MESOSCALE PRECIPITATION DISCUSSION 0021
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1038 PM EST TUE FEB 20 2018

AREAS AFFECTED...CENTRAL AND NORTHEAST TEXAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 210336Z - 210930Z

SUMMARY...FLASH FLOODING WILL BE POSSIBLE OVERNIGHT ACROSS
PORTIONS OF TEXAS. ANY TRAINING RAIN BANDS OR THUNDERSTORMS WOULD
HAVE THE POTENTIAL TO PRODUCE RAIN RATES ON THE ORDER OF 1-2
IN/HR. IF SUSTAINED FOR SEVERAL HOURS...FLASH FLOODING COULD OCCUR.

DISCUSSION...A SURFACE COLD FRONT EXTENDED IN AN ARC FROM THE
NORTHEAST CORNER OF TEXAS NEAR PARIS...TO THE SW AND THEN WSW TO
THE I-10 CORRIDOR IN TEXAS HILL COUNTRY. THE COLD FRONT WAS SLOWLY
ADVANCING...AND BY 03Z HAD GENERALLY UNDERCUT THE ONGOING
CONVECTION ACROSS NORTH TEXAS. ALTHOUGH THAT REGION (AROUND THE
DFW METRO AREA) HAS SEEN SURFACE TEMPERATURES FALL AND BOUNDARY
LAYER INSTABILITY BECOME MORE LIMITED...IT IS STILL WELL WITHIN
THE PLUME OF DEEP MOISTURE. ADDITIONALLY...OBJECTIVE RAP ANALYSIS
SHOWED 200-500 J/KG OF MUCAPE AVAILABLE AS THE COLD FRONT WAS
NEGATIVELY TILTED BACK INTO THE COLD AIRMASS...WITH THE 850MB
FRONT SITUATED WELL TO THE NORTHWEST. THEREFORE...CONVECTION WILL
REMAIN POSSIBLE FOR THE NEXT SEVERAL HOURS...ALTHOUGH RAIN RATES
SHOULD LIKELY CONTINUE TO DIMINISH BEHIND THE COLD FRONT AS
GOES-EAST IR SATELLITE SHOWS WARMING CLOUD TOPS...WITH LESS
LIGHTNING ACTIVITY IN THE PAST HOUR OR SO.

THE HIGHEST CHANCES FOR TRAINING CONVECTION...AND THUS FLASH
FLOODING...APPEAR TO BE ALONG AND JUST AHEAD OF THE COLD FRONT IN
NORTHEAST TEXAS FROM CORSICANA AND TYLER...TO NEAR TEXARKANA.
REGIONAL RADARS SHOWED CONVECTION REDEVELOPING IN THE WARM SECTOR
AHEAD OF THE COLD FRONT...AND GOES-EAST IR SATELLITE SHOWED A
CORRIDOR OF RENEWED COOLING CLOUD TOPS. THIS AREA WILL HAVE ACCESS
TO MODERATE INSTABILITY. RAP ANALYSIS HAS IT ON THE NORTHWEST
PERIPHERY OF 1000 J/KG OF MLCAPE WITH SOUTH-SOUTHEAST LOW-LEVEL
INFLOW. HI-RES MODELS ALSO GENERALLY HAVE THE STRONGEST QPF SIGNAL
BY 09-12Z OVER THIS AREA WITH SEVERAL INCHES OF RAIN LIKELY AND
LOCALIZED TOTALS POTENTIALLY UPWARDS OF 5 INCHES. THE 00Z
SHREVEPORT SOUNDING SET A FEBRUARY RECORD FOR PRECIPITABLE
WATER...AND CIRA LAYER PW PRODUCTS SHOW THAT THIS PROMINENT PLUME
OF MOISTURE EXTENDS INTO THE MID-UPPER LAYERS OF THE ATMOSPHERE.
THIS SHOULD AID IN EFFICIENT RAINFALL PRODUCTION AND HOURLY RAIN
RATES IN EXCESS OF 1 IN/HR WILL BE POSSIBLE.

A MORE CONDITIONAL THREAT FOR FLASH FLOODING WILL EXIST FURTHER TO
THE SOUTHWEST IN TEXAS HILL COUNTRY. HI-RES MODELS HAVE AN
INCREASING SIGNAL FOR CONVECTIVE INITIATION ON A MORE WIDESPREAD
SCALE IN THE 05-07Z TIMEFRAME AS THE COLD FRONT CONTINUES TO MAKE
SLOW PROGRESS TO THE SOUTHEAST INTO AN UNSTABLE ATMOSPHERE. RECENT
RUNS OF THE HRRR AND NAM NEST SHOW SOME OF THE HIGHEST RAIN RATES
IN THE ENTIRE REGION IN A SMALL AREA OF TEXAS HILL
COUNTRY...POTENTIALLY AROUND 2 IN/HR. HOWEVER...THIS WOULD BE
CONTINGENT UPON FAVORABLE ORIENTATION OF CONVECTIVE BANDS AND THUS
TRAINING FOR A MORE SUBSTANTIAL PERIOD OF TIME. 1-HR FLASH FLOOD
GUIDANCE IS GENERALLY HIGHER THAN 2 INCHES...SO IT WOULD LIKELY
TAKE SEVERAL HOURS OF HEAVY RAINFALL TO PRODUCE FLASH FLOODING.
THIS IS WHY THE THREAT IS MORE CONDITIONAL IN THIS REGION...AS
HI-RES MODELS ALSO INDICATE THAT THE CONVECTION SHOULD MOVE OFF TO
THE NORTHEAST WITHIN A FEW HOURS OF INITIATION.

LAMERS

ATTN...WFO...EWX...FWD...HGX...SHV...SJT...

ATTN...RFC...ABRFC...LMRFC...WGRFC...

LAT...LON   33979461 33769353 33059327 31989417 30549603
            29709737 29219869 29459974 30250017 31229940
            32099916 32759867 33129739 33529618 33769551