MESOSCALE PRECIPITATION DISCUSSION 0520
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
641 AM EDT SUN SEP 27 2015
 
AREAS AFFECTED...PORTIONS OF THE UPPER/MIDDLE TX COAST 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 271040Z - 271540Z
 
SUMMARY...A TRAINING BAND OF CONVECTION IS BRUSHING THE COAST NEAR
GALVESTON ISLAND WITH RAIN RATES OF 1.5" AN HOUR.  CONSIDERING THE
URBAN ENVIRONMENT AND FAVORABLE PARAMETERS, FLASH FLOODING IS
POSSIBLE OVER THE NEXT FIVE HOURS.

DISCUSSION...A LOW- TO MID-LEVEL DISTURBANCE OVER THE NORTHWEST
GULF OF MEXICO HAS LED TO A FEEDER BAND SCRAPING THE TX COAST FROM
JUST SOUTH OF HIGH ISLAND INTO GALVESTON ISLAND EXTENDING
SOUTHWEST TOWARDS FREEPORT.  UPPER DIVERGENCE IS SEEN WITH THIS
BAND ON RECENT SATELLITE IMAGERY.  MUCAPES ARE 500-1000 J/KG IN
THIS REGION, INFLOW AT 850 HPA IS 20-30 KTS (CLOSE TO DOUBLE THE
MEAN 850-400 HPA WIND), PRECIPITABLE WATER VALUES ARE 1.6-1.8",
AND A BOUNDARY JUST OFF THE COAST IS BEING REINFORCED BY A LAND
BREEZE.  

RADAR ESTIMATES SHOW HOURLY RAIN RATES UP TO 1.5" WITHIN THIS
BAND, WHICH WOULD COMPROMISE URBAN AREAS.  WITH THE FLOW OFF THE
WARM GULF OF MEXICO, INSTABILITY SHOULD REMAIN IN PLACE.  RAP
GUIDANCE INDICATES THAT 850 HPA INFLOW SHOULD BE LESSENING LATER
THIS MORNING, WHICH SHOULD ALLEVIATE THE CURRENT HEAVY RAIN
THREAT.  THE CAM GUIDANCE SHOWS SPREAD IN THIS REGION, WITH THE
00Z SPC WRF, 00Z 3 KM NAM, AND 00Z WRF4NSSL HAVING SOME CONCEPT OF
WHAT IS ONGOING.  WITH THE LOW- TO MID-LEVEL DISTURBANCE PUSHING
NORTHWARD, THERE IS A CHANCE THE BAND COULD PUSH SLIGHTLY INLAND
AND PIVOT DURING THE NEXT SEVERAL HOURS, WHICH COULD ENHANCE RAIN
RATES BETWEEN HOUSTON AND PORT ARTHUR TX.  HOWEVER, WITH LIMITED
INSTABILITY INLAND, HEAVY RAIN SHOULD NOT GET TOO FAR INLAND. 
CONSIDERING AVAILABLE MOISTURE, HOURLY RAIN RATES UP TO 2" ARE
POSSIBLE.  USING THE 3 KM NAM AS A GUIDE, AND PER RECENT RADAR
TRENDS, LOCAL AMOUNTS UP TO 3-5" ARE POSSIBLE, WHICH WOULD CAUSE
ISSUES IN URBAN AREAS.

ROTH

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

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

LAT...LON   29849391 29669376 29619405 29519439 28979517 
            28439614 28539632 29029616 29519564 29839477