Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 181846
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
146 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 LA. 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...AND AS SUCH 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
$$





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