Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 310035
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
834 PM EDT THU MAR 30 2017

...VALID 01Z FRI MAR 31 2017 - 12Z FRI MAR 31 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 E KVOA 30 E GZH 15 W ALX 15 N TCL 25 NE CBM 60 N MSL
20 SE BMG 25 ENE MIE 15 SSE FDY 20 ESE MFD 25 E ZZV 20 N I16
10 SSE ROA DAN 15 NE HBI EQY 25 SSW GRD 35 N ABY 20 W TLH
105 SSE AAF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SW BWG 30 NW LOU 10 W DAY 20 S LHQ 15 S JKL 10 SSE OQT
15 SSW CHA 20 NNW BHM 15 NE 1M4 50 NNE MSL 20 SW BWG.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ESE TRI HLX 10 NE MWK 10 NE HKY CEU 10 ENE 1A5 15 ESE TRI.


0100 UTC UPDATE

CHANGES TO THE PREVIOUS EXCESSIVE RAINFALL POTENTIAL OUTLOOK WERE
RELATIVELY MINOR...BASICALLY COSMETIC IN REFINING (TRIMMING) THE
BACK OR WESTERN EDGE OF THE MARGINAL RISK AREA BASED ON THE LATEST
COMPOSITE OF OBSERVATIONAL DATA (PARTICULARLY RADAR AND
SATELLITE). OTHERWISE--OVERALL THINKING HAS NOT CHANGED
APPRECIABLY FROM THE PREVIOUS ISSUANCE.

HURLEY


...OH/IN/KY/TN/AL/GA...

THE CYCLONE OVER THE PLAINS WILL MAINTAIN STRENGTH WHILE
PROGRESSING EASTWARD...RESULTING IN A BROAD EXPANSE OF
PRECIPITATION ACROSS THE EASTERN UNITED STATES. HEAVIER SWATHS ARE
ANTICIPATED WITHIN THE BROAD WARM SECTOR ALONG THE IMMEDIATE GULF
COAST...AND STRETCHING BOTH NORTH AND SOUTH OF OF THE MIDDLE
TENNESSEE VALLEY WHERE LOW LEVEL INFLOW / MOISTURE TRANSPORT WILL
BE MAXIMIZED.

WHILE THE LARGE SCALE PRECIPITATION PATTERN IS SIMILAR AMONG THE
MODELS...THERE ARE PLENTY OF DIFFERENCES REGARDING THE DETAILS OF
CONVECTIVE EVOLUTION...WITH BROADLY FAVORABLE SYNOPTICS AND HEIGHT
FALLS EXTENDING DOWN WELL INTO THE GULF...BUT WITH ONLY MARGINALLY
FAVORABLE EARLY-SPRING MOISTURE AND INSTABILITY ALONG MOST OF THE
WARM CONVEYOR BELT FROM JUST INLAND OF THE GULF COAST REGION UP TO
THE OHIO VALLEY. A SITUATION SUCH AS THIS SUPPORTS A BROAD
MARGINAL RISK OF EXCESSIVE RAINFALL. WE DID...HOWEVER...INTRODUCE
A SLIGHT RISK AREA STRETCHING NORTH AND SOUTH OUT OF THE MIDDLE
TENNESSEE VALLEY...WHERE MOISTURE TRANSPORT WILL BE MAXIMIZED
DURING PEAK HEATING AND EARLY EVENING. SOLUTIONS FROM THE MAJORITY
OF THE 00Z HI-RES GUIDANCE CLUSTERS TOGETHER WITH POCKETS OF
HEAVIER RAINFALL IN THIS ZONE...GENERALLY EITHER SIDE OF A LINE
FROM HUNTSVILLE TO CINCINNATI. THERE IS LITTLE SEPARATION BETWEEN
THE MEAN 0-6 KM WIND AND CORFIDI VECTORS FOR FORWARD PROPAGATING
SYSTEMS PER NAM FORECASTS...SUGGESTING GOOD ODDS OF TRAINING CELLS
FROM SSW TO NNE...WHICH IS BORN OUT IN LOOPS OF SIMULATED
REFLECTIVITY / HOURLY QPF FROM SOME OF THE HI-RES MODELS. FLASH
FLOOD GUIDANCE VALUES ARE NOT PARTICULARLY LOW...BUT ARE
ATTAINABLE...AT ABOUT 2 INCHES IN 3 HOURS. PARTS OF NORTHERN
ALABAMA...ESPECIALLY...HAVE SEEN ABOVE AVERAGE PRECIPITATION OVER
THE LAST 7 DAY PERIOD...AND MAY BE MORE SUSCEPTIBLE TO RAPID
RUNOFF.


...UPSLOPE REGIONS OF NC/SC/VA...

MODEL PRECIPITATION OUTPUT HAS ALSO TRENDED QUITE A BIT HEAVIER IN
THE UPSLOPE REGION TO THE LEE SIDE OF THE MOUNTAINS IN WESTERN
NC/SC AND SOUTHWEST VA...WHERE WE INTRODUCED A SLIGHT RISK OF
EXCESSIVE RAINFALL. PRECIPITABLE WATER IS SOMEWHAT MARGINAL...BUT
IS SUFFICIENT TO SUPPORT ROBUST CONVECTIVE UPDRAFTS AND HEAVY RAIN
RATES...WITH PW VALUES RISING TO NEAR 1.25 INCHES. OTHERWISE...THE
SETUP IS QUITE FAVORABLE FOR A PROLONGED PERIOD OF
UPSLOPE-ENHANCED PRECIPITATION MAKING LITTLE EASTWARD
PROGRESS...WHILE AWAITING THE SLOWLY APPROACHING LARGE SCALE
WEATHER SYSTEM FROM THE WEST. AGAIN...THE PW VALUES ARE SOMEWHAT
MARGINAL...AND FLASH FLOOD GUIDANCE VALUES ARE ON THE HIGH
SIDE...BUT WE ERRED ON THE SIDE OF CAUTION GIVEN THE VERY
FAVORABLE SYNOPTIC PATTERN AND EVENT TIMING...IN A REGION THAT IS
PRONE TO SLOW MOVING HEAVY RAIN FROM OFTEN SHALLOW CONVECTION THAT
CAN MAKE IT LOOK LESS THREATENING IN SOME DATA SETS.

BURKE
$$




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