Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 230913
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
512 AM EDT SUN APR 23 2017

...VALID 12Z SUN APR 23 2017 - 12Z MON APR 24 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SW AKQ 15 WSW GSB ILM 15 SSE SUT 10 SSE CRE 45 SSW MYR
30 SSE CHS HXD 20 WNW SAV 15 NW VAD 15 NNE 11J 20 NE 4A9
10 NE EKQ 15 NNW HSP OFP 15 SW AKQ.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
50 S HST 25 ESE MTH 15 N MTH 40 W TMB 30 SW SUA 20 E FPR
45 ENE SUA 40 E PMP 40 E HST 50 S HST.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 WSW I16 20 N LYH 25 SSE FVX HNZ 10 SE IGX POB 15 SSW CPC
40 SSW MYR 20 SSW CHS 30 NW NBC 10 W TBR 15 SW DBN 20 WNW MCN
RYY 15 N OQT 20 WSW I16.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 E BCT 20 ESE MIA 10 ENE HST 15 NNW TMB 15 WNW BCT 10 ESE SUA
30 ESE SUA 25 E BCT.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 SW DAN 15 N EXX 10 NE FQD 30 NNE AVL TRI 10 ENE 6V3 10 N PSK
25 SE ROA 10 SW DAN.


...SOUTHERN-CENTRAL APPALACHIANS / SOUTHEASTERN U.S. / MID
ATLANTIC...

THE POSITIVELY-TILTED UPPER TROUGH WILL CONTINUE TO DEEPEN AS IT
PIVOTS SOUTHEAST DURING DAY 1...WITH DECENT MODEL CLUSTERING OF AN
EVENTUAL CLOSED MID-UPPER LOW GRADUALLY TRAVERSING THE
SOUTHEASTERN STATES. THIS ALONG WITH THE OVERALL NEGATIVE TILT TO
THE TROUGH WILL GENERATE STRONG...FOCUSED UPPER DIFFLUENCE E-NE OF
THE UPPER LOW...WITH THE UPPER DIVERGENCE GETTING A BOOST FROM THE
RIGHT ENTRANCE REGION OF THE UPPER JET STREAK OVER THE NORTHERN
MID ATLANTIC REGION. THE EVOLVING SYNOPTIC PATTERN...WITH THE DUAL
NORTHERN AND (ESPECIALLY) SOUTHERN STREAM COMPONENTS...WILL FAVOR
A RATHER BROAD SWATH OF HEAVY RAINFALL ACROSS EASTERN PORTIONS OF
THE LOWER OH VALLEY AND TN VALLEY...ALONG WITH THE
CENTRAL-SOUTHERN APPALACHIANS... ADJACENT PIEDMONT...AND MUCH OF
THE SOUTHEAST. WPCQPF WAS INITIALIZED FROM CONTINUITY AND A
NON-NAM/NON-CMC BLEND (PER THE PMDHMD)...LED BY THE ECMWF AND
(ESPECIALLY FOR TIMING) THE WRF-ARW/NMMB. AS NOTED IN PREVIOUS
DISCUSSIONS...THE MODELS CONTINUE TO SHOW LIMITED INSTABILITY/WEAK
UPDRAFT STRENGTH POTENTIAL ACROSS THE SOUTHERN
APPALACHIANS-PIEDMONT GIVEN THE MORE OR LESS MOIST ADIABATIC LAPSE
RATES ALOFT. WHILE THIS WOULD CAP THE SHORT TERM RAINFALL RATES AT
LEAST TO SOME DEGREE...THE STRENGTHENING DYNAMICAL PROFILE WITH A
MATURING WCB/BAROCLINIC LEAF AND STRENGTHENING LOW-MID LEVEL
MOISTURE FLUX WILL SOMEWHAT COMPENSATE...ESPECIALLY OVER THE
HIGHER TERRAIN GIVEN THE UPSLOPE ENHANCEMENT. NOT WITHSTANDING THE
MEAGER INSTABILITY...THE THERMODYNAMIC PROFILE IS QUITE
IMPRESSIVE...AS POSITIVE ANOMALIES (2-2.5 STANDARD DEVIATIONS
ABOVE NORMAL) CONTINUE TO BE NOTED ON THE ENSEMBLES FOR BOTH PW
AND 850-700 MB MOISTURE FLUX. THIS AS THE SYSTEM WILL BE ABLE TO
DRAW IN BOTH GOMEX AND GULF STREAM MOISTURE...LEADING TO AN
EXPANSIVE (POLEWARD) AREA OF ANOMALOUS PW INTO THE UPPER LAYERS
(500-300 MB).

IN TERMS OF THE QPF...WPC NOTED A FAIRLY BROAD AREA WITH
AREAL-AVERAGE AMOUNTS BETWEEN 1-3"...HIGHEST ACROSS UPSTATE SC...
WESTERN NC...SOUTHWEST VA...NORTHEAST GA...AND EASTERN TN WITH
GIVEN THE UPSLOPE ENHANCEMENT. BECAUSE OF THE ANTICIPATED MINIMAL
CONVECTIVE CONTRIBUTION ACROSS MUCH OF THIS REGION (ESPECIALLY
NORTHERN AREAS)...DO NOT ANTICIPATE SIGNIFICANT DEVIATION FROM
THESE HIGHER AMOUNTS. HOWEVER CONSIDERING THE OROGRAPHY AND
ATTENDANT LOWERING FFG VALUES...WILL CONTINUE WITH A MODERATE RISK
FOR EXCESSIVE RAINFALL ACROSS THE NC-SOUTHERN VA PIEDMONT INTO THE
APPALACHIANS AND INCLUDING FAR NORTHEAST TN....WITH A SURROUNDING
BROAD SLIGHT RISK AREA.


...SOUTH FLORIDA...

THE CONVECTIVELY-AIDED SHORTWAVE LOW NEARING THE FL STRAITS EARLY
THIS MORNING WILL CONTINUE A GRADUAL EASTWARD MOTION EARLY...THEN
PIVOT MORE NE-N OF THE EAST COAST OF FL LATER TODAY AS IT BECOMES
ABSORBED BY THE AMPLIFYING BROADER SCALE TROUGH. THE
MODELS...PARTICULARLY THE HIGH-RES CAMS...HAVE BEEN PERFORMING
POORLY UP TO THIS POINT WHEN IS COMES TO THE QPF...NOT JUST IN
TERMS OF LOCATION BUT WITH AMOUNTS AS WELL (MAINLY UNDERDONE
COMPARED TO RECENT OBS). THE SOMEWHAT ORGANIZED (ALBEIT COMPACT)
LOW-MID LEVEL CIRCULATION AND THUS FOCUSED LOW-LEVEL MOISTURE
CONVERGENCE ON THE EASTERN SIDE WILL COMBINE WITH THE IMPRESSIVE
THERMODYNAMIC PROFILE (MUCAPES 1000-1500 J/KG...PWS AOA
2.00"...WARM OR >0C CLOUD LAYER ABOVE 13KFT) IN GENERATING HIGHLY
EFFICIENT SHORT TERM RAINFALL RATES. AS THE DAY PROGRESSES...THE
EXCESSIVE RAINFALL RISK WILL SHIFT FROM THE KEYS INTO SOUTH AND
SOUTHEAST FL IN RESPONSE TO THE TRACK OF THE SHORTWAVE. WHILE
MAXIMUM RAINFALL TOTALS ARE EXPECTED TO AVERAGE AROUND 2"
AREA-WIDE...SIGNIFICANTLY HIGHER LOCALIZED AMOUNTS ARE EXPECTED
CONSIDERING THE AVAILABLE DEEP-LAYER INSTABILITY...TO THE TUNE OF
4-8+ INCHES PER THE HIGH-RES CAMS. WPC NOTED A SLIGHT RISK OF
EXCESSIVE RAINFALL OVER A PORTION OF SOUTHEAST FL...LINING UP WITH
THE HIGHEST WPC AREAL-AVERAGE QPF AND CAM CONSENSUS OF HEAVIER
TOTALS. MOREOVER...THE 00Z PARALLEL HREF 40KM NEIGHBORHOOD
PROBABILITY OF THE 24 HOUR QPF EXCEEDING 5 INCHES IS AOA 50%
WITHIN THE MAJORITY OF THE SLIGHT RISK AREA.

HURLEY
$$





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