MESOSCALE PRECIPITATION DISCUSSION 0011
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
928 AM EST SAT FEB 10 2018

AREAS AFFECTED...SE TEXAS...SW LOUISIANA

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

VALID 101425Z - 102015Z

SUMMARY...LOCALIZED FLASH FLOODING WILL BE POSSIBLE THROUGH THE
MORNING AND INTO THE EARLY AFTERNOON FROM SOUTHEAST TEXAS INTO
SOUTHWEST LOUISIANA WITH SMALL TRAINING HEAVY RAIN BANDS AND
THUNDERSTORMS. HOURLY RAIN RATES COULD REACH 2 IN/HR AND RAINFALL
TOTALS COULD EXCEED 4 INCHES IN LOCALIZED AREAS.

DISCUSSION...SURFACE OBSERVATIONS AS OF 12Z INDICATED BROAD 5-10
KT SOUTHEASTERLY INFLOW FROM THE NORTHWESTERN GULF OF MEXICO INTO
COASTAL TEXAS AND LOUISIANA...WITH CALM WINDS AROUND 15-25 MILES
INLAND. THIS WAS SETTING UP A RIBBON OF BOUNDARY LAYER CONVERGENCE
FROM NEAR BAY CITY TX...TO NEAR LEAGUE CITY TX...TO NEAR BEAUMONT
TX...AND FURTHER ENE TO JUST NORTH OF LAFAYETTE LA. AS A BROAD
AREA OF RAIN CONTINUES TO FALL TO THE NORTH OF THIS AXIS...AND
CONVECTION DEVELOPS ALONG IT...THE THETAE GRADIENT AND CONVERGENCE
MAY BECOME FURTHER ENHANCED OVER THE NEXT SEVERAL HOURS. IF THAT
OCCURS...TRAINING CONVECTION WOULD BE INCREASINGLY LIKELY GIVEN
BROAD SOUTHWESTERLY FLOW IN THE 850-300MB LAYER PER THE 12Z LCH
SOUNDING. AND GIVEN OBSERVED PWATS BETWEEN 1.6 AND 1.8 INCHES ON
THAT SOUNDING AND GPS-PW ACROSS THE REGION (NEAR RECORD VALUES FOR
FEBRUARY)...ANY CONVECTION SHOULD BE EFFICIENT RAINFALL PRODUCERS.

HGX RADAR SHOWS A BAND OF HEAVY RAIN AND THUNDERSTORMS AROUND
40-50 MILES IN LENGTH THAT HAS BEEN TRAINING FOR THE PAST COUPLE
HOURS OVER PORTIONS OF BRAZORIA... GALVESTON... AND FAR SOUTHEAST
HARRIS COUNTIES. THIS APPEARS TO BE ANCHORED RIGHT ALONG THE
BOUNDARY LAYER CONVERGENCE AXIS. SURFACE OBSERVATIONS ALSO
INDICATE THE POSSIBILITY OF A WEAK MESOLOW (SLIGHTLY REDUCED
PRESSURE AND MORE FOCUSED CONVERGENCE) IN THIS AREA...AND THE 12Z
RAP ACTUALLY TRIES TO SHIFT SUCH A MESOLOW NORTHEAST INTO PORTIONS
OF LOUISIANA THIS AFTERNOON WITH A FOCUSED AREA OF BOUNDARY LAYER
CONVERGENCE. IF THIS WERE TO BECOME MORE COHERENT...IT COULD
FURTHER HELP FOCUS CONVECTION OVER THE NEXT 3-6 HOURS.

BASED ON MESONET OBSERVATIONS ACROSS PORTIONS OF BRAZORIA
COUNTY...RAIN RATES HAVE ALREADY APPROACHED 2 IN/HR IN SOME
LOCATIONS....AND THIS SEEMS REASONABLE GIVEN DUAL POL ESTIMATES
FROM THE HGX RADAR. MRMS HAS BEEN UNDERESTIMATING RAINFALL RATES
SLIGHTLY...BUT THE CREST HYDROLOGIC MODEL (DRIVEN BY MRMS
RAINFALL) IS STILL BEGINNING TO SHOW SOME HYDROLOGIC RESPONSE AND
POSSIBILITY OF FLOODING DEVELOPING FROM THIS RAIN BAND.
THEREFORE...DESPITE HIGHER FLASH FLOOD GUIDANCE IN THIS
REGION...THE POTENTIAL FOR TRAINING CONVECTION AND RAIN RATES UP
TO AROUND 2 IN/HR SHOULD SUPPORT THE POSSIBILITY OF LOCALIZED
FLASH FLOODING FROM SE TX INTO SW LA INTO THE EARLY AFTERNOON.

LAMERS

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

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

LAT...LON   31759270 31409213 30999188 30239182 29619215
            29459308 29509382 29309455 28789536 28819575
            29339588 30079535 30729462 31589362