Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 281901
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
301 PM EDT SUN SEP 28 2014

...VALID 18Z SUN SEP 28 2014 - 00Z TUE SEP 30 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
KVOA 30 NNW KMIS 10 S BIX 10 ENE PNS 20 NNW DHN 15 NE OPN
15 S 3J7 30 NE DBN AYS 30 SSW 40J 40 SW AAF KVOA.



COLORADO/WYOMING ROCKIES
~~~~~~~~~~~~~~~~~~~~~~~~

THE ANOMALOUS DEEP UPPER TROUGH/CLOSED LOW ACROSS THE LOWER GREAT
BASIN WILL SHIFT ONLY SLIGHTLY EASTWARD TODAY...THEREBY
MAINTAINING FAVORABLE DYNAMIC SUPPORT (UPPER
DIFFLUENCE/DIVERGENCE) VIA THE WARM CONVEYOR BELT (WCB) ALONG WITH
THE MOIST...S/SE LOW-LEVEL FLOW AND UPSLOPE ENHANCEMENT ACROSS THE
CENTRAL ROCKIES. THE LACK OF DEEP-LAYER INSTBY (MUCAPES 250-500
J/KG) ALONG WITH THE GRADUAL REDUCTION OF PRECIPITABLE WATER WITH
TIME (AS THE MID/UPPER LEVELS BECOME INCREASINGLY CUT OFF FROM ANY
SUBTROPICAL MOISTURE FEED) WILL LIMIT THE EXCESSIVE RAINFALL
POTENTIAL TODAY. NEVERTHELESS...THE FAVORABLE DYNAMICAL SUPPORT
(WIDESPREAD RAINFALL)...UPSLOPE ENHANCEMENT...WEAK INSTBY...AND
STILL FAVORABLE MOISTURE PROFILES (PWS 2-3 STANDARD DEVIATIONS
ABOVE THE NORM FOR LATE SEPTEMBER) WILL SUPPORT THE POTENTIAL OF
ISOLATED EXCESSIVE SHORT TERM RAINFALL RATES AND THUS RUNOFF
ISSUES...ESPECIALLY OVER THE MORE SLOPED TERRAIN WHERE THE 1-3
HOURLY FLASH FLOOD GUIDANCE WILL BE LOWER.


CENTRAL/EASTERN GULF COAST INTO PORTIONS OF THE SOUTHEAST
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

THE DIFFUSE UPPER LEVEL TROUGH WILL PUSH SLOWLY E OF THE
MISSISSIPPI DURING THE PERIOD...AS UPPER LEVEL HEIGHT FALLS DROP
AS FAR SOUTH OF THE LOWER TN VALLEY. THE WEAK QG FORCING AHEAD OF
THE BROAD-SCALE TROUGH WILL GET SOME FRONTOGENETIC ENHANCEMENT
FROM THE SUBTLE INCREASE IN UPPER LEVEL CONFLUENCE OVER THE TN
VALLEY...IN RESPONSE TO THE HEIGHT FALLS AS ANOTHER (NORTHERN
STREAM) SHORTWAVE DROPS INTO THE UPPER GREAT LAKES REGION. AS A
RESULT THE MODELS SHOW A POCKET ENHANCED UPPER DIVERGENCE/LOW-MID
LEVEL FGEN ACROSS THE ERN GULF COAST AND PTNS OF THE SOUTHEAST
WITHIN THE RIGHT ENTRANCE REGION OF AN INCREASING UPPER JET
GRADIENT TO THE NORTH. IN TERMS OF THE QPF DETAILS...MODEL
DIFFERENCES REMAIN...WITH (STILL) A GREAT DEAL OF SPREAD IN TERMS
OF HOW FAR NORTH MEASURABLE RAINFALL WILL GET (I.E. HOW FAR N INTO
THE MID ATLC REGION) THROUGH 00Z TUE...WHILE ALSO NOTING SOME
DIFFERENCES WITH THE PLACEMENT OF HEAVIEST RAIN AS WELL AS TIMING
-- BOTH OF WHICH ARE NOT AS SIGNIFICANT COMPARED TO PRIOR MODEL
RUNS. GIVEN THE UPPER LEVEL SHORTWAVE SUPPORT AND RIGHT ENTRANCE
REGION UPPER JET ENHANCEMENT BY MON AFTN...WPC DID TREND A LITTLE
FARTHER N WITH THE QPF COMPARED TO THE PREVIOUS FCST...INTO NC PER
THE ENSEMBLE OF 12Z GLOBAL GUIDANCE (QUICK PEEK AT THE 12Z ECMWF
WOULD SUPPORT THE FARTHER N TREND...PERHAPS EVEN INTO VA PER THE
12Z RGEM AS WELL).

AT THE SAME TIME...WPC CONTINUED WITH THE HIGHEST AREAL-AVERAGE
QPF ACROSS PTNS OF THE SOUTHEAST -- PARTICULARLY ALONG THE EASTERN
GULF COAST AND (ESPECIALLY) INTO SOUTHERN GA. MOST OF THE GLOBAL
AND HIGH RES GUIDANCE SHOW FAIRLY GOOD CONSENSUS WITH THE AXIS OF
MAX RAINFALL (HIGHEST PROBS OF 2+ INCHES) ACROSS SOUTHWEST
GA...THOUGH WITH THE NSSL WRF WAS A SLIGHT SOUTHERN OUTLIER (NEAR
THE SW GA/FL BORDER) WHILE THE WRF-NMMB WAS A NORTHERN OUTLIER
(CENTRAL GA). THE GFS CONTINUES TO SHOW CONVECTIVE FEEDBACK ISSUES
WITH MAX AMOUNTS OVER 6 INCHES...WHILE THE RGEM IS A BIT FARTHER
EAST OF THE CONSENSUS (SOUTH-CENTRAL AND SE GA).

DEEP-LAYER INSTBY WILL BE TOUGH TO COME BY IN THIS
ENVIRONMENT...WITH THE DEEP SUBTROPICAL MOISTURE AND (THUS) WEAK
MID LAYER LAPSE RATES (I.E. MOIST ADIABATIC). STILL...MUCAPES OF
500-1000 J/KG WILL BE MORE THAN SUFFICIENT IN GENERATING AREAS OF
HEAVY  RAINFALL (POTENTIALLY EXCESSIVE RAINFALL RATES) GIVEN THE
THERMODYNAMIC SETUP WITH PRECIPITABLE WATER VALUES AOA 2.25 INCHES
AND WET BULB ZERO LEVELS ABOVE 14000 FT (WARM CLOUD LAYERS WELL
ABOVE 10000 FEET...FAVORING OPTIMAL PRECIPITATION EFFICIENCY). THE
LOW-LEVEL INFLOW WILL REMAIN RATHER DIFFUSE AND FAR FROM ROBUST
(~15-20 KTS)...HOWEVER STILL SUFFICIENT FOR ORGANIZED AREAS OF
HEAVY (POTENTIALLY EXCESSIVE) RAINFALL GIVEN THE DYNAMICAL AND
THERMODYNAMICAL SUPPORT. ACROSS SOUTHWEST GA...SOME OF THE HIGHER
RESOLUTION GUIDANCE (12KM NAM CONEST...WRF-ARW...AND NSSL-WRF)
SHOW LOCALIZED AMOUNTS IN EXCESS OF 8 INCHES THROUGH 00Z TUE. A
SLIGHT RISK WAS NOTED GIVEN THE HIGH FLASH FLOOD GUIDANCE
(FFG)...AT LEAST INITIALLY...WHICH IS CURRENTLY 2.5-4.0
INCHES/HOUR AND 3.0-4.0 INCHES/3 HOURS.

HURLEY
$$





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