MESOSCALE PRECIPITATION DISCUSSION 0176
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
547 PM EDT SUN MAY 20 2018

AREAS AFFECTED...CNTRL TO SERN TX

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

VALID 202147Z - 210200Z

SUMMARY...PERIODS OF TRAINING WILL CONTINUE ACROSS PORTIONS OF
CENTRAL TO SOUTHEASTERN TEXAS THROUGH 02Z WITH AN ADDITIONAL 3 TO
5 INCHES OF RAIN POSSIBLE ON A LOCALIZED BASIS. FLASH FLOODING
WILL BE POSSIBLE...ESPECIALLY ACROSS THE URBANIZED HOUSTON
METROPOLITAN AREA.

DISCUSSION...21Z SURFACE OBSERVATIONS PLACED A LONG-LIVED OUTFLOW
BOUNDARY FROM JUST NORTHWEST OF THE HOUSTON BELTWAY WESTWARD TO
JUST NORTH OF COTULLA. ON THE WEST SIDE OF THIS BOUNDARY WAS A
SMALL BOWING COMPLEX MOVING EAST-SOUTHEASTWARD AT ROUGHLY 25
KT...WHILE FARTHER EAST... SLOWER MOVING CELLS WITH EMBEDDED
TRAINING WAS OCCURRING. LOCAL OBSERVATIONS HAVE CONFIRMED KHGX
RADAR ESTIMATED RAINFALL RATES OF 2-3 IN/HR LOCATED BETWEEN
COLLEGE STATION AND HOUSTON. VAD WIND PLOTS FROM CRP AND HGX
SHOWED ~30 KT AT 850 MB FROM THE SOUTH...OVERRUNNING THE OUTFLOW
BOUNDARY AMID A STRONGLY DIFFLUENT FLOW PATTERN ALOFT...WITH THE
GREATEST UPPER LEVEL DIVERGENCE/DIFFLUENCE FOCUSED OVER
SOUTHEASTERN TEXAS.

CONTINUED SLOW BUT STEADY MOVEMENT OF THE OUTFLOW BOUNDARY INTO
HOUSTON IS EXPECTED WITH A SHORT TERM FLASH FLOOD THREAT
TRANSLATING SOUTHEASTWARD INTO HARRIS AND FORT BEND COUNTIES WITH
RAINFALL RATES AS HIGH AS 2-3 IN/HR.

FARTHER WEST...THE STEADY MOVEMENT OF THE AFOREMENTIONED SMALL
BOWING CLUSTER OF THUNDERSTORMS SHOULD CONTINUE OFF TO THE
EAST-SOUTHEAST...ALONG WITH THE OUTFLOW BOUNDARY...WITH THE
POSSIBILITY OF REPEATING ROUNDS OF HEAVY RAIN ASSOCIATED WITH SLOW
MOVING STORMS ALREADY IN PLACE ALONG THE BOUNDARY.

ONE LARGE MITIGATING FACTOR FOR FLASH FLOODING IS THE VERY HIGH
FLASH FLOOD GUIDANCE...4+ INCHES IN 3 HOURS...BUT A FEW LOCATIONS
MAY EXCEED THESE VALUES AND/OR SEE RUNOFF ISSUES DESPITE THE HIGH
FFG GIVEN THE INTENSE RAINFALL RATES BEING OBSERVED.

OTTO

ATTN...WFO...BRO...CRP...EWX...HGX...LCH...SHV...

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

LAT...LON   31439467 31379418 30899406 30399421 29459503
            28469647 27799777 26799943 27249985 27959963
            28559967 29179949 29579871 29909719 30429609
            31209538