Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 202130
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
529 PM EDT SAT MAY 20 2017

...VALID 2129Z SAT MAY 20 2017 - 12Z SUN MAY 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ENE DEC 15 SW SBN 10 N DFI LCK 48I W99 15 SSW FVX 10 E HNZ
IGX 10 N UKF 10 NW GMU 25 E MLJ MGR 45 SSW PAM 20 SSW KVKY
15 N KVNP LBX CRP 50 W MFE 20 WNW MMNL 15 SSW MMPG 25 S ECU
10 NW GTU 30 SSW ELD 15 WNW AWM 15 ENE DEC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SSE SDF 45 SE MQY 15 SW LGC 20 N MAI 40 SSE DTS 25 ESE NBG
30 NW 7R4 25 WSW 5R5 HBV 30 N LRD 10 NW BAZ 30 S MLU 30 SE UTA
30 SSE CIR 15 SSW LAF 10 W FWA 30 WNW WMO 10 SSE SDF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 N MWK 10 SSE MKJ 6V3 15 WSW LNP 30 S JKL JKL 30 SW HTS
20 ESE HTS 20 SSE CRW BKW 10 ENE PSK 20 ESE BCB 20 NW MTV
15 N MWK.


...2100 UTC UPDATE...

HAVE MADE SOME ADJUSTMENTS TO THE BROAD MARGINAL AND SLIGHT RISK
OUTLOOK AREAS...MAINLY TO REMOVE THE MARGINAL OVER THE UPPER MS
VALLEY AND TO EXPAND THE RISK AREAS INTO MORE OF THE
CENTRAL-EASTERN GULF COAST REGION. IN TERMS OF THE FORMER...EVEN
IN AREAS WITH RELATIVELY LOW 1-3 HOURLY FFG...THE RAINFALL RATES
ALONG THE EXPANDING TROWAL ZONE ARE INSUFFICIENT TO POSE ANY
SHORT-TERM RUNOFF ISSUES. MEANWHILE...GIVEN THE LATEST CONVECTIVE
TRENDS ALONG WITH THE QPF FROM THE MORE RECENT MESOSCALE GUIDANCE
(CAMS)...HAVE EXPANDING THE SLIGHT AND SURROUNDING MARGINAL RISK
AREA TO CAPTURE MORE OF EASTERN LA...SOUTHERN MS/AL...AND THE
WESTERN FL PANHANDLE GIVEN THE GROWING MODEL CONSENSUS OF THE
CONVECTION BECOMING BETTER ORGANIZED AND GROWING UPSCALE INTO AN
MCS OVERNIGHT.

HURLEY

ORIGINAL DISCUSSION BELOW...


EAST TX---LOWER MS VALLEY---TN VALLEY---SOUTHEAST---OH
VALLEY---LOWER LAKES INTO THE UPPER MS VALLEY

VERY BROAD MARGINAL AND SLIGHT RISK AREAS WERE MAINTAINED FROM THE
PREVIOUS OUTLOOK FOR WHAT WILL LIKELY BE ANOTHER ROUND OF
WIDESPREAD HEAVY PRECIPITATION AHEAD OF THE STRONG CLOSED LOW
MOVING NORTHEASTWARD THIS PERIOD FROM THE CENTRAL PLAINS AT THE
BEGINNING OF THE PERIOD INTO THE NORTHERN PLAINS/UPPER MS VALLEY
REGION BY EARLY SUNDAY.  THE RECENT VERY FAVORABLE HEAVY RAIN
PATTERN WILL PERSIST INTO THE NEW DAY 1 PERIOD WITH STRONG LIFT
EXPECTED AHEAD OF THE CLOSED LOW IN AN AXIS OF PW ANOMALIES 1-2+
STANDARD DEVIATIONS ABOVE THE MEAN.  THIS WILL SUPPORT THE VERY
BROAD RISK AREAS EXTENDING FROM EAST TX INTO THE LOWER MS
VALLEY---TN VALLEY/SOUTHEAST--NORTHWARD INTO THE LOWER OH
VALLEY--LOWER LAKES AND UPPER MS VALLEY. CHANGES TO THE PREVIOUS
OUTLOOK AREA WERE TO PUSH THE SLIGHT RISK AREA FARTHER EASTWARD
INTO THE SOUTHEAST/SOUTHERN APPALACHIANS. THIS WAS TO COVER THE
GENERAL MODEL CONSENSUS THAT HEAVY RAINS COULD DEVELOP FARTHER
EAST THAN IN PREVIOUS FORECASTS--ALTHOUGH THE TIMING OF THIS
POTENTIALLY HEAVY RAINS IS VERY DIFFERENT IN THE LATEST GUIDANCE.
A SLIGHT RISK AREA WAS ALSO INTRODUCED ACROSS PORTIONS OF THE
CENTRAL APPALACHIANS FROM IN THE VICINITY OF THE KY/WV/VA
BORDER---SOUTHEAST INTO SW VA.  LOW FFG VALUES HERE AND MODEL
CONSENSUS FOR HEAVY RAINFALL POTENTIAL WARRANTED THE SLIGHT RISK
INTRODUCTION.  RECENT VERIFICATION HAS SHOWN VERY POOR SHORT TERM
MODEL SKILL---WITH THIS LIKELY TO CONTINUE DURING THE UPCOMING DAY
1 PERIOD GIVEN THE LARGE SPREAD IN QPF DETAILS.  THIS IS LEADING
TO LOW CONFIDENCE IN THE DETAILS---BUT AGAIN HIGHER CONFIDENCE
THAT ADDITIONAL VERY HEAVY TO EXCESSIVE RAINFALL AMOUNTS ARE
POSSIBLE GIVEN THE OVERALL FAVORABLE PATTERN.   WHILE THERE MAY
LIKELY BE A WEAKENING TO THE CURRENT ORGANIZED CONVECTION PRIOR TO
OR SHORTLY AFTER 1200 UTC---RE-INVIGORATION OF WIDESPREAD
CONVECTION LIKELY IN THE VERY MOIST AND UNSTABLE AIRMASS AHEAD OF
THE CLOSED LOW.  A PERIOD OF TRAINING OR SLOW CELL MOVEMENT
POSSIBLE THROUGH THE RISK AREAS---ESPECIALLY IN THE 0000 TO 1200
UTC SUNDAY TIME FRAME WHEN CONVECTION WILL LIKELY PEAK. CONFIDENCE
IN WHERE THIS WILL OCCUR IS LOW AND HAS KEPT US FROM ANY DECREASE
IN THE VERY BROAD RISK AREAS.

ORAVEC
$$





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