Quantitative Precipitation Forecast
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FOUS30 KWBC 231436
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1035 AM EDT SUN APR 23 2017

...VALID 15Z 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
70 ESE HST 20 SW HST 40 SE APF 35 ENE RSW VRB 40 ENE SUA
60 E SUA.

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.

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.

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.

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...

NO MAJOR CHANGES MADE TO THE ON-GOING EXCESSIVE RAINFALL OUTLOOK
ACROSS THIS PART OF THE COUNTRY.  RAINFALL RATES THROUGH THE
MORNING HAVE BEEN FAIRLY MODEST...WITH BETTER COVERAGE STAYING OUT
WEST SO FAR.  12Z UPPER AIR SHOWED ONLY MODEST
INSTABILITY...GENERALLY LIGHT FLOW AND PRECIPITABLE WATER VALUES
RUNNING RIGHT AROUND AN INCH.  THAT HAS HELD PRECIPITATION RATES
TO THE ORDER OF HALF INCH PER 6 HOURS ACROSS THE TN VALLEY AND
LESS ALONG THE APPALACHIANS AND IN THE CAROLINAS.

THE DECISION TO KEEP THE MODERATE IN PLACE WAS BASED ON THE
CHANGES IN THE LARGER SCALE FLOW PATTERN SHOWN BY THE MODELS FOR
LATER TODAY AND TONIGHT.  WHAT STARTS OUT AS A POSITIVELY TILTED
UPPER LEVEL TROUGH WILL DEEPEN AS IT PIVOTS TO THE SOUTHEAST WITH
A CLOSED MID LEVEL LOW EVENTUALLY TRAVERSING THE SOUTHEASTERN
STATES.  THIS PIVOTING WILL RESULT IN INCREASINGLY DIFLUENT
MID/UPPER LEVEL FLOW AND ASSOCIATED DIVERGENCE TO THE
EAST/NORTHEAST OF THE LOW.  IN ADDITION...A DUAL UPPER LEVEL JET
STRUCTURE WILL GRADUALLY GET INTO A POSITION TO ENHANCE UPPER
LEVEL DIVERGENCE OVER THE WESTERN CAROLINAS LATER THIS EVENING.

THIS SHOULD LEAD TO AN INCREASE IN AREAL COVERAGE AND RAINFALL
INTENSITY IN THE AREA OF MODERATE RISK LATER TONIGHT.  WHILE
ACKNOWLEDGING THAT...THE MODELS NEVER GENERATE A GREAT DEAL OF
INSTABILITY THIS FAR NORTH WHICH RESULTS IN ONLY WEAK TO MODERATE
UPDRAFT STRENGTH IN A REGION THAT MORE OR LESS HAS 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. INDEED THE
ENSEMBLE PRECIPITABLE WATER PROGS INCREASE ANOMALIES TO BETWEEN 2
TO 2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY AND INCREASING
MAGNITUDE OF MOISTURE TRANSPORT/FLUX.  AS A RESULT WE SUSPECT
CONDITIONS WILL BECOME BETTER FOR EXCESSIVE RAINFALL HERE GIVEN
THE INCREASING DYNAMICAL SUPPORT...POTENTIAL FOR CELL TRAINING AND
POSSIBLE INTERACTION WITH THE TERRAIN.

TO THE SOUTH OF THAT AREA...SAW LITTLE REASON TO MAKE ANY
ADJUSTMENTS TO THE SLIGHT RISK ACROSS PARTS OF GA...SOUTH CAROLINA
OR SOUTHERN NORTH CAROLINA.  THE INSTABILITY WILL BE STRONGER THAN
AREAS UP TO THE NORTH AND WEST WHICH SHOULD SUPPORT HIGHER
RAINFALL RATES.  DESPITE THE HIGHER RATES...STILL THINK A SLIGHT
RISK OF EXCESSIVE RAINFALL IS IN ORDER GIVEN HIGHER FLASH FLOOD
GUIDANCE VALUES AND THE CONVECTIVE MODE.


...SOUTH FLORIDA...

A CONVECTIVELY-AIDED SHORTWAVE LOW NEARING THE FL STRAITS 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...CONTINUE TO BE
PERFORMING POORLY UP WHEN IS COMES TO THE LOCATION AND MAGNITUDE
OF THEIR QPF AND THE ASSOCIATED FIELDS SICH AS LOW-LEVEL MOISTURE
CONVERGENCE.  THE THERMODYNAMIC PROFILES SHOWN IN AREA 12Z
SOUNDINGS INDICATED MUCAPES 1000-1500 J/KG...PWS AOA 2.00
INCHES...AND WARM OR >0C CLOUD LAYER ABOVE 13 KFT. ALL OF THOSE
POINTED TO EFFICIENT HEAVY RAINFALL PRODUCING CONVECTION.

AS THE DAY PROGRESSES...THE EXCESSIVE RAINFALL RISK WILL CONTINUE
TO PULL NORTH OF 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. 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.

BANN
$$




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