Quantitative Precipitation Forecast
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FOUS30 KWBC 171425
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1023 AM EDT FRI APR 17 2015

...VALID 15Z FRI APR 17 2015 - 12Z SAT APR 18 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 WSW HRT 40 ENE KVOA 20 NW KIKT 20 ENE S58 45 SSE KXIH
20 SSW PKV 45 SE SSF 10 NNW SAT 10 WNW AUS 25 SSW JSO 35 NW POE
35 NW BTR 30 N HSA 30 NNE BIX 15 NNE MOB 30 WNW NSE 10 WSW HRT.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNW CXO 35 SW JAS 30 NW LCH 10 NNW 7R5 15 WNW KVBS 15 W GLS
15 W 5R5 30 S 3T5 15 WSW 3T5 10 NE 62H 10 NNW CXO.


...TEXAS / LOUISIANA...

WE INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL IN THE 15Z
UPDATE...VALID THROUGH 12Z SATURDAY. AS OF THIS MORNING A
SHORTWAVE WHICH CONTRIBUTED TO A FLASH FLOOD EVENT ALONG THE TX/LA
COAST THURSDAY EVENING WAS MOVING INTO THE SOUTHEAST STATES. IN
ITS WAKE OBSERVATIONS SHOWED STABLE CONDITIONS OVER SE TX/SRN
LA...WITH DEEP CONVECTION OCCURRING OFFSHORE SOUTH OF LA ALONG A
BROAD PW AND LOW LEVEL MOISTURE GRADIENT. OUR THINKING IS IN
AGREEMENT WITH THE STORM PREDICTION CENTER OUTLOOK...THAT THE
MOISTURE/INSTABILITY GRADIENT OVER THE NORTHWEST GULF WILL ADVECT
NORTHWARD AROUND THE PERIPHERY OF OLD OUTFLOW AT THE SAME TIME
THAT A COUPLE OF RATHER WELL DEFINED SHORTWAVES WITHIN THE
SUB-TROPICAL JET STREAM APPROACH SOUTWHEST TEXAS. AHEAD OF THESE
IMPULSES...A SOUTHERLY LOW LEVEL JET INCREASES SOMEWHAT...UP TO
20-25 KNOTS TONIGHT. CONVECTION FORMING OVER SOUTHWEST TEXAS AND
THE HILL COUNTRY IS EXPECTED TO GRADUALLY GROW UPSCALE AND THE
TRACK ALONG THE LIFTING MOISTURE/INSTABILITY GRADIENT. BY 06Z
TONIGHT THE GFS PLACES THIS GRADIENT ACROSS INTERSTATE 10 FROM
JUST EAST OF SAN ANTONIO...ACROSS HOUSTON...TO NEAR LAKE CHARLES.
WPC IS FORECAST A MAXIMUM OF PRECIPITATION ALONG THIS AXIS...WITH
MODERATE CONFIDENCE...AS THIS IS IN BETWEEN THE GLOBAL MODEL
CONSENSUS AND HIGH RESOLUTION MODEL CONSENSUS...WHICH THEMSELVES
SHOWED RATHER SMALL SPREAD. IF AN ORGANIZED MCS EVOLVES IT MAY
MOVE QUICKLY PER CORFIDI VECTORS PROGGED AT 25-30 KNOTS. CELL
MERGERS AHEAD OF THE SYSTEM...HOWEVER...COULD BOOST PRECIPITATION
TOTALS...AND SHORT TERM PRECIPITATION RATES HAVE POTENTIAL TO BE
HIGH GIVEN PRECIPITABLE WATER RISING ABOVE 1.75 INCHES. RAIN RATES
OF 1-2 INCHES OVER 1-2 HOURS...AND TOTAL NEW RAINFALL OF 2-4
INCHES WILL BE POSSIBLE LOCALLY. FLASH FLOOD GUIDANCE IN THE
MODERATE RISK AREA IS VERY LOW GIVEN THE VERY HEAVY RAINFALL THAT
OCCURRED YESTERDAY...SUGGESTING A LARGE PERCENTAGE OF THE NEW
RAINFALL WILL GO INTO SURFACE RUNOFF AND PRODUCE FLASH FLOODING.


...CENTRAL GULF COAST REGION OF MS/AL/FL...

A WEAKLY DEFINED TROUGH AXIS EMBEDDED IN THE ACTIVE SUB-TROPICAL
JET FLOW WILL DRAG ACROSS THE CENTRAL GULF COAST THROUGH MID
AFTERNOON TODAY...PERHAPS CONTRIBUTING TO ENHANCED ORGANZATION OF
SEA BREEZE AND OTHER SCATTERED CONVECTION. FAVORABLY DEEP MOISTURE
PROFILES WITH PW VALUES OVER 1.5 INCHES WILL AGAIN SUPPORT LOCALLY
HEAVY DOWNPOURS. ON SMALL SCALES THE CONVECTION MAY FOCUS ALONG A
WARM FRONT NEAR THE COAST AND ALONG A DIFFERENTIAL HEATING
BOUNDARY FARTHER INLAND OWING TO CLOUD COVER OVER CENTRAL MS/AL.

BURKE
$$





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