Quantitative Precipitation Forecast
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FOUS30 KWBC 151818
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
218 PM EDT WED APR 15 2015

...VALID 18Z WED APR 15 2015 - 00Z 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
20 NE DNL MMT 30 W MYR 35 SE MYR 75 SSE MYR 95 SE CHS 80 SE HXD
30 E SSI 30 E AMG 15 NW VDI 30 S HQU 20 NE DNL.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 SSW TOI 20 SSW OZR 15 SE DTS 10 SSW KVOA 10 S KEIR 20 S KGVX
20 NW RKP 25 ESE SSF 10 E AUS 25 WNW UTS 20 SW AEX 20 WSW PIB
45 W GZH 40 SSW TOI.


...SC / GA / SAVANNAH VALLEY...

AN ORGANIZED CONVECTIVE EVENT IS LIKELY DURING THE AFTERNOON
HOURS...SQUEEZED BETWEEN A SINKING FRONTAL BOUNDARY...THE SEA
BREEZE...AND AN APPROACHING 700 MB TROUGH. MOISTURE IS
PLENTIFUL...WITH PW VALUES OF 1.5 TO 1.7 INCHES REPRESENTING ABOUT
2.5 STANDARD DEVIATIONS FROM CLIMATOLOGY. MODERATE INSTABILITY AND
DEEP LAYER WIND SHEAR OF 15-25 KNOTS SHOULD BE SUFFICIENT FOR
ORGANIZED MULTICELLS AND PERHAPS BRIEF SUPERCELL CHARACTER...WHICH
WOULD LEAD TO SOME TRAINING AND/OR SLOW DEVIANT MOTION. ALTHOUGH
THE RISK AREA FOR HEAVY RAINFALL IS LIMITED IN SIZE...THE MODELS
SHOW STRONG AGREEMENT HERE...NOTING CONSISTENCY IN CONSECUTIVE
HRRR RUNS...IN THE 12Z ARW AND NMM...AND A LOCAL MAXIMUM FROM OUR
IN-HOUSE ENSEMBLE WHICH TAKES INTO ACCOUNT THE GLOBAL MODELS AND
SREF MEMBERS. FLASH FLOOD GUIDANCE VALUES ARE HIGH...AND MAY BE
DIFFICULT TO BREACH...BUT GIVEN THE VERTICAL MOISTURE PROFILES AND
OPPORTUNITY FOR FOCUS ALONG BOUNDARIES...SOME ISOLATED LOCATIONS
COULD SEE EXCESSIVE RAINFALL AND FLASH FLOODING. THE RISK WILL
LIKELY DIMINISH BY EVENING WHEN INSTABILITY EITHER BECOMES
CONSUMED OR DECREASES OWING TO COOLING.


...GULF COAST STATES...

A CORRIDOR OF ENERGETIC UPPER FLOW REMAINED IN PLACE FROM THE
WESTERN GULF TO THE FLORIDA PANHANDLE. DRIER AIR HAD TEMPORARILY
DIPPED INTO SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA BEHIND A
FRONTAL BOUNDARY. OTHERWISE...AN AIRMASS RICH IN MOISTURE WITH
PRECIPITABLE WATER VALUES ABOVE AVERAGE FOR THE TIME OF
YEAR...UPWARD OF TO 2 TO 2.5 STANDARD DEVIATIONS...WAS STILL IN
PLACE FROM SOUTHERN LOUISIANA ACROSS SOUTHERN MS/AL AND THE
FLORIDA PANHANDLE. THIS AFTERNOON ASCENT WILL NOT BE AS ORGANIZED
AS IN PREVIOUS DAYS...BUT THERE IS A MODEL SIGNAL FOR ADDITIONAL
CONVECTION/HEAVY RAINS AS UPWARD VERTICAL VELOCITIES ARE ENHANCED
AHEAD OF AN UPPER DIFFLUENCE MAXIMUM ASSOCIATED WITH A SHORTWAVE
SKIRTING THE CENTRAL GULF COAST. CONCERN FOR A MORE ORGANIZED
PRECIPITATION EVENT IS GREATER ON THURSDAY AS A WARM ADVECTION
REGIME SETS UP FROM THE TEXAS HILL COUNTRY INTO SOUTHWEST
LOUISIANA AHEAD OF A DEEP WESTERN U.S. TROUGH. THUNDERSTORMS WHICH
DEVELOP WITHIN WARM ADVECTION THURSDAY MORNING ALONG THE TX/LA
GULF COAST MAY GROW UPSCALE AND PRODUCE HEAVY RAIN DOWNSTREAM INTO
SOUTHERN LA DURING THE DAY. THE MODELS WERE CATCHING ONTO THIS
NOTION...AND AN ASSOCIATED MID LEVEL DISTURBANCE...WITH QPF HAVING
TRENDED EASTWARD OVER THE PAST FEW RUNS OF THE GLOBAL MODELS. WE
DO NOTE...HOWEVER...THAT LOW LEVEL INFLOW WILL BE ENHANCED BACK
ACROSS CENTRAL TEXAS DURING THE DAY THURSDAY...WHEREAS RELATIVELY
MODEST 1-15 KNOT 850 MB FLOW OVER SOUTHERN LOUISIANA MAY OFFER
RESULT IN OUTFLOW DOMINANT AND PROGRESSIVE CONVECTION THERE.
CENTRAL TO SOUTHEAST TEXAS WILL LACK DEEP ASCENT...BUT IF
CONVECTION CAN BECOME INCREASINGLY SURFACE BASED WITHIN THE WARM
ADVECTION REGIME...CAPE/SHEAR COMBINATIONS WILL FAVOR ORGANIZED
STORM MODES...AND TRAINING OF LOCALLY HEAVY RAIN MAY BECOME MORE
LIKELY WITH TIME. WE DREW A BROAD SLIGHT RISK AREA TO ENCOMPASS
THE POTENTIAL FOR EXCESSIVE RAINFALL WITHIN THIS ZONE OF
MOIST...UNSTABLE...AND WEAKLY TO MODERATELY FORCED AIR MASS ALONG
THE GULF COAST AND EXTENDING TOWARD CENTRAL TEXAS. RECENT HEAVY
RAINS ALONG THE IMMEDIATE COASTAL AREAS ALSO FACTORS INTO THE RISK.

BURKE
$$




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