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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1242Z Apr 16, 2015)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
842 AM EDT THU APR 16 2015

...VALID 12Z THU APR 16 2015 - 12Z FRI APR 17 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 SSW MGM 10 E DHN 25 SSE OZR 10 ESE NSE 15 SW JKA 20 S KMIS
15 NE S58 35 E KCRH BYY 40 NNW VCT CLL 15 SSE AEX 40 WNW PIB
45 ESE NMM 25 SSW MGM.


...GULF COAST STATES...

A SLOW RETURN FLOW OF RICH GULF MOISTURE...WITH PRECIPITABLE WATER
VALUES (PWS) ABOVE 1.70 INCHES...WILL INTERACT WITH A WARM FRONT
ACROSS THIS REGION. INSTABILITY WILL BE STUNTED WITH EASTWARD
EXTENT BY THE INTRUSION OF DRIER LOW LEVEL AIR AROUND A SURFACE
RIDGE IN AL/GA. INSTABILITY OVERALL IS LACKING DUE TO POOR MID
LEVEL LAPSE RATES...BUT MID/UPPER LEVEL DIVERGENCE WILL COMPENSATE
WHERE THE SOUTHERN STREAM OF THE POLAR JET AND SUBTROPICAL JET
SPLIT. AN INCREASING THREAT OF FAIRLY WIDESPREAD THUNDERSTORMS IS
EXPECTED ACROSS MUCH OF THE GULF COAST TODAY AS A SHORTWAVE TROUGH
ARRIVES OUT OF SOUTH TEXAS AND UPPER JET LEVEL FLOW FORMS AN
INFLECTION POINT RESULTING IN JET COUPLING OVER LA/MS/AL THROUGH
THIS EVENING. THE 850 HPA INFLOW IS RATHER UNFOCUSED AND FORECAST
TO REMAIN 25 KTS OR LESS BY THE GLOBAL GUIDANCE...ALTHOUGH CLOSER
TO 35 KTS IN RECENT RAP RUNS BY AFTERNOON. THESE VALUES ARE
SIMILAR/SLIGHTLY GREATER IN MAGNITUDE TO THE 850-400 HPA MEAN
WIND.  WET BULB ZEROES ARE UP NEAR 12,000 FT...SO WARM RAIN
PROCESSES ARE EXPECTED. A BETTER DEFINED MID LEVEL SHORTWAVE IS
PREDICTED TO CONTRIBUTE TO ORGANIZATION OF THIS RAINFALL OVER
LOUISIANA. IT WAS THERE THAT WE HAD THE GREATEST CONFIDENCE TO
PLACE SOME HEAVIER AMOUNTS OF DETERMINISTIC AREAL AVERAGE
PRECIPITATION. CONVECTION INITIATING WITHIN THE WARM ADVECTION
REGIME ALONG THE TX/LA COAST EARLY IN THE DAY MAY GROW UPSCALE AND
MOVE EAST.  WEAK LOW LEVEL INFLOW MAY ENCOURAGE MORE OUTFLOW
DOMINANCE AND SPREADING OF THE HEAVIEST CONVECTION INTO THE GULF
RATHER THAN INLAND AREAS OF MS/AL, WHICH HAS BEEN SEEN ON PREVIOUS
DAYS. THE 12Z WPC QPF IN THIS REGION WAS SPATIALLY SIMILAR TO OUR
IN-HOUSE PSEUDO BIAS-CORRECTED ENSEMBLE, AS WELL AS THE 00Z RUNS
OF THE WRF4NSSL, SPCWRF, AND NAM CONEST RUNS, BUT MORE
CONSERVATIVE ON AMOUNTS. HOURLY RAINFALL RATES OF 2" REMAIN
POSSIBLE HERE CONSIDERING THE AVAILABLE MOISTURE. ALTHOUGH NOT
REFLECTED IN THE FLASH FLOOD GUIDANCE...WHICH TENDS TO RECOVER
QUICKLY...THE COASTAL AREAS FROM TX TO AL HAVE RECEIVED MUCH ABOVE
NORMAL PRECIPITATION OVER THE PAST SEVEN DAYS...AND MAY BE A
LITTLE MORE SUSCEPTIBLE TO SURFACE RUNOFF.

ROTH/BURKE