Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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170
FXUS64 KHGX 190255
AFDHGX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
955 PM CDT MON MAY 18 2015

.DISCUSSION...
WITH THE LOSS OF DAYTIME HEATING...THE SHOWERS AND THUNDERSTORMS
HAVE DISSIPATED BETWEEN HOUSTON AND COLLEGE STATION. HOWEVER...AN
ANALYSIS OF THE 00Z SURFACE AND UPPER AIR CHARTS SHOWED A SURFACE
OUTFLOW BOUNDARY LOCATED FROM HOUSTON TO BRENHAM...850 MB
DEWPOINTS BETWEEN 10 AND 12 CELCIUS AND 700 MB DEWPOINTS BETWEEN 2
AND 4 CELCIUS OVER THE SURFACE BOUNDARY...A 500 MB SHORTWAVE
TROUGH MOVING EASTWARD ACROSS WEST TEXAS...AND A DIFFLUENT ZONE
OVERHEAD SE TEXAS AT 300 AND 200 MB. THE NAMBUFR FORECAST
SOUNDINGS GIVE MU CAPE VALUES OF AT LEAST 2700 SOUTH OF THE
OUTFLOW BOUNDARY BETWEEN MIDNIGHT AND 6 AM AND INCREASED THIS TO
OVER 3000 AFTER SUNRISE. THE HIGH RESOLUTION SHORT TERM MODELS
ALL SHOWED THE DISSIPATING EVENING SHOWERS AND THUNDERSTORMS.
HOWEVER...THE RAP13 HAS DONE THE BEST JOB SO FAR AND FORECASTS A
BEGINNING OF CONVECTION ALONG THE OUTFLOW BOUNDARY ALONG THE
INTERSTATE 10 CORRIDOR AROUND AND AFTER SUNRISE TUESDAY MORNING.

FOR THE UPDATE...COULD NOT RULE OUT ISOLATED SHOWERS AND
THUNDERSTORMS OVERNIGHT TONIGHT...AND KEPT THE POPS AT 20 PERCENT
FROM LATE THIS EVENING THROUGH 7 AM. UPPED THE POPS TO 30 PERCENT
FOR THE INTERSTATE 10 CORRIDOR FOR THE FIRST HALF OF TUESDAY
MORNING AND THEN EXTENDED THE SCATTERED POPS NORTHWARD FOR THE
LAST HALF OF THE MORNING. WOULD NOT BE SURPRISED IF THE ACTIVITY
BEGINS BEFORE SUNRISE...HOWEVER IF THE RAP13 IS ON TARGET...THE
COVERAGE WILL BE ISOLATED WHICH IS CURRENTLY COVERED WITH THE 20
PERCENT POPS. WILL NEED TO MONITOR THE THREAT FOR LOCALLY HEAVY
RAINFALL. THE MODEL IS NOT INDICATING PW/S MUCH HIGHER THAN 1.6
INCHES AT THIS TIME...HOWEVER IF SLOW MOVING STORMS DEVELOP EVEN A
1 TO 2 INCH PER HOUR RAINFALL RATE WOULD POISE A PROBLEM.

UPDATED POPS AND WEATHER FOR THE REMAINDER OF THIS EVENING AND THE
TUESDAY MORNING TIME PERIODS.

40

&&

.PREV DISCUSSION... /ISSUED 723 PM CDT MON MAY 18 2015/

UPDATE...
A COUPLE OF OUTFLOW BOUNDARIES WERE MOVING SOUTHWESTWARD ACROSS
THE CENTRAL PORTIONS OF THE FORECAST AREA. UPDATE TO ADD SLIGHTER
BETTER CHANCES FOR SHOWERS AND THUNDERSTORMS OVER THE WEST-CENTRAL
SET OF COUNTIES THROUGH THE REST OF THE EVENING. THE 23Z HRRR
INITIALIZED WELL WITH THE CURRENT SHOWERS AND THUNDERSTORMS. THE
MODELS THEN KEEPS AT LEAST ISOLATED CONVECTION GOING BETWEEN 9 PM
AND MIDNIGHT.

40

PREV DISCUSSION... /ISSUED 650 PM CDT MON MAY 18 2015/

DISCUSSION...
00Z TAF DISCUSSION IN AVIATION SECTION...

AVIATION...
VIS SATELLITE AND RADAR SHOW OUTFLOW BOUNDARY PUSHING INTO THE
HOUSTON AREA FROM THE EAST. THERE HAVE BEEN SOME NEW STORMS TO
DEVELOP BUT LOOKS LIKE MOISTURE/INSTABILITY MAY HAVE BEEN WORKED
OVER JUST ENOUGH THAT THE STORMS ARE NOT AS STRONG AS THEY COULD
BE OTHERWISE. STORMS ARE MAINLY IMPACTING KCXO BUT COULD SEE A FEW
CONVECTIVE CELLS DEVELOP THROUGH THE EVENING AROUND KIAH/KUTS.
MENTION VCTS FOR MOST TAF SITES UNTIL 02Z BUT AT LEAST TEMPO TSRA
FOR CXO AND UTS.

TONIGHT EXPECT MAINLY MVFR CIGS TO DEVELOP AS HAS BEEN THE CASE
THE LAST FEW NIGHTS. REMNANT OUTFLOW BOUNDARY MAY SET UP EAST/WEST
ACROSS THE AREA. THINK LATEST RAP/HRRR SEEM TO HAVE A HANDLE ON
THIS EVOLUTION. RAP ALSO DEVELOPS CONVECTION ALONG THIS BOUNDARY
IN THE MORNING. WATER VAPOR IMAGERY SHOWS SHORT WAVE TROUGH OVER
MEXICO/W TX/BIG BEND AREA THAT COULD IMPACT THE AREA BY LATE
MORNING. WITH DAYTIME HEATING DECIDED TO PUT VCTS IN TAFS STARTING
16Z WITH VCSH PRIOR TO THAT.

39

&&

.PRELIMINARY POINT TEMPS/POPS...
COLLEGE STATION (CLL)      73  85  72  87  71 /  20  40  10  20  30
HOUSTON (IAH)              73  86  73  88  74 /  20  30  10  30  10
GALVESTON (GLS)            77  83  77  85  77 /  20  20  10  10  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&

$$

DISCUSSION...40
AVIATION/MARINE...39



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