Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS30 KWBC 270102
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
802 PM EST FRI DEC 26 2014

...VALID 03Z SAT DEC 27 2014 - 00Z SUN DEC 28 2014...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 W KCRH 20 ESE BPT ACP 20 SSE TVR 20 NE JAN 20 NW MEI
20 SSE MEI 40 E HBG 10 SSW GPT 20 W KXPY 10 NE KVQT 25 W KCRH.


...LOWER MISSISSIPPI VALLEY...

AN AMPLIFIED UPPER LEVEL TROUGH WILL MAINTAIN A POSITIVE TILT
THROUGH DAY 1 (00Z SUN). A NORTHERN STREAM SHORTWAVE WILL TIGHTEN
GRADIENTS...CONTRIBUTING TO DEVELOPMENT OF A 150-PLUS KNOT JET
STREAK...WITH THE LOWER MS VALLEY BENEATH THE RIDGE/INFLECTION
POINT IN THE UPPER FLOW AND WITHIN THE RIGHT ENTRANCE REGION OF
THE JET. THE NET AFFECT OVER TIME WILL BE AN EXPANDING AREA OF
MOD-HEAVY RAINFALL. EXPECT A SURGE IN LOW-LEVEL SOUTHERLY INFLOW
(30-40 KTS AT 850 MB) AND CORRESPONDING ANOMALOUS MOISTURE
FLUX/PRECIPITABLE WATER VALUES -- PARTICULARLY AFTER 12Z SAT. THE
ENSEMBLES SHOW BOTH THE 850-700 MB MOISTURE FLUX AND PWATS
(1.75-2.00 INCHES) PEAKING AROUND 2.5 STANDARD DEVIATIONS ABOVE
THE NORM FOR LATE DECEMBER. MEANWHILE...MUCAPES OVER THE OUTLOOK
AREA ARE EXPECTED TO CLIMB TO BETWEEN 500 TO JUST UNDER 1000
J/KG...DESPITE DEEPLY SATURATED VERTICAL PROFILES.

GIVEN THE DEEP LAYER S-SW FLOW (DIRECTIONAL SHEAR DECREASING WITH
TIME)...THE MODELS SHOW CORFIDI VECTORS FOR FORWARD PROPAGATION
BECOMING INCREASINGLY PARALLEL WITH THE 0-6 KM MEAN WIND...
SUGGESTING TRAINING OF NEW CONVECTIVE CELLS ALONG THE LEADING EDGE
OF ORGANIZED CONVECTIVE SYSTEMS. WPC AREAL-AVERAGE QPF OVER THE
OUTLOOK AREA RANGES BETWEEN 1.5 AND 3 INCHES. THE HEAVY RAIN
SIGNAL IN SOME OF THE HIGHER RESOLUTION GUIDANCE (INCLUDING THE
NAM CONEST...WRF-NMMB...AND NSSL-WRF) IS
IMPRESSIVE...THOUGH...WITH LOCALIZED AMOUNTS BETWEEN 3-6 INCHES.
THESE NUMBERS MAY BE SUPPORTED BY THE UNUSUALLY LARGE PRECIPITABLE
WATER VALUES AND LIKELIHOOD OF SOME TRAINING WITHIN A FAVORABLE
BROAD SCALE FLOW PATTERN.

IF MODELS VARY...IT IS IN THE NORTH TO SOUTH PLACEMENT OF THE
HEAVY RAIN AXIS. IN THE 00Z UPDATE WE EXPANDED THE SLIGHT RISK
AREA JUST SLIGHTLY NORTH...UP TO INTERSTATE 20 FROM JACKSON TO
MERIDIAN MS...TO ENCOMPASS SOME OF THE MIDDLE GROUND SOLUTIONS
SUCH AS THE 12Z CANADIAN GEM AND NCEP WRF NMMB. THE NAM AND OTHERS
WERE THOUGHT TO BE TOO FAR NORTH AND WEST. HIGH FLASH FLOOD
GUIDANCE VALUES AND THE EXPECTED SMALLER SCALE OF THE HEAVY
PRECIPITATION AXIS PRECLUDES INCLUSION OF A MODERATE RISK AT THIS
TIME...BUT A SMALL MODERATE RISK AREA COULD BE CONSIDERED IN LATER
DAY 1 UPDATES.

HURLEY/BURKE
$$




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