSTMS OVER PORTIONS OF SOUTH CENTRAL KS AND WESTERN OK…

A COMPLICATED…YET SIGNIFICANT SEVERE WEATHER EVENT APPEARS TO BE DEVELOPING OVER PARTS OF THE CENTRAL AND SOUTHERN PLAINS TODAY. … THE GREATEST CONCENTRATION WILL LIKELY BE ALONG THE SURFACE DRYLINE/ COLD FRONT FROM CENTRAL KS…ACROSS WESTERN OK…INTO WESTERN NORTH TX. HOWEVER…POTENTIAL FOR SIGNIFICANT SEVERE WILL BE PRESENT OVER A MUCH LARGER AREA.

…SOUTH CENTRAL KS/WESTERN OK/EASTERN TX PANHANDLE…
A VOLATILE ENVIRONMENT IS EXPECTED TO DEVELOP BY MID AFTERNOON ACROSS THIS REGION WITH MLCAPE VALUES OF 2000+ J/KG AND DEWPOINTS IN THE MID 60S. MORNING RAOBS AND MODEL FORECAST SOUNDINGS SHOW A RATHER WEAK CAPPING INVERSION OVER THIS AREA…ALLOWING SHOWERS AND THUNDERSTORMS TO DEVELOP THIS MORNING. MORE INTENSE STORMS WILL OCCUR LATER THIS AFTERNOON ALONG THE DRYLINE OVER THE EASTERN TX PANHANDLE AS SHORTWAVE TROUGH OVER NM APPROACHES. GIVEN THE WEAK CAP… IT APPEARS THAT BROKEN CLOUD COVER AND SCATTERED SHOWERS WILL HELP TO LIMIT THE NUMBER OF STORMS AND REDUCE VERTICAL MIXING OF LOW LEVEL MOISTURE. THIS WILL LEAD TO A VERY FAVORABLE THERMODYNAMIC ENVIRONMENT FOR SUPERCELLS CAPABLE OF VERY LARGE HAIL AND TORNADOES. KINEMATIC FIELDS WILL ALSO BE VERY FAVORABLE WITH LARGE…CURVED HODOGRAPHS AND EFFECTIVE HELICITY VALUES OVER 500 M2/S2. MODEL GUIDANCE IS VERY INCONSISTENT IN TIMING AND EVOLUTION OF CONVECTIVE ACTIVITY. HOWEVER…THE COMBINATION OF A POTENT ENVIRONMENT… APPROACHING UPPER TROUGH… AMPLE MOISTURE IN PLACE…AND MULTIPLE OBJECTIVE GUIDANCE MEASURES INDICATING A WIDESPREAD RISK OF SIGNIFICANT TORNADOES… FEEL THERE IS SUFFICIENT CONFIDENCE TO UPGRADE TO HIGH RISK FROM SOUTH CENTRAL KS INTO WESTERN OK.

..HART/HURLBUT.. 04/26/2009