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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1901Z Dec 18, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 PM EST THU DEC 18 2014

...VALID 18Z THU DEC 18 2014 - 00Z SAT DEC 20 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...



...TX/LA GULF COAST REGION INTO THE FAR SOUTHERN MS VALLEY...

THERE WILL BE A RELATIVELY SHORT WINDOW OF OPPORTUNITY FOR LOCALLY
HEAVY RAINFALL WITH POTENTIAL RUNOFF ISSUES DURING DAY 1 ALONG AND
JUST NORTH OF THE GULF COAST FROM EASTERN TX INTO FAR SOUTHERN
PORTIONS OF THE LOWER MS VALLEY. THIS AS A FAIRLY ROBUST INFLUX OF
SUBTROPICAL MOISTURE (PRECIPITABLE WATER VALUES CLIMBING TO 1.5+
INCHES) WITH THE SOUTHWEST FLOW ALOFT AHEAD OF THE SHORTWAVE
TROUGH CURRENTLY ACROSS THE SOUTHWEST AND WESTERN MEXICO. THE
FAVORABLE SOUTHERN STREAM FORCING... SPECIFICALLY THE WAA/WARM
CONVEYOR BELT DYNAMICS AND ATTENDANT LOW LEVEL MOISTURE FLUX FROM
THE WESTERN GULF OF MEXICO...WILL COUPLE WITH THE INCREASING UPPER
LEVEL RIGHT ENTRANCE REGION FORCING (LOW-MID LEVEL FRONTOGENESIS)
VIA THE NORTHERN JET AXIS TO FAVOR A FAIRLY ELONGATED AREA OF
MOD-HEAVY RAINFALL AMOUNTS. WHILE THE PRECIPITABLE WATER
AVAILABILITY (ABOVE THE 75TH PERCENTILE ACCORDING TO CLIMO) WILL
BE SUPPORTIVE FOR POTENTIALLY EXCESSIVE SHORT TERM RAINFALL... THE
DEGREE/DEPTH OF INSTABILITY (OR LACK THEREOF) ALONG WITH THE
STRENGTH OF THE DEEP-LAYER SHEAR WILL LIMIT THE FLASH FLOODING
THREAT. SPECIFICALLY...THE MODELS SHOW MUCAPES LESS THAN 500 G/KG
(HIGHER OVER THE GULF)... WHILE THE STRONG VERTICAL SHEAR WILL
PROMOTE GOOD FORWARD/DOWNWIND PROPAGATION AND THUS FAIRLY
PROGRESSIVE CELL MOTION WITH THE HEAVIER RAIN BANDS. THE UPSHOT IS
AREAL-AVERAGE RAINFALL AMOUNTS BETWEEN 1.5 AND 3.0 INCHES OVER A
FAIRLY BROAD AREA FROM EASTERN TX NEAR THE GULF EASTWARD THROUGH
CENTRAL LA AND SOUTHWEST PORTIONS OF MS. HOWEVER... PER THE
ENSEMBLE OF HIGHER RESOLUTION GUIDANCE...THERE IS NOT MUCH
STANDARD DEVIATION AMONG THE MEAN ACROSS THE OUTLOOK AREA...WHILE
THE MODELS CONTINUE TO STRUGGLE WITH THE AXIS OF HEAVIEST RAINFALL
(LOWER RES GUIDANCE CONTINUES TO BE FARTHER N COMPARED TO THE HIGH
RES). THE MORE FAVORABLE THERMODYNAMICS WOULD SUPPORT THE HEAVIER
RAINFALL POTENTIAL CLOSER TO THE GULF COAST AND OFFSHORE... AS PER
THE RGEM... WRF-ARW... AND NSSL WRF... THEREFORE THE LATEST WPC
QPF SHIFTED THE AXIS OF HEAVIEST RAINFALL A LITTLE FARTHER SOUTH
FROM CONTINUITY -- WITH FURTHER SHIFTS TO THE SOUTH (CLOSER TO THE
GULF COAST) CERTAINLY POSSIBLE.

HURLEY