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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.

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.

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.

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.

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.

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%.

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.

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.

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.

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.

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.

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.

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

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.

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

Another potentially big tornado day seems possible today as west Conus upper-level trough continues its slowly movement eastwards and creates favorable environment for a tornado outbreak across Central Plains. During the next 24hrs a short-wave trough moves into southern high Plains and continues across central Plains overnight to Monday. At low levels, low surface pressure from NM slowly moves NEwards into OK Panhlandle in the afternoon, while a surface frontal boundary retreats NWwards as a warm front across NWern OK and central KS. A dryline is expected to mix Ewards during the mid afternoon hours across TX Panhandle extending SWward into W TX.

Rich low-level moisture continues to move northwards into central Plains as strong SSW LL flow is present in front of ejecting trough, surface dewpoints in mid 60s are spreaded as north as s-cntrl KS already and will be pushed NEwards as well as into eastern NE an IA along with NWward retreating warm frontal boundary. Strong mid/upper-level jet streak with effective 40-50kt bulk shear ahead of the trough will overlap with the unstable enviroment in boundary layer and create favorable conditions for significant severe weather in warm sector between frontal boundary and dryline.

Storm Prediction Center has issued a large SLGT risk area extending all the way from S WI SWwards across IA, NE, KS, OK into much of TX. Additionally, a MDT risk was issued for parts of eastern TX Panhandle and NW TX, W OK and S-CNTRL KS where conditions favour an outbreak of significant severe weather, including strong tornadoes and very large to giant hail. 15% of area in W OK into S KS has been hatched with the risk of significant tornadoes possible and 45% for significant hail.

Again as it happened a day before, models are not so confident with the position of frontal boundaries and rather big differences exist on the models, RUC and WRF having a triple point more south in OK Panh/SW KS while GFS has it slightly more northwards east of DDC. But given the latest surface observations, RUC/WRF predictions seems more reasonable. A triple point as WF/DL intersection is then expected in Eern OK Panhlandle with WF extending NEwards across central KS into SEern NE and CNTRL IA, while dryline extends SWwards across central TX Panh into west TX as it slowly mixes Ewards during the day, creating a dryline bulge over NEern TX Panh. This region certainly has a high potential for extremely severe weather today.

Storm initiation has some uncertainties due to possibility of early convection as CINH is rather weak and cap is likely not strong enough to surpress on going convection. Mostly elevated storms and showers are expected to develop from Eern TX Panh across OK, KS NEwards by late morning which will be in very favorable environment to maintain themselves for large hail and damaging winds, but also tornadoes remain possible. Given the fact that this morning activity will be mainly elevated, not much of effects on boundary layer mixing is expected, on the other hand these storms might be even helpul for moistening the boundary layer even more.

During the early to mid-afternoon when dryline starts to mix Ewards and strong surface heating will be in place, rapid evolution of severe storms should take place first near intersection of dryline and warm front in Eern OK Panh down along the dryline bulge into Eern TX Panh and more Swards along the dryline later on. Every storm that will form in such environment will very likely quickly become severe as explosive environment will be in place from Eern Tx Panh into W OK. Some uncertainties still exist due to current high cloud cover for strong diurnal heating, but likely this won’t have a big effects as its clearing out rapidly.

Due to high surface dewpoints and approaching steeper lapse rates aloft, high SBCAPE will be in place, RUC 12z has it around 4500-5000 J/kg, but usually RUC is having some problems with overestimating surface dewpoints for a couple of degrees, so likely around 3500 J/kg of SBCAPE can be expected ahead of dryline from west TX NEwards into Wern OK. MLCAPE predictions are as well pretty high, AOA 2000 J/kg which is indeed favorable for rapid storm development. Given the favorable low-level and bulk 0-6km shear and enhanced SR helicity, storms should create rotating updrafts pretty fast as they move ENEwards into WNWern OK.

Both high CAPE and strong wind shear (LLS 30-40kts and DLS AOA 50kts) along with enhanced SR helicity (0-1km near 300m^2/s^2 while 0-3km SRH could exceed 500m^2/s^2 locally) will become extremely favorable for strong long-lived tornadic supercells capable of producing large and violent tornadoes, along with very large to giant hail up to size of baseballs or larger. Major tornado outbreak remains possible through late afternoon and evening hours when LLJ southerly jet intensifies and kicks in and LLS/SRH values enhance across central Plains.

Some limiting factors for this upcoming severe weather event could be an early start of initiation and large coverage of storms due to weak CINH which would not give time o create a stronger cap for preventing storms initiation until mid-afternoon, while widespread elevated convection could also limit the diurnal surface heating on the other hand. If the coverage of severe storms later will become more widespread than its simulated attm, some parts of MDT risk area can be upgraded to HIGH risk. An update will be added in the next couple of hours if conditions improve significantly.

Stay tuned for future updates!

As mentioned earlier in the previous post, things are shaping up together for a severe weather outbreak across south-central Plains today with the main focus for extremely severe weather from extreme E TX Panh into western OK. Very large to giant hail, tornadoes and strong winds are expected, including possibility for a few isolated strong tornadoes after dark when LLJ strenghtens!

Classic upper level pattern with long-wave trough over western Conus favours healthy LL Gulf moisture advection onto central Plains with mid 60s SFC dewpoints as north as northern OK. At surface, strong cold front continues to race southwards across KS into NW OK and TX Panh during the day, becoming Q-stationary in the afternoon. A short-wave through shifting east from AZ towards NM/CO which will result in pressure falls over NM towards TX/OK Panh.

Additionally increasing and veering LLJ will take place ahead of this surface low, resulting in dryline mixing Ewards across TX Panh and stronger moisture advection into western OK. Given the increasing LLJ, surface front will tend to retreat slowly NWwards during th afternoon, while models simulate a dryline bulge forming over eastern TX Panhlandle.

SPC has issued a MDT risk from eastern TX Panh into WNW OK NEwards into extreme S KS with 15% area hatched for strong tornadoes and 45% area for very large to giant hail.

The right position of the front with triple point is somehow uncertain due to model differences as GFS was focusing the front more to the north than WRF, but according to latest runs it seems that frontal boundary will be pushed south as low as Amarillo on the west and extending NEwards over NW OK into south KS, before it starts to retreat backwards. Dryline should be located over central TX Panh and oriented SW-NEwads in the afternoon, having an intersection with the front just east of Amarillo, while dryline bulge should form as dryline mixes eastwards.

Diurnal heating and steep lapse rates aloft will yield high values of near or above 3000J/kg SBCAPE and weaken the cap, storm initiation should take place near tripple point and along/ahead of dryline by late afternoon, while earlier storm initiation is expected all along the cold front with isentrophic lifting at frontal boundary. SSE LL winds ahead of front/dryline will favour rotating updraft, especially for very large hail and few tornadoes.

Towards the evening and after dark, LLJ strenghtens due to approaching short-wave trough and enhance SR helicity values to above 500m^2/s^2 in 0-3km layer, which will rapidly increase tornado potential across western OK. Potential for strong tornado(-es) will take place as more than 50kt LL SSEerly jet kicks in.

Isolated cells that will be in place in this envoronment will have all potential to become cyclic tornadic supercells with quite high possibility for strong long-lived tornadoes before they merge into one or more large MCS clusters overnight. Giant hail will also be a big threat with such cells. Just for imagination how extreme tornadic potential is forecasted with some models, here is the forecasted sounding for Clinton, OK showing an insanely tornadic hodograph with extremely high values of indices; SBCAPE almost 2000J/kg, 3km EHI above 7, 0-1km SRH near 500 and 0-3km SRH at 700m^2/s^2, 40kt of 0-1km shear, etc. Certainly an incredible sounding and textbook tornadic hodograph!

According to models, it looks like the highest potential for strong tornadoes could be in area inside triagle Woodward-Altus-El Reno after dark.

Overnight clusters will definatelly have a role in the future evolution of severe storms tomorrow as there will definatelly be some sort of outflow boundaries that might intersect with the mixing dryline in the afternoon. As today, potential for giant hail and tornadoes will be in place across much od central Plains again. SPC has already issued MDT risk with 45% hatched area for significant weather possible across much of OK and KS.

Stay tuned for future updates!