Flash Flood Warning 17, NWS Houston/Galveston, TX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
910 PM CST TUE NOV 11 2008

.DISCUSSION...
NUMEROUS SHRA/TSRA PERSIST ALONG AND NORTH OF A WEST TO EAST
ORIENTED FRONTAL BOUNDARY WHICH EXTENDS FROM NEAR COLUMBUS TO
RICHMOND TO ALVIN TO ANAHUAC. CELLS OVER OUR WESTERN ZONES
CONTINUE TO GRADUALLY BECOME MORE INTENSE AND MORE
NUMEROUS...LIKELY DUE TO THE STRONG SHORTWAVE OVER WEST TEXAS
BEGINNING TO AFFECT OUR AREA. KHGX STORM TOTAL PRECIP SHOWS
WIDESPREAD 2-4 INCH RAIN AMOUNTS ACROSS THE CENTRAL THIRD OF THE
CWA...WITH ISOLATED TOTALS OF 5-6 INCHES OVER SOUTHERN GRIMES AND
MONTGOMERY COUNTIES. THE POTENTIAL EXISTS FOR ADDITIONAL TRAINING
OF STORMS AND HEAVY RAINFALL ACROSS THIS AREA THROUGH THE
OVERNIGHT HOURS. LATER TONIGHT...THE BOUNDARY AND ASSOCIATED
CONVECTION SHOULD BEGIN SHIFTING SOUTHWARD AS THE SHORTWAVE GETS
CLOSER TO THE AREA. HEAVY RAINFALL THREAT WILL BE GREATEST OVER
THE SOUTHERN THIRD OF THE AREA VERY LATE TONIGHT THROUGH THE
MORNING HOURS WEDNESDAY. LAPS ANALYSIS SHOWS THE SFC-BASED
INSTABILITY IS CONFINED TO THE AREA SOUTH OF THE FRONT...GENERALLY
SOUTH OF A LINE FROM COLUMBUS TO RICHMOND TO GALVESTON. STORMS
NORTH OF THE BOUNDARY ARE ELEVATED IN NATURE WITH LARGE HAIL BEING
IN THE MAIN THREAT. OVERALL...THE SEVERE THREAT IS EXPECTED TO BE
REMAIN ISOLATED IN NATURE. HAVE ADJUSTED POP FORECAST BASED ON
CURRENT TRENDS...AND WILL ISSUE AN UPDATE TO THE HWOHGX.

35

&&

.PREV DISCUSSION... /ISSUED 710 PM CST TUE NOV 11 2008/

UPDATE.../ISSUED 529 PM CST TUE NOV 11 2008/
HAVE ISSUED A FLOOD WATCH FOR MOST OF SE TX UNTIL 18Z WEDNESDAY.
SHRA/TSRA CONTINUE TO FOCUS ALONG AND JUST NORTH OF A WELL-DEFINED
SFC BOUNDARY WHICH IS LOCATED FROM LA GRANGE TO HOUSTON TO NORTH
OF BEAUMONT. CONVECTION HAS BEEN ON AN INCREASING TREND OVER OUR
FAR WRN ZONES WITH TRAINING OF ECHOES TAKING PLACE. KHGX RADAR
INDICATES THAT 1 TO 3 INCHES OF RAIN HAS FALLEN TODAY OVER MOST OF
THE CENTRAL THIRD OF THE CWA...WITH TOTALS AS HIGH AS 5 INCHES
OVER MONTGOMERY COUNTY. SE TX WILL REMAIN IN THE RIGHT REAR
QUADRANT OF A 100-110 KT JET MAX MOVING ACROSS TEXAS WHICH WILL
ENHANCE UPPER DIFLUENCE OVER THE AREA. WATER VAPOR IMAGERY SHOWS A
STRONG DISTURBANCE MOVING INTO FAR WEST TX. AS THIS SYSTEM
APPROACHES LATER TONIGHT...COOLING MID LEVELS AND INCREASING LIFT
WILL RESULT IN AN ENHANCEMENT OF CONVECTIVE ACTIVITY. SFC BOUNDARY
WILL PUSH SLOWLY SOUTH TONIGHT/WEDNESDAY MORNING...WITH THE
POTENTIAL FOR HEAVY RAINFALL/FLOODING GRADUALLY SHIFTING SOUTHWARD
WITH TIME. FLASH FLOOD GUIDANCE SHOWS ONLY 2.5-3 IN/3 HRS
NECESSARY FOR FLASH FLOODING. HAVE MADE SOME ADJUSTMENTS TO POPS
BASED ON CURRENT TRENDS AND THE 18Z MODEL RUNS.

35

PREV DISCUSSION... /ISSUED 248 PM CST TUE NOV 11 2008/

DISCUSSION...
AFTERNOON SHOWERS AND THUNDERSTORMS HAVE ORIENTED THEMSELVES FROM THE
NORTHEAST TO THE SOUTHWEST FROM AROUND THE JASPER AREA TO BETWEEN
HOUSTON AND CONROE TO BETWEEN SEALY AND BRENHAM. COOLING CLOUD
TOPS...MODERATE TO STRONG LOW LEVEL SOUTHERLY WINDS...AND A
DIFFLUENT FLOW ALOFT SUPPORT LOCALLY HEAVY RAINFALL FROM THIS
ACTIVITY AS IT TRAINS ACROSS THE AREA. RADAR HAS BEEN GENERALLY
ESTIMATING HOURLY RAINFALL RATES BETWEEN 1 AND 2 INCHES. WILL
CONTINUE TO WATCH THIS AREA CLOSELY FOR POSSIBLE FLOOD ADVISORIES OR
FLASH FLOOD WARNINGS. THINK WE CAN HANDLE THESE LOCALLY HEAVY RAINS
WITHOUT A FLASH FLOOD WATCH AT THIS TIME...BUT WILL NEED TO CLOSELY
WATCH THE RADAR TREND. THIS ACTIVITY SHOULD TAPER OFF THIS
EVENING...THEN MORE SHOWERS AND THUNDERSTORMS ARE EXPECTED TO DEVELOP
OVERNIGHT AND ON INTO TOMORROW MORNING AS THE NEXT SHORTWAVE HEADS
THIS WAY FROM THE WEST. LOCALLY HEAVY RAINFALL WILL CONTINUE TO BE
THE PRIMARY SEVERE WEATHER THREAT...BUT CANNOT TOTALLY RULE OUT SOME
OF THE STRONGER STORMS POSSIBLY CONTAINING SOME STRONG WINDS...HAIL
OR EVEN TORNADOES. BEFORE TONIGHTS/TOMORROWS BATCH OF STORMS MOVE
OFF TO THE EAST...WOULD NOT AT ALL BE SURPRISED IF SOME SPOTS
RECEIVE ANOTHER 3 TO 5 INCHES OF RAIN. A DRIER AIRMASS BEGINS TO
FILTER INTO THE AREA WEDNESDAY NIGHT AND THURSDAY...SO WILL CARRY
DECREASING RAIN CHANCES FROM WEST TO EAST DURING THIS TIME PERIOD.
FOR TEMPERATURES...EXPECT COOLER NIGHTS AND WARMISH DAYS BEHIND ALL
THESE RAINS AS WE AWAIT THE PASSAGE OF THE STRONG COLD FRONT. THIS
FRONT SHOULD ROAR ON THROUGH FRIDAY EVENING WITH STRONG COLD AIR
ADVECTION BEHIND THE BOUNDARY. IT LOOKS LIKE COOLER TEMPERATURES AND
A DRY AIRMASS WILL PREVAIL SATURDAY THROUGH MUCH OF NEXT WEEK AS A
NORTHWEST FLOW ALOFT ALLOWS A SERIES OF REINFORCING COLD FRONTS
THROUGH OUR AREA.  42

MARINE...
SOUTHERLY WINDS HAVE SLACKED OFF SOME SO WILL ONLY CARRY AN SCA FOR
THE WATERS OUT 20NM TO 60NM.  WINDS NEARSHORE HAVE COME DOWN
ALLOWING SEAS TO DROP TO NEAR OR BELOW 7FT.  SCA WILL CONTINUE
OVERNIGHT BUT POSSIBLE THAT CONDITIONS WILL BE BELOW CRITERIA.
ANOTHER SHORTWAVE TROUGH PASSING THROUGH TOMORROW WILL KEEP
THUNDERSTORMS POSSIBLE FOR THE COASTAL AREAS AND PUSH MORE INTO THE
GULF.  WINDS AND SEAS SHOULD REMAIN NEAR MODERATE LEVELS WITH SCEC
CONDITIONS POSSIBLE.  THE OTHER CONCERN WILL BE THE POSSIBLE
DEVELOPMENT OF A COASTAL LOW WED/THU AS AN UPPER LEVEL TROUGH MOVES
OVER THE NW GULF.  THIS MAY ENHANCE WINDS AND SEAS FOR A PERIOD BUT
SCA CONDITIONS ARE NOT EXPECTED.  OVER THE WEEKEND...GFS/ECMWF
CONSISTENT WITH A FAIRLY STRONG COLD FRONT PUSHING OFF THE COAST
LATE FRI.  SCA WILL CERTAINLY BE NEEDED BUT WILL MONITOR FUTURE
MODEL RUNS FOR THE POSSIBILITY OF GALE CONDITIONS.  THE FORECAST
WILL CARRY WINDS AND SEAS BELOW GALE LEVELS.

39

&&

.PRELIMINARY POINT TEMPS/POPS...
COLLEGE STATION (CLL)      64  78  55  78  55 /  80  50  30  10  10
HOUSTON (IAH)              66  77  57  78  56 /  80  80  50  10  10
GALVESTON (GLS)            70  76  63  76  63 /  70  80  50  20  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...FLOOD WATCH THROUGH WEDNESDAY MORNING FOR THE FOLLOWING ZONES:
     AUSTIN...BRAZORIA...CHAMBERS...COLORADO...FORT BEND...
     GALVESTON...GRIMES...HARRIS...JACKSON...LIBERTY...
     MATAGORDA...MONTGOMERY...WALLER...WASHINGTON...WHARTON.

GM...SMALL CRAFT ADVISORY UNTIL 6 AM CST WEDNESDAY FOR THE FOLLOWING
     ZONES: WATERS FROM FREEPORT TO THE MATAGORDA SHIP CHANNEL
     20 NM TO 60 NM...WATERS FROM HIGH ISLAND TO FREEPORT 20 TO
     60 NM.

&&

DISCUSSION...35

HAZARDOUS WEATHER OUTLOOK...UPDATED
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
931 PM CST TUE NOV 11 2008

GMZ330-335-350-355-370-375-TXZ163-164-176>179-195>200-210>214-226-
227-235>238-121200-
AUSTIN-BRAZORIA-BRAZOS-BURLESON-CHAMBERS-COLORADO-FORT BEND-
GALVESTON-GALVESTON BAY-GRIMES-HARRIS-HOUSTON-JACKSON-LIBERTY-
MADISON-MATAGORDA-MATAGORDA BAY-MONTGOMERY-POLK-SAN JACINTO-
TRINITY-WALKER-WALLER-WASHINGTON-
WATERS FROM FREEPORT TO THE MATAGORDA SHIP CHANNEL 20 NM TO 60 NM-
WATERS FROM FREEPORT TO THE MATAGORDA SHIP CHANNEL OUT 20 NM-
WATERS FROM HIGH ISLAND TO FREEPORT 20 TO 60 NM-
WATERS FROM HIGH ISLAND TO FREEPORT OUT 20 NM-WHARTON-
931 PM CST TUE NOV 11 2008

...THREAT FOR FLOODING AND ISOLATED SEVERE THUNDERSTORMS TONIGHT
AND WEDNESDAY MORNING...

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF SOUTHEAST TEXAS.

.DAY ONE...TONIGHT AND WEDNESDAY

SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS WILL PERSIST ACROSS
THE NORTHERN TWO-THIRDS OF SOUTHEAST TEXAS AS DEEP GULF MOISTURE
CONTINUES TO MOVE ONSHORE AND INTERACTS WITH A NEARLY STATIONARY
FRONT DRAPED ACROSS THE REGION. A STRONG UPPER LEVEL DISTURBANCE
WILL APPROACH FROM THE WEST LEADING TO A FURTHER ENHANCEMENT OF
THUNDERSTORM ACTIVITY OVERNIGHT. AS THE DISTURBANCE
APPROACHES...HEAVY THUNDERSTORMS WILL GRADUALLY SPREAD SOUTHWARD
INTO THE SOUTHERN THIRD OF THE AREA.

THE PRIMARY THREAT WITH THESE STORMS WILL BE HEAVY RAINFALL AND
FLOODING. A FLOOD WATCH IS IN EFFECT FOR ALL BUT THE FAR NORTHERN
COUNTIES OF SOUTHEAST TEXAS THROUGH NOON WEDNESDAY. ADDITIONAL
RAINFALL AMOUNTS WILL AVERAGE 1 TO 3 INCHES...WITH ISOLATED TOTALS
OF 5 TO 7 INCHES POSSIBLE. WITH THE GROUND ALREADY SATURATED...ANY
ADDITIONAL HEAVY RAINFALL IS LIKELY TO CAUSE FLOODING.

THE POTENTIAL EXISTS FOR ISOLATED SEVERE THUNDERSTORMS MAINLY
TONIGHT. THE PRIMARY THREAT WILL BE PENNY TO QUARTER SIZED
HAIL...WITH A LESSER THREAT OF DAMAGING WINDS OR ISOLATED
TORNADOES.

THE THUNDERSTORMS ARE EXPECTED TO GRADUALLY DIMINISH FROM NORTH TO
SOUTH BEGINNING MID TO LATE MORNING WEDNESDAY.

.DAYS TWO THROUGH SIX...WEDNESDAY NIGHT THROUGH MONDAY

HAZARDOUS WEATHER IS NOT EXPECTED.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION IS LIKELY TONIGHT AND WEDNESDAY MORNING.