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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1902Z Apr 19, 2015)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
302 PM EDT SUN APR 19 2015

...VALID 18Z SUN APR 19 2015 - 00Z TUE APR 21 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 ESE GSP 20 WSW AND 20 ENE GVL 20 E DNN 10 NNE CHA 15 SW CSV
30 N CSV 20 N SME 10 NW LEX 30 N FFT CVG 20 SSE ILN 30 W 3I2
30 SE HTS 10 W I16 30 NW LWB 25 NNE EKN 20 NE AOO 30 W AVP
15 WNW POU 10 WNW HFD 20 ESE IJD BID 25 S FOK 20 ESE MJX
15 ESE ESN 25 NW PTB 20 NNE TDF BUY VUJ UZA 30 ESE GSP.


...SPINE OF THE APPALACHIANS EAST TO THE MID ATLANTIC COASTAL
PLAIN AND SOUTHERN NEW ENGLAND...

WITH THE VALID PERIOD NOW RUNNING THROUGH MONDAY
EVENING...21/00Z...WE EXPANDED THE SLIGHT RISK AREA FARTHER NORTH
AND EAST TO CAPTURE MORE OF PA/DE/NJ/NY/CT. WHILE THE PRIMARY
LARGE SCALE CYCLONE WILL FORM OVER THE GREAT LAKES...A HEALTHY
LEAD SHORTWAVE WILL INDUCE STRONG WIND FIELDS FROM THE
APPALACHIANS TO THE EASTERN SEABOARD. PRECIPITATION WITHIN A BROAD
RAIN SHIELD WILL GROW HEAVIER WITH THE APPROACH OF A SURFACE WARM
FRONT. EMBEDDED CONVECTIVE ELEMENTS WILL INCREASE NEAR THE WARM
FRONT...WITH THE MORE AGGRESSIVE MEMBERS OF THE SREF BUILDING
MUCAPE VALUES TO A FEW HUNDRED J/KG IN ADVANCE OF THE FRONT.
STRONG LOW LEVEL UPSLOPE ON SOUTHEASTERLY 850 MB WINDS OF 40-50
KNOTS WILL ALSO CONTRIBUTE TO ENHANCED LIFT AND PRECIPITATION
EFFICIENCY ALONG THE EASTERN SLOPES OF THE MOUNTAINS...WITH THE
CORE OF STRONGEST WINDS LIFTING OUT OF WESTERN SC/NC THIS EVENING
INTO VA/WV/MD OVERNIGHT.

ALTHOUGH THE LARGER INSTABILITY WILL NOT BE CO-LOCATED WITH THE
BETTER FORCING...THE AIRMASS AHEAD OF THE WARM FRONT AND MID LEVEL
WAVE WILL BE CHARACTERIZED BY PW VALUES OF AT LEAST 1.25
INCHES...WITH 1.50 PLUS AT THE COAST...REPRESENTING ABOUT 2.5
STANDARD DEVIATIONS FROM CLIMATOLOGY. GIVEN MODERATELY LOW FLASH
FLOOD GUIDANCE IN THIS REGION...WE DREW A BROAD SLIGHT RISK OF
EXCESSIVE RAINFALL WHERE GENERALLY 1-3 INCH AND ISOLATED 4 INCH
TOTALS ARE EXPECTED.


...EASTERN END OF THE TN/OH VALLEYS INTO THE WESTERN FOOTHILLS OF
THE APPALACHIANS...

ARRIVAL OF BROAD SYNOPTIC ASCENT AND TRANSPORT OF LOWER/MIDDLE
LEVEL MOISTURE INTO THIS REGION WILL YIELD WIDESPREAD RAIN THROUGH
EARLY SUNDAY AFTERNOON. AT THE TAIL END OF THIS SYNOPTICALLY
FORCED REGIME ENOUGH HEATING SHOULD TAKE PLACE TO PRODUCE CAPE OF
500-1000 J/KG THIS AFTERNOON DEVELOPING FIRST IN EASTERN TN AND
SPREADING NORTHWARD. THIS WILL SUPPORT A MORE CONVECTIVE
PRECIPITATION REGIME WITHIN THE ENTRANCE REGION OF A JET STREAK
LIFTING TOWARD EASTERN KY/OH BY 00Z. INDIVIDUAL CELL MOTIONS WILL
BE RATHER QUICK...BUT HIGH RESOLUTION GUIDANCE FORECASTS SOME
SW-NE ORIENTED SWATHS OF TRAINING CONVECTION. THE HRRR HAS BEEN
VERY CONSISTENT IN DEPICTING ROBUST CONVECTION OVER EASTERN TN/KY
THIS AFTERNOON...AND THE 00Z SSEO MEAN 24-HOUR PRECIPITATION SHOW
A SIGNAL FOR LOCAL 1-2 INCH RAINFALL AMOUNTS...SOME OF WHICH COULD
OCCUR QUICKLY. GIVEN RECENT WET CONDITIONS AND MARGINALLY LOW
FLASH FLOOD GUIDANCE VALUES IN THIS REGION WE MAINTAINED A SLIGHT
RISK OF EXCESSIVE RAINFALL.


...EASTERN GA / SOUTHERN SC / NORTHERN FL...

CONVECTIVE LINE SEGMENTS PROGRESSING THROUGH THIS REGION WILL
PRODUCE BRIEF INTENSE RAIN RATES AT A GIVEN LOCATION. CONVECTION
HAD EVOLVED UPSCALE DURING THE MORNING HOURS...AND WAS MOVING
STEADILY EAST IN TANDEM WITH A MID LEVEL SHORTWAVE TROUGH. FLASH
FLOOD GUIDANCE IS HIGH IN THIS REGION...SUGGESTING AT LEAST 2.5 TO
3.0 INCHES IN ONE HOUR WOULD BE NEEDED TO BRING STREAMS TO
BANKFUL. THE MAIN AREA OF CONCERN WOULD BE SOUTHEAST GA AND
SOUTHERN SC THROUGH ABOUT 21Z...AS CLUSTERS OF THUNDERSTORMS
WITHIN A WARM ADVECTION REGIME WILL COMBINE WITH THE EVENTUAL
PASSAGE OF LINE SEGMENTS TO LOCALLY BOOST RAIN DURATION AND
RAINFALL TOTALS.

BURKE