MESOSCALE PRECIPITATION DISCUSSION 0591
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
556 PM EDT TUE OCT 20 2015
 
AREAS AFFECTED...SOUTHEAST NM & NORTHWEST TX 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 202155Z - 210355Z
 
SUMMARY...THUNDERSTORMS ARE EXPECTED TO INCREASE IN COVERAGE NEAR
A THERMAL/OUTFLOW BOUNDARY ACROSS THE AREA.  HOURLY RAIN RATES UP
TO 1.5" COULD LEAD TO FLASH FLOODING.

DISCUSSION...WIDELY SCATTERED THUNDERSTORMS HAVE CAUSED A THERMAL
GRADIENT/OUTFLOW BOUNDARY TO FORM AND SHIFT SOUTHEAST ACROSS
SOUTHEAST NM AND THE TX PANHANDLE OVER THE PAST FEW HOURS.  THIS
IS OCCURRING WITHIN A DIVERGENT REGION ALOFT AHEAD OF AN UPPER LOW
IN SOUTHWEST AZ.  MLCAPES ACROSS SOUTHEAST NM HAS INCREASED TO
1000-2000 J/KG, WHILE SURFACE TO 850 HPA WINDS OF 15-20 KTS ARE
FOCUSING PRECIPITABLE WATER VALUES OF 1-1.2" INTO THE BOUNDARY. 
CIN ACROSS THE AREA IS DECREASING DUE TO DAYTIME HEATING. 
DIRECTIONAL WIND SHEAR ALONG WITH DEWPOINTS IN THE MID 50S ASSURE
FORWARD PROPAGATION TO ACTIVE CONVECTION.

THE EXPECTATION IS FOR CONVECTION TO INCREASE IN COVERAGE AND
INTENSITY OVER THE NEXT SEVERAL HOURS, PRIMARILY BEYOND 23Z. 
INFLOW AT 850 HPA INCREASES ABOVE 25 KTS BY 00Z, NEAR THE
MAGNITUDE OF THE 850-400 HPA WIND THOUGH AT A RIGHT ANGLE, WHICH
SHOULD INCREASE PRECIPITATION EFFICIENCY.  PRECIPITABLE WATER
VALUES ARE EXPECTED TO BREACH 1.25", 2-2.5 SIGMAS ABOVE THE MEAN
FOR LATE OCTOBER, WHICH IN THE HIGH PLAINS IS NEAR THE THRESHOLD
FOR A BROADER SCALE HEAVY RAIN THREAT.  WHILE UNORGANIZED
CONVECTION SHOULD CONTINUE TO MOVE NORTHEAST AT ROUGHLY 25 KTS,
ORGANIZED CELLS SHOULD TRACK MORE TO THE EAST-NORTHEAST PARALLEL
TO 1000-850 HPA THICKNESS LINES AS THE THERMAL GRADIENT/OUTFLOW
BOUNDARY SHIFTS SLOWLY EASTWARD AND PERHAPS SOUTHWARD.  AFTER 02Z,
CONVECTION COULD BECOME MORE ELEVATED OVER THE BOUNDARY AS CIN
REDEVELOPS.  

CELL TRAINING IS EXPECTED TO BE THE MAIN HEAVY RAIN THREAT IN THIS
REGION.  HOWEVER, THE GALVEZ-DAVISON INDEX BASED ON THE 12Z GFS
INDICATES THAT THUNDERSTORMS SHOULD REMAIN SCATTERED RATHER THAN
NUMEROUS IN COVERAGE, WHICH COULD MAKE THIS A MARGINAL FLASH FLOOD
THREAT -- USED THE POSSIBLE CATEGORY BECAUSE OF THE ABOVE.  THE
CAM GUIDANCE ADVERTISES LOCAL AMOUNTS IN THE 2-4" RANGE, WHICH
COULD FALL QUICKLY.  THIS MAGNITUDE OF RAINFALL IS CLOSE TO THE 10
YEAR RECURRENCE INTERVAL OVER A SIX HOUR PERIOD.  AVAILABLE
MOISTURE SUPPORTS HOURLY RAIN RATES UP TO 1.5".  BASED ON THE
LOCATION OF THE SURFACE BOUNDARY AND EXPECTATIONS OF CONVECTIVE
MOVEMENT THROUGH 04Z, BELIEVE A COMPROMISE OF THE 12Z SPCWRF AND
18Z NAM CONEST APPEARS THE MOST REALISTIC WITH CONVECTIVE
PLACEMENT.  FLASH FLOODING IS CONSIDERED POSSIBLE.

ROTH

ATTN...WFO...ABQ...AMA...EPZ...LUB...MAF...

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

LAT...LON   35220249 35220115 34520079 33410124 32280315 
            32020542 32230597 32590622 33310612 33330613 
            33330613 33640562 34330443