MESOSCALE PRECIPITATION DISCUSSION 0258
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
352 AM EDT MON MAY 22 2017

AREAS AFFECTED...COASTAL TEXAS

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

VALID 220752Z - 221352Z

SUMMARY...AN UPTICK IN CONVECTIVE COVERAGE ACROSS PORTIONS OF
COASTAL TEXAS WILL LEAD TO A FLASH FLOOD THREAT OVER THE NEXT
SEVERAL HOURS.

DISCUSSION...RECENT RADAR IMAGERY AND SURFACE WIND FIELDS SUGGEST
THAT AN MCV HAS FORMED WITH THE SMALL THUNDERSTORM COMPLEX MOVING
EAST ACROSS SOUTH TEXAS. THIS COMPLEX IS ALSO IN AN AREA OF
INCREASED UPPER LEVEL DIVERGENCE ON THE NOSE OF THE SUBTROPICAL
JET. AS THE MCV AND INCREASED UPPER LEVEL DIVERGENCE MOVES
EAST...CONDITIONS SHOULD CONTINUE TO BECOME MORE FAVORABLE FOR
CONVECTIVE DEVELOPMENT OVER PORTIONS OF SOUTHEAST TEXAS. EARLIER
THIS EVENING A BOUNDARY WAS EVIDENT OFFSHORE...WITH TRAINING
CONVECTION EVIDENCE OF THIS BOUNDARY. HOWEVER THIS CONVECTION HAS
BEEN FORCED EASTWARD WITH TIME...AND WIND FIELDS SUGGEST THE
BOUNDARY IS SLOWLY LIFTING BACK NORTH INTO THE COAST FROM CORPUS
CHRISTIE TO PORT LAVACA. THE COMBINATION OF FAVORABLE UPPER
DYNAMICS MOVING IN...AND INCREASED LOW LEVEL MOISTURE TRANSPORT
INTO THIS BOUNDARY...WILL SET THE STAGE FOR AN INCREASE IN ROBUST
CONVECTION OVER THE NEXT SEVERAL HOURS.

HIGH RES MODEL RUNS ARE GENERALLY IN GOOD AGREEMENT ON THE
EVOLUTION...WITH CELL MERGERS AND TRAINING CONVECTION LIKELY FROM
THE MCV TOWARDS THE COASTAL FRONT. PRECIPITABLE WATER VALUES ARE
RIGHT AROUND 2" PER GPS...WHICH COMBINED WITH A DEEP WARM LAYER
SUPPORTS EFFICIENT RAINFALL. THUS ANTICIPATE ISOLATED TO SCATTERED
4-6" TOTALS INTO THE EARLY MORNING HOURS...WITH THE MAIN FOCUS
THROUGH 12Z ALONG THE COAST IN BETWEEN CORPUS CHRISTIE AND
HOUSTON. 

CONFIDENCE IN AN EXCESSIVE RAINFALL THREAT IS LOWER FURTHER UP THE
COAST IN THE VICINITY OF HOUSTON...THUS HAVE OPTED TO NOT EXTEND
THE MPD INTO THIS AREA AT THIS TIME. THIS AREA IS CURRENTLY
FURTHER INTO THE MORE STABLE AIR PER RADAR IMAGERY AND SPC RAP
MESOANALYSIS. HOWEVER DO ANTICIPATE THAT THE FRONT WILL SLOWLY
LIFT BACK NORTH WITH TIME HERE AS WELL. JUST A QUESTION AS TO HOW
FAR NORTH THE COASTAL FRONT AND BETTER INSTABILITY CAN GET NEAR
HOUSTON. THIS WILL DETERMINE WHETHER THE HEAVIEST TOTALS END UP
ONSHORE OR JUST OFFSHORE AS THE CONVECTION MOVES UP THE COAST
EARLY THIS MORNING. WILL CONTINUE TO MONITOR THIS REGION...AND
ISSUE ADDITIONAL MPDS AS NEEDED.

CHENARD

ATTN...WFO...CRP...EWX...HGX...

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

LAT...LON   29269671 29049566 29049523 28679525 28549544
            28189608 28129621 28019663 28179698 28259721
            28479805 28589857 29089823 29239767