MESOSCALE PRECIPITATION DISCUSSION 0784
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
708 PM EST SAT DEC 03 2016
 
AREAS AFFECTED...UPPER TX COAST...SOUTHWEST LA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 040005Z - 040605Z
 
SUMMARY...HEAVY RAINS WILL CONTINUE THROUGH THIS EVENING WITH
INCREASING CONCERNS FOR FLASH FLOODING/RUNOFF PROBLEMS FROM
REPEATING/TRAINING SHOWERS AND THUNDERSTORMS.

DISCUSSION...IR SATELLITE IMAGERY HAS BEEN SHOWING AN EXPANSION OF
COLD CONVECTIVE TOPS OVER THE LAST COUPLE OF HOURS ALONG A
WELL-DEFINED INSTABILITY GRADIENT INVOLVING A QUASI-STATIONARY
FRONTAL ZONE AND WAVE OF LOW PRESSURE JUST OFF THE MIDDLE AND
UPPER TX COAST AND EXTENDING NORTHEAST TOWARD COASTAL AREAS OF
SOUTHWEST LA. THE CONVECTION IS BEING FACILITATED BY AN AXIS OF
ENHANCED FRONTAL CONVERGENCE AND MOISTURE TRANSPORT...AND IS
COUPLING WITH LARGER SCALE ASCENT FROM NOTABLE 250 MB
RIGHT-ENTRANCE REGION UPPER JET DYNAMICS AHEAD OF AN ANOMALOUSLY
DEEP CLOSED/LOW TROUGH OVER NORTHWEST MX.

A VERY WELL-DEFINED DUAL FEED OF MOISTURE TRANSPORT IS NOTED OVER
THE REGION WITH THE CIRA-LPW PRODUCT INDICATING A PRONOUNCED
700/300 MB LAYER SOUTHWESTERLY SUBTROPICAL MOISTURE FEED FROM THE
EASTERN TROPICAL PACIFIC AHEAD OF THE TROUGH...WHILE A LOW LEVEL
SOUTHERLY FETCH OF GULF OF MEXICO MOISTURE IS ADVANCING NORTH INTO
THE AFOREMENTIONED FRONT. THE COMBINATION OF THIS DUAL MOISTURE
FETCH IS FOSTERING TPW VALUES OF 1.9 TO 2.0 INCHES ALONG THE UPPER
TX COAST AND INTO SOUTHWEST LA WHICH IS A SOLID 2.5 SIGMAS ABOVE
THE MEAN. CONSEQUENTLY...THE BANDS OF CONVECTION ALONG AND
ADJACENT THE FRONTAL ZONE ARE QUITE EFFICIENT IN PRODUCING HEAVIER
RAINFALL RATES.

THE HRRR AND HRRR-TLE GUIDANCE SUGGESTS A VERY SLOW EASTWARD
ADVANCE OF THE CONVECTION OVER THE NEXT SEVERAL HOURS WHICH WILL
ALLOW AREAS OF THE UPPER TX COAST AND SOUTHWEST LA TO SEE
ADDITIONAL HEAVY TRAINING BANDS OF SHOWERS/THUNDERSTORMS. BY LATE
IN THE EVENING...AREAS FARTHER EAST ACROSS SOUTHERN LA WILL NEED
TO CLOSELY MONITOR THE POTENTIAL FOR TRAINING CONVECTION. EXPECT
ADDITIONAL RAINFALL AMOUNTS OF AS MUCH AS 3 TO 6 INCHES THROUGH
06Z...WITH ISOLATED HEAVIER AMOUNTS POSSIBLE...AND ESPECIALLY OVER
SOUTHWEST LA. FLASH FLOODING AND/OR LOCALLY SIGNIFICANT RUNOFF
PROBLEMS WILL BE POSSIBLE.

ORRISON

ATTN...WFO...HGX...LCH...

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

LAT...LON   30699271 30569206 30049201 29629238 29609298 
            29649364 29409437 29149489 29339510 29949453 
            30489361