Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 201304
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
904 AM EDT WED MAY 20 2015

...VALID 15Z WED MAY 20 2015 - 12Z THU MAY 21 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 S CDS 35 WNW CDS 40 NNW CDS 40 W CSM CSM 20 ESE CSM
20 WNW CHK TIK 15 SSE CQB 15 SE OKM 10 NNE JSV 20 NE FSM
35 ENE FSM 15 NW MWT 15 SW MWT 15 N TXK OSA 10 W 3T1 RBD
10 WSW MWL 35 W RPH 65 S CDS.



...RIO GRANDE VALLEY OF TX FROM THE BIG BEND SOUTHEASTWARD INTO
THE LOWER RIO GRANDE

LATE WEDNESDAY NIGHT INTO EARLY THURSDAY---CONCERNS FOR HEAVY TO
POSSIBLY ISOLATED EXCESSIVE RAINFALL AMOUNTS SHIFT TO THE VICINITY
OF THE BIG BEND AND SOUTHEASTWARD INTO THE LOWER RIO GRANDE.
SHORTWAVE ENERGY MOVING OUT OF NORTHERN MEXICO ON THE SOUTHERN
EDGE OF THE WESTERLIES MAY IGNITE MORE ORGANIZED CONVECTION NEAR
THE BIG BEND---WITH THIS ACTIVITY THEN PUSHING SOUTHEASTWARD ALONG
THE RIO GRANDE IN THE EARLY HOURS OF THU.  THERE IS SUPPORT FOR
THIS SOLUTION IN THE HI RES ARW--NMMB---ECMWF---NAM CONEST AND CMC
GEM---ALTHOUGH THEY DO DIFFER WITH RESPECT TO HOW FAR TO THE
SOUTHEAST CONVECTION PROPAGATES.  THE SLIGHT RISK WAS DRAWN TO
ENCOMPASS THE VARYING GEOGRAPHICAL LOCATIONS OF THE HEAVY RAINFALL
FORECASTS IN THE HI RES RUNS.  HEAVY TO ISOLATED EXCESSIVE
RAINFALL AMOUNTS POSSIBLE WITH THIS CONVECTION IN THE EARLY HOURS
OF THURSDAY FROM ISOLATED SHORT TERM PRECIP TOTALS OF 2-4"+.

CAN NOT RULE OUT ADDITIONAL CONVECTION FIRING ALONG THE SOUTHWARD
DRIFTING COLD FRONT ACROSS PORTIONS OF CENTRAL TX THAT COULD
PRODUCE LOCALLY HEAVY TO ISOLATED EXCESSIVE RAINS AS MOIST
SOUTHERLY INFLOW CONTINUES INTO THE BOUNDARY.  WITH NO CLEAR MODEL
SIGNAL FOR PLACEMENT---NO RISK AREA IS DENOTED--BUT WITH RECENT
RAINS LOWERING FFG VALUES---THIS AREA WILL NEED TO BE MONITORED.

...SOUTHERN AR AND NORTHERN MS...

DIURNAL WEAKENING OF OVERNIGHT CLUSTERS OF HEAVY SHOWERS AND
THUNDERSTORMS HAS OCCURRED THIS MORNING IN CENTRAL AR.  PERSISTENT
HIGH AVAILABLE MOISTURE REMAINS IN PLACE WITH AVAILABLE
PRECIPITABLE WATER VALUES 1.5-2 STD DEVIATIONS ABV NORMAL IN
SOUTHERN AR TO NORTHERN MS BEFORE DRYING ALOFT REDUCES THESE
VALUES LATER TODAY AND TONIGHT.  PRE-FRONTAL MOISTURE CONVERGENCE
WILL HELP INDUCE ASCENT AND HIGH AVAILABLE MOISTURE SUPPORTS
DEVELOPMENT OF BRIEF HEAVY DOWNPOURS. THE LOW LEVEL JET IS FAIRLY
WEAK AND GRADUAL VEERING OF WINDS ALOFT FAVORS FORWARD PROGRESSION
OF CELLS.  WITHOUT A STRONG SIGNAL FOR MAINTAINING CELLS FOR AN
EXTENDED PERIOD...WE DECIDED TO DOWNGRADE THE EARLIER SLIGHT RISK.

PETERSEN/ORAVEC
$$





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