Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX

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000
FXUS64 KEWX 211148
AFDEWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE AUSTIN/SAN ANTONIO TX
548 AM CST FRI NOV 21 2014

.AVIATION DISCUSSION...FOR 12Z TAF PACKAGE...
A COMPLEX FORECAST FOR THIS TAF PERIOD AS LOW LEVEL MOISTURE IS
PLENTIFUL RESULTING IN MVFR/IFR/LIFR CATEGORIES IN VIS AND CIG
THROUGHOUT THE NIGHT. THESE CONDITIONS WILL LIKELY PERSIST THROUGH
THE MORNING HOURS BEFORE ALL TERMINALS GENERALLY IMPROVE TO MVFR
CIGS BY MID DAY. KAUS/KSSF/KSAT TERMINALS SHOULD EXPERIENCE
ISOLATED TO SCATTERED SHOWER ACTIVITY THROUGHOUT THE DAY AS WELL
WITH POSSIBLE -TSRA IN THE AFTERNOON. BY NIGHTFALL...BR SHOULD
RETURN FOR ALL TERMINALS TO AT LEAST MVFR.

AN APPROACHING STORM SYSTEM WILL IMPACT ALL TERMINALS TOMORROW
MORNING WITH HIGHEST CHANCE AT KAUS SO INCLUDED PROB30 LATE IN THE
PERIOD FOR PLANNING PURPOSES BEYOND 24 HOURS.

TB3


&&

.PREV DISCUSSION... /ISSUED 409 AM CST FRI NOV 21 2014/

SHORT TERM (TODAY THROUGH SATURDAY)...
CLOUDY SKIES AND PATCHY FOG HAVE OVERSPREAD SOUTH CENTRAL TEXAS
OVERNIGHT AS CONTINUED SOUTHERLY FLOW PUMPS IN WARMER TEMPERATURES
AND HIGHER MOISTURE INTO THE AREA. THE WEAK LIFT ASSOCIATED WITH
THE WARM AIR ADVECTION HAS LED TO ISOLATED SHOWER DEVELOPMENT
SINCE YESTERDAY AND THIS WILL CONTINUE MAINLY ACROSS THE EASTERN
ZONES EARLY THIS MORNING. CURRENTLY...THE LOW-LEVEL FLOW IS SSW IN
THE LOWEST 4 KM OF THE ATMOSPHERE. BY THE AFTERNOON MOST GUIDANCE
MEMBERS ARE BACKING THE WINDS TO THE SSE AND INCREASING THE LOW-
LEVEL JET IN RESPONSE TO THE APPROACHING UPPER LOW WHICH IS
CURRENTLY OVER SOUTHERN CALIFORNIA PER LATEST WATER VAPOR
ANALYSIS. THE BACKING OF THE WIND AND INCREASING OF THE SPEED WILL
LEAD TO A STRONG PUSH OF POSITIVE THETA-E ADVECTION BEGINNING
IN THE SOUTHEAST ZONES...NEAR THE COASTAL PLAINS THIS AFTERNOON.
THE NOSE OF THE INCREASING THETA-E VALUES WILL THEN PUSH NORTHWEST
TO THE I-35 CORRIDOR BY 6 PM AND THEN THE HILL COUNTRY AND EDWARDS
PLATEAU BY MIDNIGHT. POSITIVE THETA-E ADVECTION IS A DIRECT
MEASURE OF INCREASING TEMPERATURE/MOISTURE WHICH IS LIFT. THIS
LIFT CAN ALSO BE SEEN ON ISENTROPIC CHARTS ON CONSTANT THETA
SURFACES. EXPECT ABOUT A 50 TO 60 PERCENT COVERAGE OF SHOWERS AND
THUNDERSTORMS IN THE FORM OF A BROKEN LINE BEGINNING THIS
AFTERNOON ACROSS THE SOUTHEAST AND THEN MOVING NORTHWEST THROUGH
MIDNIGHT.

AS WE GET INTO TOMORROW MORNING...CONTINUED WEAK LIFT ON
ISENTROPIC CHARTS WILL RESULT IN ISOLATED TO SCATTERED ACTIVITY
ACROSS MUCH OF THE AREA. ALSO BY TOMORROW MORNING...THE UPPER LOW
IS PROGGED TO BE NEAR NORTHERN MEXICO...ENTERING WEST TEXAS.
MODELS ARE IN FAIRLY GOOD AGREEMENT WITH THE PLACEMENT AND
STRENGTH OF THE LOW AS IT PROGRESSES THROUGH THE AREA. THIS LOW IS
EXPECTED TO TRANSITION FROM A POSITIVE TILT TO EITHER A NEUTRAL OR
SLIGHTLY NEGATIVE TILT AS IT MOVES OVER THE AREA SATURDAY
AFTERNOON. A NEGATIVE TILT TO AN UPPER TROUGH IS MORE CONDUCIVE
FOR THUNDERSTORM DEVELOPMENT AS UPPER DIFFLUENCE COMMENCES AHEAD
OF THE TROUGH. MODELS ARE FORECASTING THE CENTER OF THE STRONGEST
LIFT OF THE UPPER LOW TO MOVE ACROSS THE HEART OF OUR CWA AS IS
EVIDENT BY THE PEAK TROPOPAUSE FOLD TO 430 MB OVER SAN ANTONIO
SATURDAY NIGHT. THIS SHOULD EQUATE TO SUBSTANTIAL UPPER FORCING
FOR ASCENT AND WILL LEAD TO WIDESPREAD SHOWER AND THUNDERSTORM
ACTIVITY ACROSS MUCH OF IF NOT ALL THE CWA. THE QUESTION THEN
TURNS TO THE INSTABILITY/SHEAR PARAMETERS AND ANY MESOSCALE
FEATURES THAT COME INTO PLAY FOR SEVERE WEATHER CHANCES.

MODELS ARE IN FAIRLY GOOD AGREEMENT THAT THERE WILL BE AROUND 750
TO 1500 J/KG OF SURFACE BASED CAPE ACROSS MUCH OF THE AREA
SATURDAY. THIS AMOUNT OF INSTABILITY IN ADDITION TO 40 KNOTS OF
0-6 KM SHEAR WILL BE SUFFICIENT FOR ORGANIZED CONVECTION. THE MAIN
LIMITING FACTOR WILL BE THE AMOUNT OF ACTIVITY AND WHAT EFFECT
THAT WILL HAVE ON SEVERE CHANCES. IF THERE IS TOO MUCH OF A
COVERAGE OF ACTIVITY...IT WILL BE DIFFICULT TO REALIZE THE PEAK
INSTABILITY VALUES. NEVERTHELESS...THE MOST LIKELY SCENARIO WILL
BE WIDESPREAD ACTIVITY WITH EMBEDDED ROTATING THUNDERSTORMS AS THE
ACTIVITY MOVES FROM WEST TO EAST DURING THE DAY SATURDAY. LARGE
HAIL AND DAMAGING WINDS WILL BE THE MAIN THREATS. THE SHERB INDEX
IS A FORECAST TOOL TO HELP WITH THESE HIGH SHEAR/LOW CAPE EVENTS
AND CURRENT FORECASTED VALUES ARE ABOVE 1.0 WHICH IS FAVORABLE FOR
SEVERE CONVECTION...MAINLY IN THE FORM OF A QLCS OR INDIVIDUAL
LINE SEGMENTS. THIS SEEMS REASONABLE AND CAN SOMEWHAT BE SEEN ON
THE HIGH-RES MODELS. THE TORNADO THREAT IS NON-ZERO GIVEN THE
AMOUNT OF LOW-LEVEL SHEAR EXPECTED OVER THE AREA. WOULD LIKE TO
SEE MORE DISCRETE ACTIVITY AND A FOCUSED SURFACE BOUNDARY OVER THE
AREA TO EXPECT ANYTHING GREATER THAN ISOLATED TORNADOES.

MODELS MOSTLY AGREE THAT THE ACTIVITY WILL SHUT OFF FROM WEST TO
EAST PRETTY RAPIDLY IN THE AFTERNOON. BY 6 PM...THE BACK EDGE OF
THE CONVECTION SHOULD BE JUST EAST OF THE I-35 CORRIDOR. DRY WEST
WINDS FROM THE SURFACE TO 850 MB WILL MOVE IN AHEAD OF THE UPPER
LOW AND THIS WILL END THINGS. THE OUTLIER IS THE NAM WHICH IS
DEVELOPING ANOTHER LINE OF STORMS AROUND 6 PM WEST OF THE I-35
CORRIDOR...BUT THE NAM 850 WINDS ARE UNREALISTIC. IT KEEPS THE
WEST WIND SHIFT DIRECTLY UNDERNEATH THE 500 MB LOW...WHILE THE
REST OF THE GUIDANCE MEMBERS HAVE THE WIND SHIFT AHEAD OF THE LOW
WHICH IS MORE REALISTIC.

THE LAST THING TO FOCUS ON IS THE HEAVY RAIN POTENTIAL ACROSS THE
AREA. THE HIGHEST RAIN TOTALS WILL BE ACROSS THE EASTERN AND
NORTHERN ZONES. MOISTURE SHOULD POOL UP NICELY IN THAT CORRIDOR
AS A WEAK SURFACE LOW DEVELOPS JUST NORTH OF OUR CWA. PW VALUES
WILL BE AROUND 1.5 OR ABOUT 2 STANDARD DEVIATIONS ABOVE THE MEAN
FOR THIS TIME OF THE YEAR. THESE AREAS WILL LIKELY RECEIVE 1 TO 2
INCHES OF RAIN WITH ISOLATED LOCATIONS PICKING UP 4 INCHES.

LONG TERM (SATURDAY NIGHT THROUGH THURSDAY)...
THE REST OF THE THUNDERSTORM ACTIVITY WILL BE EXITING THE AREA BY
MIDNIGHT SATURDAY NIGHT. DRY WEST WINDS WILL BE LEFT IN PLACE
SUNDAY AND THIS SHOULD BE A WARM AND SUNNY DAY FOR THE AREA WITH
HIGHS REACHING LOWER TO MIDDLE 70S. A LONGWAVE TROUGH WILL PASS
THROUGH THE PLAINS SUNDAY AND WILL SEND A DRY FRONT INTO THE
REGION BY MONDAY MORNING COOLING THINGS OFF ONCE AGAIN. THE REST
OF THE WEEK SHOULD REMAIN DRY WITH HIGHS IN THE 60S AND LOWS IN
THE 30S/40S. THANKSGIVING LOOKS DRY AND SEASONAL.

HAMPSHIRE

&&

.PRELIMINARY POINT TEMPS/POPS...
AUSTIN CAMP MABRY              72  63  70  55  76 /  50  50 100  20  -
AUSTIN BERGSTROM INTL AIRPORT  73  62  71  53  77 /  50  50 100  30  -
NEW BRAUNFELS MUNI AIRPORT     73  64  73  53  77 /  50  50  80  20  -
BURNET MUNI AIRPORT            71  61  68  51  74 /  30  50 100  10  -
DEL RIO INTL AIRPORT           71  62  74  50  77 /  -   40  50  10   0
GEORGETOWN MUNI AIRPORT        72  62  69  53  75 /  50  50 100  30  -
HONDO MUNI AIRPORT             72  62  73  48  75 /  20  50  60  10  -
SAN MARCOS MUNI AIRPORT        72  63  72  52  77 /  50  50  90  20  -
LA GRANGE - FAYETTE REGIONAL   73  63  74  57  76 /  60  30  90  60  -
SAN ANTONIO INTL AIRPORT       73  65  74  54  77 /  30  50  70  10  -
STINSON MUNI AIRPORT           74  65  75  53  77 /  30  50  70  10  -

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

MESOSCALE/AVIATION...03
SYNOPTIC/GRIDS...29




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