Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 030833
QPFERD

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

...VALID 12Z 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
30 E KMIU 10 WNW KMIU 45 N ALI 10 NW ERV 35 N 6R9 15 SSW PRX
15 NE M89 20 SSE UTA 35 SW TUP 45 WSW GTR 25 SE TVR 15 SSE ACP
30 W KCRH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNW KMZG 20 SSW PKV 15 SW PKV 40 SW 3T5 25 ENE GTU 15 S CRS
30 ESE OSA 20 WNW LLQ 10 NW GLH 40 NNE TVR 25 NNW HEZ 10 ESE DRI
15 SSE BPT 25 SSE KXIH.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SE SGR 10 S UTS 40 SW PSN 15 WNW JSO 20 WNW DTN 25 S ELD
10 WNW MLU 35 N ESF 10 SW POE 25 NNE KXIH 20 SSW GLS 15 ESE LBX
10 SE SGR.


...SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...

...MODERATE RISK OF EXCESSIVE RAINFALL ACROSS EASTERN TEXAS INTO
NORTHWEST LOUISIANA...

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 WEST-NORTHWEST
LA...SOUTHERN AR...AND NORTHWEST MS. A LARGE AND DEEP TROUGH
CONTINUES TO EVOLVE OVER THE SOUTHWESTERN U.S. AND NORTHWEST
MX...WITH A MID LEVEL CIRCULATION NOW NOTED OVER THE NORTHERN SEA
OF CORTEZ PER THE LATEST SATELLITE WV IMAGERY. HEIGHTS WILL NOT BE
TERRIBLY LOW...BUT WITH THE TROUGH DIGGING TO SUCH A LOW
LATITUDE...THE 500 MB HEIGHT ANOMALIES DIP TO A RARELY SEEN MINUS
5.0. 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...MULTI-DAY PRECIPITATION SHIELD.
DESPITE THE INHERENT UNCERTAINTY THERE IS INCREASING MODEL
CONSENSUS AS TO SOME OF THE DETAILS. WPC QPF WAS BASED ON A NEAR
EVEN BLEND OF CONTINUITY (PREVIOUS FORECAST) ALONG WITH THE 00Z
GFS / NSSL-WRF / AND OUR IN-HOUSE ENSEMBLE BIAS-CORRECTED QPF. 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.

AS WAS NOTED YESTERDAY...SUCH A HIGHLY-FAVORABLE THERMODYNAMIC
SETUP COUPLED WITH A DYNAMIC COOL-SEASON SYNOPTIC SETUP WILL ALLOW
FOR AN EXPANSIVE AREA OF HEAVY RAINFALL...THOUGH AGAIN EXPECT
ELEVATED CONVECTION TO POTENTIALLY DISRUPT WHAT WILL OTHERWISE BE
OPTIMAL DEEP-LAYER MOIST ISENTROPIC ASCENT WELL NORTH OF THE
SURFACE BOUNDARY. THIS ESPECIALLY NEAR THE GULF COAST WITH THE
HIGHER DEEP-LAYER INSTABILITY (MUCAPES ABOVE 1000 J/KG) AVAILABLE
JUST OFFSHORE. EXCEPT FOR THE NAM (WELL NORTH WITH THE MAX
RAINFALL AXIS)...THE GLOBAL AND HIGH-RES CAM GUIDANCE ARE A BIT
BETTER CLUSTERED IN DEPICTING A MORE CONSOLIDATED AREA OF HEAVY
RAINFALL DURING THE NEW DAY 1 PERIOD...WITH THE AFOREMENTIONED
TROUGH AMPLIFICATION AND STRENGTHENING S-N THETA-E AND MOISTURE
SURGE INDICATIVE OF LESS CONVECTIVE INTERFERENCE FARTHER NORTH
WITH TIME. HOWEVER...STILL LIKE THE IDEA PER THE LATEST CAMS OF
CONFINING THE HIGHER PROBABILITIES OF >5 INCHES FARTHER SOUTH
ALONG 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. CHANGES
TO THE PREVIOUS WPC QPF WERE MINOR WITH RESPECT TO THE MAGNITUDE
OF FORECAST RAINFALL...WITH A SWATH OF 3-4+ INCH AREAL-AVERAGE
TOTALS STILL ON TARGET ACROSS THE MIDDLE AND UPPER TX
COAST...NORTH TO AROUND LONGVIEW IN EASTERN TX...THEN EASTWARD
INTO NORTHWEST LA. THE EXPANSE OF THE 1+ INCH QPF IS RATHER
IMPRESSIVE...EXTENDING WESTWARD INTO SOUTH-CENTRAL TX AND AS FAR
EAST AS NORTHWEST AL.

IN TERMS OF THE DAY 1 EXCESSIVE RAINFALL THREAT...BASED ON BOTH
THE DETERMINISTIC QPF GUIDANCE (INCLUDING HIGH-RES CAMS) ALONG
WITH THE PROBABILITIES OF EXCEEDANCE PER THE ENSEMBLES...HAVE
OPTED TO EXPAND THE "MODERATE" RISK AREA FARTHER SOUTH AND EAST
FROM THE PREVIOUS (YESTERDAY`S DAY 2) ISSUANCE...TO INCLUDE MUCH
OF THE UPPER TX COAST (INCLUDING GLS) AND AREAS FARTHER EAST
ACROSS NORTHWEST LA TOWARD MLU AND POW. RELATIVELY HIGHER FFG
VALUES INITIALLY OVER THESE AREAS (2-3" IN 3 HOURS) ARE EXPECTED
TO LOWER DURING THE DAY...PERHAPS SIGNIFICANTLY IN SOME
LOCATIONS...GIVEN THE ANTICIPATED MULTIPLE ROUNDS OF HEAVY
RAINFALL.

HURLEY
$$




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