MESO: Nws Weather Prediction Center College Park Md

AWUS01 KWNH 122010
FFGMPD
MEZ000-MAZ000-NHZ000-VTZ000-CTZ000-NYZ000-122309-

MESOSCALE PRECIPITATION DISCUSSION 0240
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
410 PM EDT THU SEP 12 2013

AREAS AFFECTED...EASTERN NY...PORTIONS OF SOUTHERN NEW ENGLAND

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 122009Z - 122309Z

SUMMARY...TRAINING THUNDERSTORMS ACROSS AN AREA WITH LOW FLASH
FLOOD GUIDANCE COULD LEAD TO REGIONAL FLASH FLOODING.

DISCUSSION...THUNDERSTORMS FORMING NEAR AND AHEAD OF A PROGRESSIVE
COLD FRONT HAVE BEGUN TO TRAIN ACROSS EASTERN NY, NORTHWEST MA,
SOUTHERN VT, NORTHWEST MA, AND SOUTHERN NH. THE AREA IS DIVERGENT
ALOFT TO THE EAST OF AN UPPER LEVEL TROUGH MOVING THROUGH THE
GREAT LAKES. THE BAND OF THUNDERSTORMS IS ALIGNED WITH THE
850-400 HPA MEAN LAYERED WINDS, WHICH INDICATES CELL TRAINING
WOULD BE LIKELY EVEN IF THUNDERSTORMS DID NOT ORGANIZE. HOWEVER,
CAPES STARTING OUT IN THE 1000-2000 J/KG RANGE WHILE 0-3 KM
VERTICAL WIND SHEAR IS IN THE 30-35 KNOT BALLPARK, WHICH INDICATE
ORGANIZED CONVECTION IN THE FORM OF WET MACROBURST MULTICELLS,
THOUGH SOME FORWARD PROPAGATION IS POSSIBLE DUE TO A BUILDING COLD
POOL AT THE SURFACE. PRECIPITABLE WATER VALUES IN THE REGION ARE
AROUND 1.75 INCHES, WHICH IS IN THE 95TH PERCENTILE FOR
MID-SEPTEMBER FOR THIS REGION.

CORFIDI VECTORS FROM THE 12Z GFS INDICATE SLOW PROPAGATION TO
ORGANIZED CONVECTION IN THIS REGION, BARRING SUBSTANTIAL INFLUENCE
FROM ANY BUILDING COLD POOLS. FLASH FLOOD GUIDANCE VALUES ARE
NEAR TWO INCHES IN THREE HOURS, AND RECENT RADAR IMAGERY INDICATES
RAINFALL RATES OF 1-2 INCHES PER HOUR, WHICH WOULD EASILY EXCEED
GUIDANCE. THE GUIDANCE IS SHOWING MIXED SIGNALS HERE, SHOWING
FADING CAPE VALUES ALMOST IMMEDIATELY IN THE 12Z ARW YET THE 15Z
HRRR INDICATES THE POSSIBILITY OF CELL TRAINING FOR SIX HOURS, SO
USED THREE HOURS AS THE HORIZON FOR THIS ISSUANCE. DUE TO THE
UNCERTAINTY OF ITS DURATION, REGIONAL FLASH FLOODING IS CONSIDERED
POSSIBLE HERE.

ROTH

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...GYX...BOX...BTV...ALY...BGM...

ATTN...RFC...MARFC...NERFC...

LAT...LON 44217058 43357310 41807504 41607379 42547114 43936922
44217058

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