Severe Weather Thread (December 19)

[David Reimer]

Having chased a lot of cold season events last winter out east, I've come to recognize that 3 KM CAPE is pretty important when determining the tornado potential in cool season events. We don't need a lot of instability to get mini-supercells going with low level circulations. This is a pretty concerning graphic and actually represents a 3 KM CAPE value I would expect to see in spring events.

As is normally the case with winter events, the incoming system is quite amplified and certainly will have enough wind shear to support organized convection.

I am seeing that the 12Z NAM does have the LLJ a bit displaced to the east of the primary instability axis, but I'm thinking that may have something to do with convective feedback as the NAM has a line of convection along/west of the LLJ. I'm not expecting a major event, unless we get more instability then forecasted, but I wouldn't be surprised to see a few mini supercells take off. I'm carefully watching this and am planning to chase on Monday. Here's a great discussion from Fort Worth written earlier this morning.

LARGE UPPER LEVEL LOW NEAR SAN DIEGO WILL BEGIN TO HEAD EAST

DURING THE DAY AND WILL TRACK ACROSS THE TEXAS PANHANDLE MONDAY

EVENING BEFORE CURLING NORTH INTO THE CENTRAL PLAINS ON TUESDAY.

THIS WILL BE A POWERFUL STORM SYSTEM...BRINGING HEAVY SNOW FROM

THE PANHANDLE INTO KANSAS...AND A ROUND OF POSSIBLY SEVERE

THUNDERSTORMS WITH THE POTENTIAL FOR TORNADOES IN OUR AREA.

FOR TODAY...MID LEVEL CLOUDS WILL BE INCREASING ACROSS THE REGION

FROM THE SOUTHWEST AS ISENTROPIC LIFT BEGINS TO ORGANIZE. HIGHS

WILL BE NEAR 60 DESPITE THE LACK OF SUNSHINE AS SOUTHERLY WINDS

BRING WARMER AIR NORTHWARD. SATURATION OF THE LOW LEVELS OCCURS

TONIGHT...WITH SOME WEAK SHOWERS BY DAYBREAK MAINLY WEST OF I-35.

WIND SPEEDS THROUGH THE ENTIRE COLUMN INCREASE DRAMATICALLY ON

MONDAY AS THE UPPER LOW EDGES CLOSER. VERY HIGH VALUES OF SHEAR

ARE EXPECTED WITH 0-6KM SHEAR OF 55KT...0-1KM SHEAR OF 35KT AND

0-3KM HELICITY EXCEEDING 500M2/S2. 850MB WINDS ARE ALSO DUE

SOUTHERLY...WHICH IS MORE BACKED THAN NORMAL. THIS SHOULD ALLOW

FOR EXCELLENT TRANSPORT OF MOISTURE/WARM AIR NORTHWARD AND A RAPID

MODIFICATION OF OUR AIR MASS DURING THE DAY. THE NAM/HI-RES WRF

ARE FAVORED FOR INSTABILITY FORECASTS AS THEIR SURFACE TEMPS

APPEAR MOST REASONABLE. ALTHOUGH CAPE LIKELY WILL NOT EXCEED 1000

J/KG...IT SHOULD BE NOTED THAT 100-300J/KG OF THE TOTAL CAPE IS

LOCATED BELOW 3KM DUE TO LFC OCCURRING WITHIN 1KM OF THE GROUND.

THIS SYSTEM HAS SOME SIMILARITIES TO THE DECEMBER 29 2006 SYSTEM

THAT PRODUCED 2 DOZEN TORNADOES OVER OUR AREA. THE 2006 SYSTEM WAS

A LITTLE BIT STRONGER AT 500 MB...TRACKED FARTHER SOUTH...AND THE

SURFACE LOW AND WARM FRONT WERE ALSO FARTHER SOUTH. I DONT EXPECT

A SEVERE WEATHER EVENT OF THAT CALIBER THIS TIME AROUND...BUT

THERE ARE ENOUGH SIMILARITIES THAT IT RAISES THE CONCERN FOR

TORNADOES DESPITE MANY MODEL FORECASTS SAYING THE INSTABILITY WILL

BE LIMITED. THE MODELS IN 2006 ALSO UNDERPLAYED THE INSTABILITY

AND THIS TIME WE WILL BE A LITTLE MORE WARY. SINCE IT IS DECEMBER

AND SEVERE WEATHER IS NOT USUALLY ON PEOPLES MIND...WE ARE GOING

TO ADVERTISE THE THREAT NOW DESPITE LOWER THAN NORMAL CONFIDENCE.

THE 2006 CASE HAD NUMEROUS DISCRETE SUPERCELLS AHEAD OF THE COLD

FRONT...AND I DONT EXPECT THAT THIS TIME AROUND. EXPECT MOST OF

THE CONVECTION TO DEVELOP ALONG A NORTH-SOUTH ORIENTED COLD FRONT

WHICH SHOULD MOVE INTO THE WESTERN CWA BY 3 PM. A SOLID SQUALL

LINE SHOULD DEVELOP ALONG THE FRONT...AND REACH THE I-35 CORRIDOR

AROUND 6PM MONDAY. STORM MOTION AT 45KT MEANS ANY STORM WILL HAVE

THE POTENTIAL FOR WIND DAMAGE DUE TO DOWNWARD MOMENTUM TRANSPORT

OF 55KT 850MB WINDS. TORNADO THREAT WILL BE LIMITED BY THE STORM

MODE BEING MAINLY LINEAR...BUT WILL BE POSSIBLE DUE TO THE HIGH

LOW LEVEL SHEAR AND CAPE. TORNADO THREAT IS HIGHEST SOUTH OF A

GOLDTHWAITE...HILLSBORO...ATHENS LINE WHERE INSTABILITY PARAMETERS

ARE GREATEST. SQUALL LINE SHOULD CLEAR THE CWA BY 2 AM TUESDAY.

[andyhb]

NAM has increased the SBCAPE over the past few runs, which has been a common trend with models for these events lately.

[David Reimer]

Afternoon discussion from FWD

.DISCUSSION...

12Z UPPER AIR ANALYSIS INDICATED THAT A CUT OFF LOW REMAINED IN

PLACE OVER FAR SOUTHERN CA WITH RIDGING OVER THE GULF OF MEXICO.

THIS LEFT SOUTHWEST FLOW ALOFT OVER THE SOUTHERN PLAINS OUT AHEAD

OF THE UPPER LOW. SOUTHWEST FLOW ALOFT HAD LED TO THE ADVECTION OF

STEEP LAPSE RATES INTO WEST TX THIS MORNING WITH A TEMPERATURE

DIFFERENCE IN THE H700 TO H500 LAYER OF 22 DEG C. A POOL OF 10 DEG

C DEW POINTS WAS OBSERVED AT THE H850 LEVEL OVER FAR SOUTH TX AND

THE EFFECTS OF LEE SIDE CYCLOGENESIS WERE BECOMING EVIDENT WITH

THE DEVELOPMENT OF A CYCLONIC CIRCULATION OVER FAR WEST TX. 21Z

SFC ANALYSIS INDICATED THAT A MOISTURE WAS RAPIDLY RETURNING TO

WESTERN AND CENTRAL TX WITH 45 DEG F DEW POINTS OBSERVED AS VAR

NORTH AS CHILDRESS.

TONIGHT THROUGH TUESDAY...AFTERNOON WATER VAPOR IMAGERY INDICATED

THAT THE CUT OFF LOW AS ALREADY BEGINNING TO MOVE EAST AND THE

CENTER OF CIRCULATION HAD JUST CROSSED OVER THE SOUTHERN AZ BORDER

AS OF 21Z. A PLUME OF PACIFIC MOISTURE HAS ALREADY BEEN ADVECTED

OVER MUCH OF THE SOUTHERN PLAINS...AND SOUTHWEST FLOW ALOFT

CONTINUES THIS TRANSPORT OF MID TO UPPER TROPOSPHERIC MOISTURE

INTO THE REGION. THE PROGRESSION OF SURFACE DEW POINTS WAS THE

MOST INTERESTING PIECE OF INFORMATION THAT COULD BE OBSERVED ABOUT

THE POTENTIAL THERMODYNAMIC ENVIRONMENT THIS AFTERNOON AHEAD OF

THIS UPPER LOW. BASED ON 12Z GUIDANCE...THE NAM WAS THE ONLY MODEL

TO SUFFICIENTLY HANDLE THE RAPID INFLUX OF LOW LEVEL MOISTURE INTO

TX. THE NAM HAD ROUGHLY AN ACCURATE REPRESENTATION OF 21Z DEW

POINTS ACROSS TX WITH THE GFS LAGGING BEHIND BY 6 HRS AND THE

ECMWF COMING IN A FEW DEG F TOO LOW FOR ITS 00Z FORECAST THIS

EVENING. EVEN THE 18Z INITIALIZATION OF THE RUC IS SLOW ON THIS

MOISTURE TRANSPORT WHICH POINTS TO A POTENTIALLY SIGNIFICANT

SOURCE OF ERROR WITH REGARDS TO HANDLING THE THERMODYNAMIC

ENVIRONMENT THAT WILL SET UP OVER NORTH TX TOMORROW.

DID NOT CHANGE MUCH WITH REGARDS TO THE TIMING OF EXPECTED

PRECIPITATION ACROSS NORTH TX WITH THIS FORECAST PACKAGE. THE

UPPER LEVEL SYSTEM AND ASSOCIATED FRONTAL BOUNDARIES ARE EXPECTED

TO MOVE THRU NORTH TX AT ROUGHLY THE SAME TIME AS THE PREVIOUS

FORECAST INDICATED. THE UPPER LOW IS EXPECTED TO MOVE INTO THE

SOUTHERN PLAINS TOMORROW EVENING AND THEN BECOME NEGATIVELY TILTED

AS IT MOVES NORTHEAST OVER NORTH CENTRAL TX AND OVER OK MONDAY

NIGHT. STRONG LARGE SCALE FORCING FOR ASCENT WILL SPREAD OVER

NORTH TX AHEAD OF THIS UPPER LOW...WITH MAXIMUM ASCENT SPREADING

OVER THE FORECAST AREA FROM 21Z MONDAY TO 06Z TUESDAY. THE

FORECAST STRENGTH OF THE UPPER LOW AS IT MOVES OVER THE SOUTHERN

PLAINS IS HIGH WITH H500 HEIGHTS REGISTERING AS A -2.5 STANDARD

DEVIATION ANOMALY PER HPCS ANALYSIS OF THE 12Z GFS. PRECIPITABLE

WATER VALUES CONTINUE TO BE FORECAST TO HOLD IN THE 99TH

PERCENTILE OVER NORTH TX FOR DECEMBER. THE 0 TO 6 KM BULK SHEAR

MAGNITUDE IS EXPECTED TO BE IN EXCESS OF 55 KTS DURING THE PERIOD

OF MAXIMUM FORCING FOR ASCENT WHICH SPEAKS TO THE STRENGTH OF THE

WIND FIELDS OUT AHEAD OF THIS SYSTEM. INSTABILITY CONTINUES TO BE

THE MOST DIFFICULT PARAMETER TO FORECAST...AND THE DEGREE OF

INSTABILITY WILL LARGELY DETERMINE OUR SEVERE WEATHER CHANCES AND

COVERAGE TOMORROW AFTERNOON.

BASED ON 21Z SURFACE OBS...THE NAM APPEARS TO HAVE THE BEST HANDLE

ON LOW LEVEL MOISTURE MOISTURE RETURN OUT AHEAD OF THIS SYSTEM.

MOISTURE TRANSPORT IS EXPECTED TO INCREASE IN INTENSITY TONIGHT

AND DURING THE DAY MONDAY AS THE UPPER LEVEL LOW MOVES OVER THE

SOUTHERN PLAINS. THE LOW LEVEL JET IS FORECAST TO INCREASE TO AT

LEAST 50 KTS OVER NORTH TX TOMORROW...WITH OUR LOCAL 4 KM WRF

INDICATING A POTENTIAL JET CORE OF 60 KTS AFTER 21Z. STRATUS IS

STILL EXPECTED TO BE IN PLACE OVER MOST OF THE OPEN WARM SECTOR

TOMORROW WHICH WILL BE A LIMITING FACTOR FOR LOW LEVEL

DESTABILIZATION. HOWEVER WITH MOISTURE TRANSPORT LIKELY

UNDERESTIMATED IN MOST OPERATIONAL GUIDANCE...THINK THAT SURFACE

BASED INSTABILITY IS LIKEWISE UNDERESTIMATED. 12Z MOS GUIDANCE IS

NOW CONSISTENT WITH MID 60S HIGHS FOR DFW TOMORROW. MOS GUIDANCE

ALSO INDICATES HIGHS IN THE UPPER 60S TO LOWER 70S FOR WACO

TOMORROW. THIS IS A FEW DEGREES WARMER THAN HIGHS WERE EXPECTED TO

CLIMB FROM YESTERDAYS FORECAST.

CONFIDENCE IN THESE HIGHER TEMPERATURES IS INCREASING DUE TO

SIGNIFICANT BREAKS IN CLOUD COVER OVER NORTH TX AND THE TX GULF

COAST. WARMER AFTERNOON TEMPERATURES TODAY WILL THEORETICALLY LEAD

TO WARMER OVERNIGHT LOWS ONCE CLOUD COVER BUILDS BACK IN TONIGHT.

THIS WOULD MEAN THAT LESS HEATING IS NECESSARY TO REACH HIGHER

TEMPERATURES TOMORROW AFTERNOON. THE 12 AND 18Z NAM RUNS BOTH

CONTINUE TO AGGRESSIVELY ADVERTISE FREE CONVECTION POTENTIAL

TOMORROW AFTERNOON AND EVENING. WITH THE NAM HAVING THE BEST

HANDLE ON LOW LEVEL MOISTURE AND HIGH TEMPERATURES LIKELY CLIMBING

A FEW DEGREES HIGHER TOMORROW THAN PREVIOUS EXPECTED...CONFIDENCE

IN THE NAMS THERMODYNAMIC SOLUTION HAS INCREASED SUBSTANTIALLY.

THE NAM ADVERTISES A CORRIDOR OF SURFACE BASED CAPE OF 1000 J/KG

EXTENDING FROM THE METROPLEX SOUTH...NEAR THE INTERSTATE 35

CORRIDOR BY 00Z TUE OR TOMORROW AT 6 PM.

AS PREVIOUS FORECAST MENTIONED...LOW LCL HEIGHTS INDICATE THAT A

SIGNIFICANT PORTION OF THE FREE CONVECTION POTENTIAL ENERGY /CAPE/

WILL LIE IN THE LOWEST 3 KMS. 0 TO 3 KM CAPE VALUES ARE FORECAST

TO BE IN EXCESS OF 200 J/KG WITHIN THIS SAME GENERAL CORRIDOR.

THESE VALUES ARE QUITE HIGH EVEN FOR A SPRING SEVERE WEATHER

EPISODE...AND HIGH LOW LEVEL CAPE VALUES SPEAK TO THE POTENTIAL

FOR LOW LEVEL STRETCHING AND TILTING OF HORIZONTAL VORTICITY INTO

THE VERTICAL. LOW LEVEL WIND PROFILES CONTINUE TO VEER WITH HEIGHT

AND CONTAIN SIGNIFICANT SPEED SHEAR DUE TO THE PRESENCE OF THE

STRONG LOW LEVEL JET. 0-1 KM HELICITY VALUES IN EXCESS OF 200

M2/S2 ARE PRESENT WITHIN THE CORRIDOR OF HIGH LOW LEVEL CAPE. THIS

COMBINATION OF PARAMETERS INDICATES THAT ANY UPDRAFT WILL HAVE A

STRONG LIKELIHOOD TO INGEST HORIZONTAL VORTICITY WHICH GREATLY

INCREASES THE SEVERE WEATHER THREAT TOMORROW AFTERNOON. THE

GENERAL STORM MODE IS STILL EXPECTED TO BE LINEAR WITH SIGNIFICANT

FORCING FOR ASCENT EXPECTED ALONG A PACIFIC TYPE COLD FRONT THAT

WILL MOVE TROUGH NORTH TX JUST AHEAD OF THE CORE OF THE UPPER LOW.

THIS NORTH/SOUTH ORIENTED FRONT IS ORIENTED NEARLY PARALLEL TO THE

0-6 KM BULK SHEAR VECTORS WHICH GENERALLY FAVORS SHARED COLD POOLS

LEADING TO A LINEAR STORM MODE.

EVEN WITH A LINEAR STORM MODE STRETCHING AND TILTING OF HORIZONTAL

VORTICITY INTO THE VERTICAL CAN LEAD TO THE DEVELOPMENT OF

MESOVORTICIES CAPABLE OF PRODUCING STRONG LOCALIZED WIND

DAMAGE...AND TO THE DEVELOPMENT OF BRIEF TORNADOES. ANY DISCRETE

STORMS OUT AHEAD OF LINE SEGMENT CONVECTION WILL LIKELY BE IN THE

FORM OF LOW-TOPPED AND VERY FAST MOVING SUPERCELLS. ANY LOW-TOPPED

SUPERCELL AWAY FROM THE MAIN LINE OF CONVECTION WILL BE CAPABLE OF

PRODUCING A TORNADO...ESPECIALLY SOUTH OF THE METROPLEX TOMORROW

AFTERNOON. THE MOST WIDESPREAD SEVERE WEATHER HAZARD IS THOUGHT

TO BE IN THE FORM OF STRAIGHT LINE WIND DAMAGE...AND THIS IS

MOSTLY DUE TO THE POTENTIAL FOR STRONG ENVIRONMENTAL WINDS TO BE

DRIVEN DOWN TO THE SURFACE. DEEP MOISTURE MEANS THAT THERE IS

LITTLE THETA-E DIFFERENCE WITH HEIGHT...AND THEREFORE LITTLE

POTENTIAL FOR ENHANCED DOWNDRAFT SPEEDS OR MICROBURSTS.

OVERALL CONFIDENCE IN AN ORGANIZED STORM MODE IS HIGH NOW FOR

TOMORROW AFTERNOON. DAMAGING STRAIGHT LINE WINDS WITH A HIGH

POTENTIAL FOR LOCALLY ENHANCED DAMAGE IS THE MOST PREVALENT SEVERE

WEATHER HAZARD. THERE IS AN ATTENDANT TORNADO THREAT TOMORROW

AFTERNOON...ESPECIALLY IF ANY DISCRETE STORMS ARE ABLE TO DEVELOP

OUT AHEAD OF THE SQUALL LINE/PACIFIC FRONT. 99TH PERCENTILE PWATS

ALWAYS RAISE THE POTENTIAL FOR LOCALIZED FLOODING ISSUES...HOWEVER

THE VERY FAST STORM MOTION IS EXPECTED TO MITIGATE SIGNIFICANT

FLOODING CONCERNS.

THE PACIFIC TYPE COLD FRONT IS EXPECTED TO MOVE EAST THRU MOST OF

NORTH TX BY MIDNIGHT /06Z TUE/ MONDAY NIGHT. THE ATTENDANT SQUALL

LINE WILL LIKELY BE JUST EXITING THE FORECAST AREA AT THIS TIME

SO HAVE LINGERED HIGH POPS OVER THE FAR EASTERN CWA AFTER

MIDNIGHT. BY TUESDAY MORNING THERE IS SOME LOW END POTENTIAL FOR

LIGHT RAIN ASSOCIATED WITH MOISTURE WRAPPED AROUND THE UPPER

LOW...ESPECIALLY AS SOME RESIDUAL FRONTOGENETIC FORCING BUILDS

INTO THE NORTHERN CWA AS THE CANADIAN AIRMASS RESPONSIBLE FOR THE

BLIZZARD PORTION OF THIS SYSTEM /OVER THE SOUTHERN HIGH PLAINS/

MOVES SOUTH INTO NORTH TX. NO WINTRY PRECIPITATION IS EXPECTED AT

THIS TIME AS THERE WILL BE VERY LITTLE MOISTURE LEFT TO WORK WITH

AND TEMPERATURES WILL GENERALLY REMAIN ABOVE FREEZING EARLY

TUESDAY MORNING BEHIND THE FRONT.

EXTENDED...NO SIGNIFICANT CHANGES MADE TO THE EXTENDED AT THIS

TIME. KEPT THE LOW END RAIN CHANCES OVER THE SOUTHEASTERN PORTION

OF THE CWA ON THURSDAY AS ANOTHER SHORTWAVE TROUGH MOVES OVER

NORTH TX. THE 12Z ECMWF WAS THE MOST AGGRESSIVE WITH PRECIPITATION

BECAUSE IT BRINGS THE WAVE OVER NORTH TX THE FURTHEST NORTH AND

STRONGER THAN THE GFS ADVERTISES. EITHER WAY LIMITED MOISTURE WILL

PREVENT SIGNIFICANT COVERAGE OF RAIN OVER NORTH TX ON THURSDAY.

MAINTAINED A DRY FORECAST FOR THE HOLIDAY WEEKEND AS RUN TO RUN

AND MODEL TO MODEL CONSISTENCY EVEN ON LARGE SCALE FLOW REMAINS

POOR AT THIS TIME. GENERAL CONSENSUS IS THAT TEMPERATURES WILL BE

BELOW NORMAL...SO FOR NOW A COOL AND DRY HOLIDAY WEEKEND IS

EXPECTED.

[andyhb]

Very in-depth discussion.

Judging by the wind fields described in this and in the models, it won't take much of an underestimate in instability to create a problem...

[Ed Lizard]

Any storm at all in Dallas would look to come close to severe wind gusts base don forecast 0Z sounding. Houston before the storms arrive, unstable, and shear, but NAM forecasts less deep moisture. Dallas area looks like mainly speed shear where actual instability exists, as a guess, the low level directional shear/helicity is 'wasted', as it were, and it is more a wind/hail than tornado threat.

[Ed Lizard]

KIAH sounding, I'm not editing that yet because I'm going to link to that post on the forum tech forum...

[Ed Lizard]

NAM for Dallas looks similar to 0Z, very good low level shear may go wasted by weak near surface instability, but decent (TT ~50) mid level instability, near 30 knot winds almost to the deck, 50 knot winds just above, and 30 knots speed shear where there is instability, it has potential.

[tornadotony]

Don't look to the RUC for any help forecasting today. It decided to take a dump in the Gulf by painting <40°F dew points down there and completely screwing up its initialization the past few runs.

[Ed Lizard]

Don't look to the RUC for any help forecasting today. It decided to take a dump in the Gulf by painting <40°F dew points down there and completely screwing up its initialization the past few runs.

The HRRR hasn't replaced the RUC yet, right? I haven't actually started looking at it yet, but would it have the same errors?

Edit- ok, looked...

[David Reimer]

The HRRR doesn't look like it's affected by the RUC. The Rapid Refresh is scheduled to replace the RUC in January. I've been using that for the past couple of weeks after it handled one of our events pretty well.

[Srain]

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1025 AM CST MON DEC 19 2011

VALID 191630Z - 201200Z

...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT

ACROSS PORTIONS OF NRN/CNTRL/ERN TX...

...SYNOPSIS...

EWD-MOVING UPPER LOW CENTERED NEAR EPZ WILL TURN MORE NEWD THIS

PERIOD IN RESPONSE TO THE PROGRESSION OF 80-90 KT MIDLEVEL JET

STREAK FROM NRN MEXICO INTO NRN TX/ERN OK. THIS EVOLUTION WILL

RESULT IN A CORRIDOR OF 12-HR HEIGHT FALLS /500 MB/ AOA 100 M

SPREADING FROM ERN NM/WRN TX INTO KS/OK. IN THE LOW LEVELS...

SURFACE LOW OVER ERN NM WILL DEVELOP ACROSS THE SRN TX PNHDL THIS

AFTERNOON INTO CNTRL OK BY 20/12Z. THIS LOW PRESSURE WILL BE

ACCOMPANIED BY A PACIFIC FRONT THAT WILL ADVANCE EWD ACROSS

WRN/CNTRL TX AND SWRN/S-CNTRL OK.

...SRN PLAINS...

SCATTERED ELEVATED TSTMS OVER PARTS OF W-CNTRL/NWRN TX INTO

WRN/CNTRL OK WILL MOVE/DEVELOP EWD/NEWD TODAY IN CONJUNCTION WITH A

LEAD VORTICITY MAXIMUM EJECTING NEWD IN ADVANCE OF PRIMARY

UPPER-LEVEL SYSTEM. LIMITED ELEVATED INSTABILITY SHOULD PRECLUDE A

SEVERE HAIL THREAT WITH THIS ACTIVITY.

BY MID/LATE AFTERNOON...LATEST CONVECTION-ALLOWING AND MESOSCALE

MODEL GUIDANCE INDICATES AN INCREASED LIKELIHOOD FOR SURFACE-BASED

TSTM DEVELOPMENT ALONG PACIFIC FRONT INVOF OF THE I-35 CORRIDOR OF

NRN/CNTRL TX. HERE...INCREASING BOUNDARY LAYER MOISTURE CONTENT

WILL BE OFFSET TO A DEGREE BY WIDESPREAD CLOUDS WITH MLCAPE VALUES

RISING TO 500-1000 J/KG. BUT...GIVEN THE STRONG FORCING FOR ASCENT

AND STRENGTHENING LOW- AND DEEP-LAYER SHEAR /I.E. ESRH OF 200-300 M2

PER S2 AND EBS OF 45-55 KT/...THE SETUP WILL FAVOR THE DEVELOPMENT

OF ORGANIZED STORM MODES INCLUDING SUPERCELLS WITH A RISK FOR A FEW

TORNADOES...DAMAGING WINDS AND HAIL.

ELSEWHERE...ISOLATED...LOW-TOPPED TSTMS WILL BE POSSIBLE THIS

AFTERNOON OVER THE SRN TX PNHDL INTO FAR SWRN OK BENEATH MIDLEVEL

COLD CORE AND INVOF OCCLUDING SURFACE LOW. HERE...STEEPENING LAPSE

RATES WILL YIELD WEAK CONVECTIVE INSTABILITY WITH THE STRONGEST

STORMS CAPABLE OF SOME HAIL...POSSIBLY APPROACHING SEVERE LEVELS.

[David Reimer]

AREA FORECAST DISCUSSION...UPDATED

NATIONAL WEATHER SERVICE FORT WORTH TX

1039 AM CST MON DEC 19 2011

.UPDATE...

LATEST WATER VAPOR IMAGERY SHOWS THE LARGE UPPER LOW CONTINUING TO

SLOWLY MOVE EAST THIS MORNING AND MAY NOW BE STARTING TO SHIFT TO

THE NORTH. THIS CONTINUED EASTWARD MOVEMENT OVERNIGHT WASNT

FORECASTED BY THE MODELS AND THE OVERALL TRACK OF THE UPPER LOW

AND SUBSEQUENT SURFACE LOW WILL BE A BIT FURTHER SOUTH THAN

ANTICIPATED. A LEAD VORTICITY LOBE HAS DETACHED FROM THIS TROUGH

AND A BAND OF THUNDERSTORMS HAVE DEVELOPED WITHIN THIS SMALL AREA

OF ENHANCED LIFT THIS MORNING JUST WEST OF THE FORECAST AREA.

EXPECT THIS AREA OF SHOWERS AND THUNDERSTORMS TO CONTINUE TO MOVE

TO THE NE AND IT COULD AFFECT OUR NORTHWESTERN ZONES LATER THIS

MORNING.

16Z SURFACE ANALYSIS CONTINUES TO SHOW MOISTURE STREAMING

NORTHWARD. 60 DEWPOINTS ARE NOW JUST SOUTHWEST OF THE CWA AND

SHOULD SEE THESE VALUES IN OUR CWA SHORTLY. A WARM FRONT IS

ANALYZED ACROSS THE SOUTHWESTERN ZONES AND THIS BOUNDARY IS

EXPECTED TO MOVE NORTHEAST WITH THE SURFACE LOW.

THE SET UP FOR SEVERE WEATHER THIS AFTERNOON AND EVENING CONTINUES

TO COME TOGETHER...BUT THERE ARE STILL SEVERAL FACTORS THAT HAVE

TO PLAY OUT IN THE NEXT 4-6 HOURS TO DETERMINE THE OVERALL CHANCES

FOR SEVERE THUNDERSTORMS. THE LARGE AREA OF PRECIP TO OUR WEST

CONTINUES TO SLOWLY MOVE TO THE EAST AND WOULDNT BE SURPRISED TO

SEE SCATTERED SHOWERS AND THUNDERSTORMS WEST OF INTERSTATE 35 IN

THE LATE MORNING AND EARLY AFTERNOON HOURS AS THE LEAD IMPULSE

MOVES NEAR THE AREA. EXPECT THAT WE WOULD HAVE SOME TIME BETWEEN

THIS LEAD PRECIP AND WHEN THE LINE OF THUNDERSTORMS MOVES THROUGH

THE AREA IN THE LATE AFTERNOON AND EVENING HOURS. WITH A 50-60

KNOT LOW LEVEL JET IN PLACE...THE AIRMASS SHOULD BE ABLE TO

QUICKLY RECOVER FROM ANY LINGERING EFFECTS OF PRECIPITATION.

THE SECOND FACTOR WILL BE THE AVAILABLE INSTABILITY IN PLACE THIS

AFTERNOON. THE 12Z NAM SEEMS TO HAVE THE BEST HANDLE REGARDING

SURFACE T/TD AND INSTABILITY VALUES. EXPECT THE 60 DEWPOINT

ISODROSOTHERM TO BE AS FAR NORTH AS HILL COUNTY BY 21Z AND INTO

THE METROPLEX BY 00Z THIS EVENING. WE SHOULD SEE SOME SURFACE CAPE

WITH THE 60 DEWPOINTS...BUT HIGHER VALUES NEAR 1000 J/KG WILL BE

CONFINED TO AREAS WITH 63+ DEWPOINTS...LOCATED TO THE SOUTH OF

WACO. THESE LOCATIONS WILL SEE THE BEST CHANCES OF SEVERE

WEATHER.

THE THIRD FACTOR WILL BE THE STORM MODE. LATEST HIGH RESOLUTION

MODEL OUTPUT FORMS A SOLID LINE OF CONVECTION IN OUR WESTERN ROW OF

COUNTIES NEAR 19Z OR 1 PM AS STRONG FRONTOGENETICAL FORCING

DEVELOPS ALONG THE EASTWARD MOVING FRONTAL BOUNDARY. THERE ARE

SOME HINTS IN THE MODEL OUTPUT THAT AS THE LINE MOVES EAST THERE

MAY BE SOME BREAKS IN THE LINE AND SEPARATE STORM SEGMENTS WOULD

BE THE DOMINANT MODE. IF THIS WERE TO OCCUR...THE POTENTIAL

FOR TORNADOES WOULD INCREASE AS THESE SEGMENTS WOULD LIKELY BE

SUPERCELLS GIVEN THE STRONG SHEAR IN PLACE. LOCATIONS SOUTH AND

EAST OF A KILLEEN TO DFW TO PALESTINE LINE HAVE THE BEST CHANCES

FOR A TORNADO TODAY PER LATEST SPC DAY 1 OUTLOOK. THESE LOCATIONS

WILL HAVE THE BEST LOW-LEVEL INSTABILITY THIS AFTERNOON...BUT

AGAIN...THE FURTHER SOUTH YOU ARE...THE BETTER CHANCE FOR A

TORNADO. TEMPERATURE/DEWPOINT FIELDS WILL NEED TO BE WATCHED

CLOSELY TODAY AS A FEW DEGREES OFF OF FORECASTED VALUES EITHER WAY

COULD HAVE A BIG IMPACT ON THE SEVERE/TORNADO POTENTIAL.

85/NH

[Jim Martin]

Looking like broken lines with possible supercells later today.

[Ed Lizard]

[Ed Lizard]

Fort Worth- nice lapse rates above 850 mb.

Per the SPC discussion, a little farther South and a little warmer...

[SouthernNJ]

Between 5:20 and 5:30 CST, the radar looked interesting northeast of Waco around the Hubbard and Coolidge areas.

The reflectivity had some inflow into the line as well as a circulation in the vicintity. Not sure if anything happened, but

a feature to make note of moving forward.

[Ed Lizard]

've only seen 1 severe warning myself in the FWD CWA. Might have missed one, but its not a big outbreak, at least not yet and HGX is pulling down severe headlines.

[David Reimer]

Well, this was a total waste of time. NEXT!