Quantitative Precipitation Forecast
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478
FOUS30 KWBC 040101
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
800 PM EST SAT DEC 03 2016

...VALID 01Z SUN DEC 04 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
55 ESE KMIU 35 SSE KOPM 10 E HRL EBG 25 WNW MFE 50 WNW MFE
60 SSE MMNL 35 S MMNL MMNL 20 NNW LRD 15 SW COT 30 NE COT
10 SE SSF 15 E HYI 15 W 11R 30 E CXO JAS IER 20 NW TVR
20 NNW HKS 15 ENE JAN 30 WNW PIB 30 N HSA 15 SE NBG 20 WSW 9F2
KEIR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SSE KBBF 25 ESE KMIU KOPM 30 N HRL 20 SSE BKS 15 WSW BKS
15 SSW HBV 25 W HBV 20 ENE LRD 30 SE COT 35 E COT 45 SE SSF
30 SSW 3T5 30 ESE 3T5 15 NNW SGR 30 ENE IAH 25 SSE JAS
25 NNE ESF 10 N HEZ 30 SSE HEZ 25 SW BTR 20 SSW KVNP.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WSW 7R4 10 N KCMB KVBS 20 WSW GLS 10 SW LVJ 15 NE EFD BPT
15 ESE DRI 20 ESE ESF 35 NW BTR 25 NE LFT 10 WSW 7R4.


01Z UPDATE...

VERY HEAVY RAINS CONTINUE TO IMPACT PORTIONS OF THE UPPER TX COAST
AND SOUTHWEST LA AS A STRONG DUAL FETCH OF MOISTURE INVOLVING THE
EASTERN TROPICAL PACIFIC AND THE GULF OF MEXICO CONVERGE ON THE
AREA...AND OUT AHEAD OF WHAT CONTINUES TO BE A STRONG UPPER
LOW/TROUGH OVER NORTHWEST MX WHICH IS SLOWLY MOVING EAST. A
FRONTAL ZONE JUST OFF THE MIDDLE/UPPER TX COASTA AND SOUTHERN LA
WILL REMAIN A FOCUS FOR STRONG SURFACE CONVERGENCE OVERNIGHT AND
SHOULD REMAIN CONVECTIVELY ACTIVE GIVEN LARGE SCALE 250 MB
RIGHT-ENTRANCE REGION JET DYNAMICS AND PROXIMITY OF A WELL-DEFINED
INSTABILITY GRADIENT. THE DUAL FETCH OF MOISTURE AS IDENTIFIED IN
CIRA-LPW IMAGERY SUPPORTS ENHANCED RAINFALL EFFICIENCY WITH RATES
THAT COULD APPROACH 2 INCHES/HR. THE BIGGER CONCERN IS MORE IN
LINE WITH THE POTENTIAL FOR PERSISTENT/TRAINING SHOWERS AND
THUNDERSTORMS AS THE CONVECTION BECOMES BETTER ALIGNED WITH THE
MEAN FLOW OUT OF THE SOUTHWEST AHEAD OF THE TROUGH. MPD #784
ADDRESSES THE PRIMARY FOCUS FOR EXCESSIVE RAINS AND FLASH FLOODING
OVER THE NEXT SEVERAL HOURS. THE MODERATE RISK AREA CONFORMS WITH
THE LATEST MPD THREAT AREA AND THE GREATEST POTENTIAL FOR
TRAINING. THE SLIGHT RISK AREA HAS BEEN ADJUSTED TO HIGHLIGHT
WHERE THE LATEST HRRR GUIDANCE FOCUSES THE MAIN AREAS OF
ADDITIONAL HEAVY RAIN POTENTIAL OTHERWISE

...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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