Quantitative Precipitation Forecast
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FOUS30 KWBC 271836
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
136 PM EST SAT DEC 27 2014

...VALID 18Z SAT DEC 27 2014 - 00Z MON DEC 29 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 WSW TOI NBG 25 NW KVNP 15 SW GLS 10 NNE SGR 45 E OCH
25 NNE GWO MDQ 30 SW RHP 20 S LZU 25 WSW TOI.


...LOWER MISSISSIPPI VALLEY INTO THE LOWER TENNESSEE VALLEY...

THE PREVIOUS SLIGHT RISK AREA OVER THE LOWER MS VALLEY REGION WAS
EXPANDED A LITTLE FARTHER DOWNSTREAM (NORTHEASTWARD) ACROSS THE
LOWER TN VLY AND INTO NORTHWEST GA. THE MODELS CONTINUE TO SUPPORT
AN AREA OF HEAVY TO POTENTIALLY EXCESSIVE RAINFALL ACROSS THIS
AREA...ALTHOUGH STILL SHOW NOTABLE DIFFERENCES IN TERMS OF WHERE
THE AXIS OF HEAVIEST RAIN WILL BE (ESPECIALLY THE HIGHER
RESOLUTION GUIDANCE). WPC CONTINUED TO SIDE WITH THE SOUTHERN
FRINGES OF THE GUIDANCE SPREAD... CLOSER TO THE ENSEMBLE OF HIGHER
RESOLUTION GUIDANCE (INCLUDING THE RGEM)...BUT MOST NOTABLY
CORRELATING CLOSEST TO THE 4 KM NSSL-WRF AND NAM CONEST RUNS IN
TERMS OF THE AXIS OF HEAVIEST RAINFALL (LOCATION)... THOUGH NOT AS
ROBUST AS THESE MODELS WITH RESPECT TO AMOUNTS.

CONVECTION SHOULD CONTINUE TO EXPAND AND INTENSIFY ACROSS THE
LOWER MS VLY AND INTO THE LOWER TN VLY THIS AFTN AND
EVENING...ALONG AND AHEAD OF A SLOWLY EASTWARD MOVING FRONTAL
BOUNDARY. PW VALUES AND 850-700 MOISTURE FLUX WILL CONTINUE TO
RISE ACROSS THE AREA...AND AS WITH PRIOR EXPECTATIONS...PEAK
AROUND 2.5 STANDARD DEVIATIONS ABOVE THE NORM OR ABOVE THE 75TH
PERCENTILE. DYNAMICAL FORCING WILL ALSO BE FAVORABLE... WITH
COUPLED CONTRIBUTIONS VIA THE THE RIGHT ENTRANCE REGION OF THE NRN
STREAM JET STREAK (ENHANCING THE LOW-MID LAYER FGEN)... WHILE THE
FORCING AND FAVORABLE THERMODYNAMIC PROFILE CONTINUES TO BUILD
OVER THE AREA OUT AHEAD OF THE SOUTHERN STREAM TROUGH. A KEY
CONTRIBUTOR TO THE EXCESSIVE RAINFALL THREAT WILL BE THE POTENTIAL
FOR TRAINING CONVECTION... AS FOR A WHILE (THIS EVENING AND
OVERNIGHT AHEAD OF THE HEIGHT FALLS) SOUTHWESTERLY LOW LEVEL FLOW
(30-40 KTS) WILL BE ALIGNED NEARLY PARALLEL TO THE MEAN 850-300 MB
WIND...THUS SUPPORTING WEAKENING CORFIDI VECTORS AND ENHANCED
UPWIND PROPAGATION. THE MAIN INHIBITOR TO THE EXCESSIVE RAINFALL
THREAT -- AT LEAST IN TERMS OF SHORT TERM RAINFALL RATES (1-3
HOURS) WILL BE THE LACK OF DEEP LAYER INSTABILITY... PARTICULARLY
OVER THE NORTHERN PORTION OF THE OUTLOOK AREA WHERE MUCAPES WILL
STRUGGLE TO REACH 500 J/KG. THIS IS THE KIND OF SETUP THAT WOULD
LIKELY FAVOR LONGER DURATION FLOODING (6-12+ HOURS)... THOUGH THE
DECISION TO MAINTAIN A SLIGHT RISK FOR SHORT TERM EXCESSIVE
RAINFALL WAS BASED ON CONTINUITY AS WELL AS THE LATEST SSEO 40 KM
NEIGHBORHOOD PROBABILITIES OF 3-6 HR QPF EXCEEDING 3-6 HOUR FFG
RESPECTIVELY (GENERALLY 20-30 PERCENT OVER A LARGE PORTION OF THE
OUTLOOK AREA).

HURLEY

$$





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