Quantitative Precipitation Forecast
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FOUS30 KWBC 031440
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
939 AM EST SAT DEC 03 2016

...VALID 15Z SAT DEC 03 2016 - 12Z SUN DEC 04 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 ESE KOPM 20 N PIL 20 NNW EBG 15 NNE HBV 35 SE COT COT
40 W COT 10 NW UVA 15 NW GTU 20 SE CRS 10 NE LLQ 20 W TUP
15 SSE TUP 30 WNW GTR 25 E TVR 25 SE HEZ 25 ENE LFT KEHC
25 ESE KVAF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE KBBF 20 NNE KMIU NGP NQI 10 SSW ALI 25 N RBO 35 WNW 5R5
11R 35 WNW LFK 10 ESE BAD 15 SW GLH 10 SSW GWO 20 W TVR
40 SSW HEZ 25 WNW 7R4 15 NNE KCRH 30 SW KCRH.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WSW JAS 20 NW POE AEX 35 S ACP 7R5 GLS 30 ENE IAH 15 WSW JAS.


...CENTRAL AND SOUTHEAST TEXAS INTO THE LOWER MISSISSIPPI VALLEY...

...MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS AND
SOUTHWEST LOUISIANA...

VERY HEAVY RAIN WAS ONGOING AND IS EXPECTED TO PERSIST OFF AND ON
THROUGH TONIGHT...PARTICULARLY IN THE AREA STRETCHING BETWEEN
HOUSTON TX AND LAKE CHARLES LA. MOST MODEL GUIDANCE WAS HANDLING
THE DEEP CONVECTION NEAR THE COAST WELL...BUT WAS TOO FAR NORTH
WITH THE FRONTOGENETICALLY FORCED BAND OF MODERATE TO HEAVY RAIN
FARTHER INLAND. OVER TIME...IT LOOKS AS THOUGH THE 00Z WRF-ARW AND
06Z GEM REGIONAL MODELS ARE CLOSEST TO REALITY IN HANDLING BOTH
AREAS...WITH ADDITIONAL SUPPORT FROM THE GFS. RAIN RATES ALONG THE
COAST HAVE BEEN IMPRESSIVE...WITH A NEW DAILY RECORD ALREADY SET
AT GALVESTON WHERE MORE THAN 3 INCHES HAS FALLEN. VERY HEAVY RAIN
ALSO FELL IN THE SAN ANTONIO AREA...WITH MORE THAN 4 INCHES
OBSERVED...BUT AS A SHORTWAVE TROUGH PUSHES EASTWARD THE DEEP
LAYER ASCENT WILL MOVE FARTHER AWAY FROM THE INSTABILITY
GRADIENT...ESPECIALLY OVER NORTHEAST TX AND NORTHWEST LA. THE MORE
PERSISTENT OVERLAP OF DEEP LAYER ASCENT WITH MODERATE INSTABILITY
FOR PARCELS IN THE EFFECTIVE INFLOW LAYER...WILL OCCUR OVER
SOUTHEAST TX AND SOUTHWEST LA...IN LINE WITH THE AFOREMENTIONED
MODEL QPFS. RAIN RATES WITHIN THE FRONTOGENETICALLY FORCED BAND
EAST OF SAN ANTONIO HAD DECREASE CONSIDERABLY WITH EASTWARD
EXTENT...AND FOR THESE REASONS WE CHOSE TO REPOSITION THE MODERATE
RISK AREA...CONFINING IT TO COASTAL TX/LA. ELSEWHERE SOME RISK OF
EXCESSIVE RAINFALL REMAINS...BUT WITH A REDUCTION OF THE POINT
PROBABILITIES GIVEN THE SCARCITY OF INSTABILITY AND LESSER
LIKELIHOOD OF INTENSE SHORT TERM RATES. WE DID EXPAND THE SLIGHT
RISK SOUTHWESTWARD...HOWEVER...INTO THE CORPUS CHRISTI
AREA...WHERE OUR PREFERRED MODELS POINT TO REDEVELOPMENT OF
VIGOROUS CONVECTION IN THE OVERNIGHT PERIOD AS THE UPPER LOW
BEGINS TO SWING EASTWARD. PLEASE REFERENCE ACTIVE MESOSCALE
PRECIPITATION DISCUSSIONS FOR FURTHER DETAILS.


...PREVIOUS DISCUSSION...

WIDESPREAD SOAKING RAIN REMAINS ON TARGET DURING THE DAY 1
PERIOD...THE HEAVIEST OF WHICH IS EXPECTED ACROSS EASTERN TX
(INCLUDING THE MIDDLE TO UPPER TX COAST) INTO WESTERN LA.
UNUSUALLY RICH LOW-LEVEL MOISTURE WORKING UP FROM THE WESTERN GULF
OF MEXICO...COMBINED WITH A SUB-TROPICAL FEED WITHIN SOUTHWESTERLY
FLOW ALOFT...WILL BOOST PRECIPITABLE WATER VALUES TO WELL ABOVE
AVERAGE FOR DECEMBER... WIDESPREAD 1.5 TO 2.5 STANDARD DEVIATION
PW VALUES...WITH THE ABSOLUTE MAGNITUDES OF 1.75-2.00" ALONG THE
TEXAS COAST RANKING NEAR THE RECORD DECEMBER VALUES. AS WE HAVE
ALREADY SEEN EARLY THIS MORNING...MESOSCALE INFLUENCES WILL LIKELY
RESULT IN SEVERAL AREAS OF ENHANCED PRECIPITATION EMBEDDED WITHIN
THE LARGER PRECIPITATION SHIELD. THE AMPLIFICATION OF THE WARM
CONVEYOR BELT (WCB) WILL LEAD TO STRENGTHENING AND DEEPENING
SOUTHERLY FLOW AND MOISTURE FLUX IN THE LOWER-MID LAYERS...AS
850-700 MB MOISTURE FLUX ANOMALIES CLIMB BETWEEN +3 AND +4
STANDARD DEVIATIONS OVER THE WESTERN GULF COAST REGION. THE EXTENT
TO WHICH THIS FAVORABLE INFLOW PENETRATES INLAND...HOWEVER...WILL
BE INHIBITED BY ONGOING CONVECTION AS WELL AS VEERING WINDS
ASSOCIATED WITH A LOW AMPLITUDE MID LEVEL WAVE PUSHING THROUGH
EASTERN TX THIS MORNING.

WE STILL LIKE THE IDEA PER THE LATEST HI-RES MODELS OF CONFINING
THE HIGHER PROBABILITIES OF >5 INCHES FARTHER SOUTH ALONG THE GULF
COAST...I.E. CLOSER TO THE SURFACE FRONT AND WHERE THE
THERMODYNAMIC PROFILE WILL BE MORE FAVORABLE (NEAR THE AXIS OF
GREATER INSTABILITY AND HIGHER PWS). A BEVY OF HIGH-RES CAMS
INDICATE 24 HOUR TOTALS OF 5-8+ INCHES OVER THIS
REGION...INCLUDING THE NSSL-WRF...WRF-ARW...AND WRF-NNMB.

BURKE/HURLEY
$$




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