Area Forecast Discussion
Issued by NWS Dallas/Fort Worth, TX

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990
FXUS64 KFWD 220456
AFDFWD

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
1156 PM CDT THU MAY 21 2015

.AVIATION...
THE PREVIOUS THINKING REMAINS THE SAME...WITH THE EXCEPTION OF THE
INTRODUCTION OF VFR CIGS OVER THE DFW AREA TERMINALS IN THE SHORT
TERM. THESE CIGS ARE EXPECTED TO BE SHORT LIVED...AS MVFR FILTERS
BACK INTO THE AREA WITHIN THE NEXT HOUR OR TWO.

AJS

.../PREVIOUS AVIATION DISCUSSION/...
/ISSUED 719 PM CDT THU MAY 21 2015/
THE MAIN AVIATION CONCERNS FOR THIS FORECAST PERIOD ARE THREE-
FOLD: LOW CIGS THROUGHOUT THIS TAF CYCLE...A CHANCE FOR SHOWERS
ON FRIDAY...AND A RETURN TO SOUTHERLY FLOW.

WIDESPREAD MVFR CIGS REMAIN IN PLACE ACROSS MUCH OF NORTH
TEXAS...WITH THE EXCEPTION OF AREAS ALONG THE RED RIVER...WHO
CURRENTLY HAVE OBS IN THE VFR CATEGORY. REGARDLESS...EXPECT MVFR
TO PREVAIL FOR OUR TAF SITES THIS EVENING. SOME GUIDANCE HINTS AT
IFR CIGS COMING IN OVERNIGHT AND CLIMATOLOGY REVEALS THIS IS A
DEFINITE POSSIBILITY...AS WE WILL BE UNDER WEAK EASTERLY WINDS
DURING THE OVERNIGHT HOURS. THEREFORE...HAVE DECIDED TO PLACE IFR
CIGS IN THE DFW AREA TERMINALS BY 09Z AND FOR WACO BY 04Z. CIGS
SHOULD LIFT BACK TO MVFR BY LATE MORNING ON FRIDAY THOUGH.

AS FAR AS WINDS GO...WEAK NORTHEASTERLY WINDS ARE THE CURRENT
TREND...BUT EXPECT VEERING TO OCCUR OVERNIGHT AND WINDS TO WIND
UP SOUTHEASTERLY BY MIDDAY FOR THE DFW AREA TERMINALS. WACO WILL
BE SLOWER TO RESPOND TO THE APPROACHING SHORTWAVE AND MAY NOT END
UP IN SOUTHEASTERLY FLOW UNTIL FRIDAY EVENING.

AS HINTED AT A MOMENT AGO...A SHORTWAVE WILL APPROACH THE AREA
OVER THE NEXT 12 HOURS. THIS WILL BRING RENEWED LIFT FOR
CONVECTION. WHILE THE CONVECTION...CURRENTLY IN THE TEXAS
PANHANDLE THAT DEVELOPED IN RESPONSE TO THIS UPPER LEVEL
SUPPORT...WILL LIKELY STAY NORTHWEST OF THE TERMINALS DURING THE
OVERNIGHT HOURS...HAVE IT MENTIONED IN THE TAFS DURING THE
MORNING HOURS TOMORROW. THIS CORRESPONDS TO THE TIME WHEN THE
UPPER LEVEL SUPPORT WILL BE STRONGEST OVER OUR AREA. NOT EXPECTING
A WIDESPREAD RAIN EVENT DURING THIS TIME FRAME LIKE WAS OBSERVED
THIS MORNING...SO KEPT IT AT VCSH.

IN THE EXTENDED...CONDITIONS LOOK FAVORABLE FOR A WIDESPREAD RAIN
EVENT TO OCCUR THIS WEEKEND AND HEAVY RAIN AND OCCASIONAL
LIGHTNING WILL BE THE PRIMARY CONCERNS FOR THE TERMINALS. MORE
DETAILS TO COME AS TIME PROGRESSES.

AJS



&&

.UPDATE...

ISSUED A FLASH FLOOD WATCH FOR A LARGE PORTION OF NORTH TEXAS WITH
THE EVENING FORECAST UPDATE.

THE PREVIOUS FORECAST DISCUSSION HIGHLIGHTED MOST OF THE
SCIENTIFIC REASONING IN SUPPORT OF A POTENTIALLY HIGH IMPACT FLASH
FLOODING EVENT THIS WEEKEND ACROSS PARTS OF NORTH TEXAS, SO WILL
NOT REHASH THOSE EFFORTS HERE. THE REASONS WE WENT AHEAD WITH A
FLASH FLOOD WATCH ARE AS FOLLOWS:

1. FOLKS ARE MAKING PLANS FOR MEMORIAL DAY WEEKEND RIGHT NOW. WE
WANTED TO RAISE AWARENESS FOR THE POTENTIAL FOR HIGH IMPACT
FLOODING WHERE TENS OF THOUSANDS OF PEOPLE ARE LIKELY GOING TO BE
SPENDING TIME, OR PLANNING ON SPENDING TIME, THIS WEEKEND. WE
WANTED TO GET THIS OUT BEFORE THE EVENING NEWS CYCLE TO HELP RAISE
AWARENESS.

2. THE EVENING MODEL DATA CONTINUES TO SUPPORT THE OBSERVATIONS OF
THE DAY SHIFT`S DISCUSSION. THE LARGE SCALE ENVIRONMENT SEEMS VERY
SUPPORTIVE FOR A HEAVY RAINFALL EVENT ACROSS MUCH OF NORTH TEXAS
FROM SATURDAY AFTERNOON THROUGH SUNDAY EVENING.

3. EVENING MODEL DATA INDICATED THAT NORTH TEXAS WILL HAVE
PRECIPITABLE WATER VALUES OF 1.75 TO 2 INCHES FOR AT LEAST 36
HOURS. THESE VALUES RANK IN THE 99TH PERCENTILE OF ENVIRONMENTAL
CLIMATOLOGY FOR THIS REGION AND TIME OF YEAR.

4. WHILE LARGE SCALE DYNAMICS SUPPORT LIFT AND THE THERMODYNAMIC
ENVIRONMENT SUPPORTS HEAVY RAIN, THE CORFIDI VECTORS FROM SATURDAY
AFTERNOON THROUGH SUNDAY EVENING ARE LESS THAN 5 KTS. THIS VECTOR
CALCULATION SIMPLY REINFORCES THE POTENTIAL FOR TRAINING STORMS
THAT WILL BE EXTREMELY SLOW TO MOVE AWAY FROM A SPECIFIC AREA.
THESE VECTORS PREDICT STORM MOTION FOR CONVECTIVE STORM SYSTEMS
AS OPPOSED TO INDIVIDUAL STORM MOTIONS. INDIVIDUAL STORMS MAY MOVE
FROM SOUTH TO NORTH AT 30-35 MPH...HOWEVER THE CLUSTER OF STORMS
IS EXPECTED TO HARDLY MOVE AT ALL, WHICH IS WHAT THE 5 KT OR LESS
CORFIDI VECTOR CALCULATION HELPS PREDICT.

THE NEW MODEL DATA THAT CAME IN THIS EVENING COMBINED WITH THE
ANTICIPATED POTENTIAL SOCIETAL IMPACTS ASSOCIATED WITH MEMORIAL
DAY WEEKEND PROMPTED THE FLASH FLOOD WATCH AT THIS TIME. BECAUSE
THE WATCH DOESN`T EVEN GO INTO EFFECT UNTIL SATURDAY AFTERNOON, IT
IS VERY LIKELY THAT THE COVERAGE AREA OF THE WATCH WILL CHANGE
WITH TIME, AS WE GET A BETTER UNDERSTANDING ON EXACTLY WHERE THE
LARGE CLUSTER OF STORMS WILL SET UP AND SLOWLY DRIFT ON SATURDAY
AFTERNOON THROUGH SUNDAY EVENING.

AS THE PREVIOUS FORECAST MENTIONED, THERE IS A CHANCE FOR SEVERE
STORMS. HOWEVER THE LARGE SCALE ENVIRONMENT SUGGESTS THAT FLOODING
WILL BE A SIGNIFICANTLY GREATER CONCERN THAN SEVERE WEATHER AT
THIS TIME. A WIDESPREAD 1 TO 3 INCHES OF RAIN IS EXPECTED WITHIN
THE WATCH AREA. ISOLATED RAINFALL TOTALS OF UP TO 6 INCHES ARE
POSSIBLE, AND THOSE SMALLER POCKETS OF HEAVIER RAINFALL ARE
LIKELY TO RESULT IN HIGH IMPACT FLASH FLOODING.

RAINFALL IS STILL EXPECTED LATE TONIGHT INTO FRIDAY MORNING AS
WELL. AT THIS TIME, THE FRIDAY MORNING RAINFALL DOES NOT LOOK
INTENSE ENOUGH TO RESULT IN WIDESPREAD FLOODING, SO DID NOT START
THE FLASH FLOOD WATCH TOMORROW MORNING.

CAVANAUGH

&&

.PREV DISCUSSION... /ISSUED 329 PM CDT THU MAY 21 2015/
THE CONVECTION THAT FIRED UP THIS MORNING HAS DISSIPATED AND
DRIER AIR ALOFT CONTINUES TO MOVE INTO THE REGION. NORTHERLY WINDS
AND CLOUD COVER HAVE KEPT TEMPERATURES UNSEASONABLY COOL ACROSS
THE AREA TODAY. SURFACE WINDS WILL BECOME MORE EASTERLY TONIGHT
AND WITH THE CONTINUED CLOUD COVER...LOW TEMPS WILL NOT FALL
MUCH...BUT STILL EXPECT BELOW NORMAL READINGS IN THE UPPER 50S TO
LOWER 60S. WINDS ABOVE THE SURFACE WILL BECOME SOUTHEASTERLY
OVERNIGHT WHICH WILL BEGIN TO BRING THE MOISTURE BACK NORTHWARD
AND RESULT IN WEAK ISENTROPIC LIFT. THIS IS ALL IN RESPONSE TO A
SHORTWAVE TROUGH THAT WILL TRACK GENERALLY NORTHWEST OF THE
REGION...THROUGH WEST TEXAS AND INTO OKLAHOMA...FRIDAY MORNING.
LIFT AHEAD OF THIS SYSTEM SHOULD RESULT IN INCREASING
SHOWERS/STORMS ACROSS WESTERN PARTS OF THE CWA EARLY FRIDAY
MORNING...WITH ACTIVITY GENERALLY SPREADING NORTHEASTWARD AND
AFFECTING THE NORTHERN PORTIONS OF THE CWA DURING THE DAY. WILL
RETAIN 30-40 POP FOR THE REMAINDER OF THE NORTH TEXAS AREA FOR
SCATTERED SHOWERS AND STORMS THAT MAY DEVELOP DURING THE DAY ALONG
THE TAIL END OF THE SHORTWAVE TROUGH AXIS. THIS ACTIVITY WILL BE
ELEVATED SO THE SEVERE WEATHER THREAT IS VERY LIMITED...BUT
LOCALLY HEAVY RAINFALL MAY LEAD TO MINOR FLOODING ISSUES IN
SATURATED AREAS. HIGHS TOMORROW WILL RETURN TO THE 70S GENERALLY
AREA WIDE AS SOUTHERLY WINDS BRING WARMER AND MORE HUMID AIR
NORTHWARD. IN THE WAKE OF THE SHORTWAVE TROUGH FRIDAY
NIGHT...SUBSIDENCE WILL OVER SPREAD THE REGION WITH GENERALLY
DRY WEATHER EXPECTED.

A CLOSED UPPER LEVEL LOW WILL MOVE INTO THE FOUR CORNERS REGION
ON SATURDAY...AND SET THE STAGE FOR A POTENTIALLY SIGNIFICANT
FLOODING EVENT OVER THE REGION SATURDAY NIGHT THROUGH SUNDAY
EVENING. IN ADDITION TO THE HEAVY RAINFALL THREAT...SEVERE WEATHER
WILL BE POSSIBLE DUE TO AMPLE INSTABILITY AND SUFFICIENT WIND
SHEAR. UNLIKE THE LAST FEW ROUNDS OF CONVECTION...THE SHEAR
PROFILES ARE MORE UNIDIRECTIONAL WHICH WOULD HELP TO LIMIT THE
TORNADO THREAT...BUT THIS TYPE OF WIND PROFILE INCREASES THE
POTENTIAL FOR QUASI-STATIONARY NORTH-SOUTH ORIENTED BANDS WITH
CELL TRAINING. EXTREME AMOUNTS OF RAINFALL ARE POSSIBLE DUE TO
UNSEASONABLY HIGH PRECIPITABLE WATER THAT WILL BE IN PLACE AND THE
40-50KT LOW LEVEL FLOW WHICH WOULD PROVIDE EXCELLENT MOISTURE
FLUX TO REPLACE THE WATER VAPOR LOST TO PRECIPITATION.

IT IS IMPOSSIBLE TO PINPOINT THE EXACT LOCATION OF WHERE THESE
BANDS SET UP...BUT WE CAN SAY THAT THE NW CWA IS THE MOST LIKELY
REGION WHERE 6+ INCHES OF RAIN COULD FALL...WITH AVERAGE VALUES OF
3-5 INCHES NORTHWEST OF A COMANCHE TO FORT WORTH SHERMAN LINE. BY
SUNDAY MORNING...A COLD POOL SHOULD DEVELOP WITH THE BAND OF RAIN
AND HELP TO PUSH IT SLOWLY EASTWARD ACROSS THE CWA. RAINFALL
AMOUNTS WILL PROBABLY AVERAGE 2-4 INCHES ACROSS THE CENTRAL AND
SOUTHWEST ZONES..AND 1-3 INCHES EAST OF THE I-35 CORRIDOR. SOILS
ARE SATURATED OVER THE REGION...AND MOST OF THE RAIN WILL RUN OFF
LEADING TO FLASH FLOODING. IN ADDITION...MANY OF THE LAKES AND
RESERVOIRS ARE NEARLY FULL AND WILL NOT BE ABLE HOLD ANY MORE
WATER WHICH COULD LEAD TO SIGNIFICANT RIVER FLOODING. A FLASH
FLOOD WATCH WILL BE ISSUED FOR THE WEEKEND TIME PERIOD...JUST IT
IS A LITTLE TOO EARLY TO DO SO AT THIS TIME.

THE MAIN TROUGH AXIS AND PRIMARY BAND OF RAINFALL WILL SHIFT INTO
EAST AND SOUTHEAST TEXAS SUNDAY NIGHT. THIS PROBABLY MEANS THAT
THE HEAVY RAINFALL THREAT WILL COME TO AN END FROM WEST TO EAST
SUNDAY EVENING. WILL MAINTAIN HIGH POPS INTO SUNDAY NIGHT ONLY
BECAUSE THE ECMWF IS A LITTLE SLOWER IN CLEARING THINGS OUT. ON
MONDAY SIGNIFICANT CONVECTION IS EXPECTED TO PERSIST ALONG THE
TEXAS COAST AND INTO LOUISIANA WHICH SHOULD HELP TO STABILIZE
NORTH TEXAS. POPS WILL REMAIN HIGHEST OVER THE EASTERN ZONES WHERE
SOME LIGHT RAIN FROM STORM ANVILS MAY CONTINUE INTO THE DAY. IN
ADDITION...THE WESTERN ZONES BECOME VERY UNSTABLE DURING THE
DAY...WHICH MAY RESULT IN ISOLATED-SCATTERED STORMS POPPING UP
DURING THE AFTERNOON HOURS. DEEP LAYER SHEAR LOOKS TOO LIMITED FOR
AN ORGANIZED SEVERE WEATHER THREAT...BUT HIGH INSTABILITY MAY
ALLOW FOR SEVERE PULSE TYPE CONVECTION.

OTHERWISE TEMPERATURES WILL BEGIN WARMING INTO THE LOW-MID 80S
NEXT WEEK WITH LOWS IN THE UPPER 60S AND LOWER 70S. A LOW CHANCE
FOR MAINLY AFTERNOON AND EARLY EVENING SHOWERS AND STORMS WILL
CONTINUE TUESDAY THROUGH THURSDAY AS WEAK TROUGHING REMAINS WEST
OF THE REGION. LIMITED DEEP LAYER WIND SHEAR WILL KEEP THIS
ACTIVITY UNORGANIZED...BUT ISOLATED POCKETS OF HEAVY RAINFALL IS
POSSIBLE.

TR.92



&&


.PRELIMINARY POINT TEMPS/POPS...
DALLAS-FT. WORTH, TX  60  73  67  81  67 /  20  40  20  50  70
WACO, TX              61  78  69  82  68 /  20  30  20  40  60
PARIS, TX             58  70  61  81  68 /  20  40  20  40  70
DENTON, TX            59  72  66  80  66 /  20  50  20  60  80
MCKINNEY, TX          59  72  65  80  67 /  20  40  20  50  70
DALLAS, TX            61  75  68  83  67 /  20  40  20  40  70
TERRELL, TX           60  75  67  83  68 /  20  40  20  40  60
CORSICANA, TX         61  78  69  83  68 /  20  30  20  40  50
TEMPLE, TX            63  78  70  82  69 /  20  30  20  40  60
MINERAL WELLS, TX     59  74  67  80  66 /  40  50  20  60  90

&&

.FWD WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM SATURDAY AFTERNOON THROUGH SUNDAY EVENING
FOR TXZ091>095-100>107-115>120-129>133.


&&

$$



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