Quantitative Precipitation Forecast
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FOUS30 KWBC 230026
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
825 PM EDT SUN OCT 22 2017

...VALID 01Z MON OCT 23 2017 - 12Z MON OCT 23 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 N KMDJ 10 SSW KDLP 45 SE MEI 15 WNW 1M4 25 N MKL CGI
25 SSE DEC 20 ESE VPZ 15 WSW HNB 25 SW CSV 10 SE DNN 35 WNW AVL
25 NNW FQD FQD 25 NNE AHN 20 E FFC 15 SSE MGR 55 WSW CTY
115 W PIE 125 W PIE.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 NNE CYD 40 S PQL 50 W MGM 10 W 8A0 15 E 4A9 10 WSW CCO
20 ENE 11J 10 S TLH 30 ESE AAF 60 S AAF.


...CENTRAL AND EASTERN GULF COAST...

SEE ANY VALID MESOSCALE DISCUSSIONS FOR DETAILED EXPECTATIONS. A
SLIGHT RISK OF EXCESSIVE RAINFALL REMAINS IN PLACE FOR THE FLORIDA
PANHANDLE...MUCH OF EASTERN ALABAMA...AND FAR WESTERN GEORGIA
THROUGH 12Z. CONFIDENCE IN EXCESSIVE RAINS IS GREATEST NEAR THE
COAST FROM EAST OF MOBILE TOWARD THE AREA WEST OF TALLAHASSEE.
HEIGHT FALLS WILL PROGRESS ONLY SLOWLY EAST...WITH A CLOSED LOW
FORMING IN THE MID LEVELS OVER ARKANSAS. DEEP MERIDIONAL FLOW AND
INFLUX OF TROPICAL MOISTURE AT LOW LEVELS / ASSOCIATED LIMITED
DOWNDRAFT CAPE...WILL ALSO CONTRIBUTE TO VERY SLOW EASTWARD
PROGRESS. THUS...A QUASI-LINEAR SYSTEM FEATURING SOME
TRAINING...AND INTERCEPTING NEW CELLS FROM WITHIN THE
INFLOW...COULD LEAD TO VERY INTENSE RAIN RATES GIVEN THE
AVAILABILITY OF PW VALUES WELL ABOVE 1.50 INCHES. HI-RES MODELS IN
ADDITION TO THE WPC MANUAL FORECAST INDICATE 1 TO 3 INCHES OF
AREAL AVERAGE RAINFALL...BUT EMBEDDED SWATHS OF 5-PLUS
INCHES...INCLUDING RATES ABOVE 2 INCHES IN AN HOUR...WILL BE
POSSIBLE...OR EVEN EXPECTED ALONG THE COAST.

AREAS FARTHER NORTH MAY BECOME CUT OFF FROM UNSTABLE INFLOW...BUT
SYNOPTIC DEEP LAYER ASCENT WILL INCREASE ACROSS NORTHERN
MISSISSIPPI / ALABAMA...AND CENTRAL AND WESTERN
TENNESSEE...POSSIBLY COMPENSATING TO YIELD SMALLER SCALE HEAVY
RAINFALL AND A MARGINAL FLASH FLOOD THREAT.


...TENNESSEE VALLEY UP THROUGH EASTERN ILLINOIS...

MARGINAL RISK OF EXCESSIVE RAINFALL IS MAINTAINED UP THROUGH
CENTRAL AND WESTERN TENNESSEE TOWARD EASTERN ILLINOIS. THE SPC
MESOANALYSIS AND RAP FORECASTS SHOW THIS REGION TO BE NEARLY VOID
OF MEANINGFUL INSTABILITY. ANY HEAVY RAIN WILL NEED TO BE FORCED
THROUGH THE IMPRESSIVE SYNOPTIC ASCENT...AS A MID LEVEL CLOSED LOW
FORMS...YIELDING 120 METER 12-HOUR HEIGHT FALLS OVER WESTERN
TENNESSEE AND FORMING A SHARP DEFORMATION ZONE / ALMOST A TROWAL /
THROUGH EASTERN ILLINOIS. AT 00Z THIS EVOLUTION WAS ALREADY
EVIDENT...WITH DEEP CONVECTIVE CELLS LINED UP OVER FAR WESTERN
KENTUCKY...AND ASSOCIATED DUAL POL RADAR ESTIMATES OF RAIN RATES
NEARING 0.70 INCHES PER HOUR. THE STRONGEST RAIN RATES OVERNIGHT
ARE EXPECTED FROM HERE NORTHWARD WITHIN THE DEFORMATION
ZONE...WHILE RAINFALL FARTHER SOUTH AND EAST WILL BE MORE
PROGRESSIVE AND LESS FOCUSED ACROSS TENNESSEE. STILL...RATHER THAN
GETTING TOO DETAILED GIVEN THE UNCERTAINTY...AND WITH STILL A
NUMBER OF MODELS INDICATING HEAVY RAIN IN TENNESSEE...WE
MAINTAINED A NARROW MARGINAL RISK TO COVER ALL THESE POSSIBILITIES.


...SOUTHERN APPALACHIANS / GA / NC...

A WING OF MARGINAL RISK EXTENDS INTO THE MOUNTAIN UPSLOPE REGIONS
IN NORTH GEORGIA AND THE WESTERN CAROLINAS. THROUGH 12Z UPSLOPE
FLOW AND ASSOCIATED MOISTURE FLUX VALUES ARE EXPECTED TO
SKY-ROCKET WITH THE APPROACH OF THE DEEP SYNOPTIC SYSTEM OVER THE
MISSISSIPPI VALLEY. MUCAPE VALUES ARE FORECAST TO INCREASE OVER
THE WESTERN CAROLINAS BY MORNING...SUGGESTING GRADUALLY DEEPER AND
MORE EFFICIENT UPDRAFTS. THIS RISK OF RAINFALL EXCEEDING FFG WILL
BE FAIRLY LIMITED SPATIALLY...CONFINED TO THE LOCATIONS THAT ARE
MOST SUSCEPTIBLE TO UPSLOPE ENHANCEMENT.

BURKE



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