MESOSCALE PRECIPITATION DISCUSSION 0800
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
401 PM EST SAT DEC 17 2016
 
AREAS AFFECTED...AR/MS BORDER THROUGH WRN TN/KY INCLUDING MO
BOOTHEEL 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 172100Z - 180300Z
 
SUMMARY...INCREASING COVERAGE OF TRAINING CONVECTION PAST SUNSET
MAY CAUSE FLASH FLOODING IN A FEW PLACES. HOURLY RATES OF 1-2
IN/HR MAY LEAD TO 6 HOUR RAINFALL OF 3-4 INCHES.

DISCUSSION...AN AXIS OF SCATTERED SHOWERS AND THUNDERSTORMS HAS
BEEN EXPANDING IN COVERAGE OVER THE PAST HOUR FROM NERN AR TO THE
LOWER OHIO VALLEY. THESE STORMS ARE OCCURRING ALONG THE NWRN EDGE
OF MLCAPE VALUES RANGING FROM 500-1000 J/KG...LOCATED JUST AHEAD
OF A STRONG COLD FRONT. SWLY 850 MB WINDS OF ROUGHLY 50 KT HAVE
ALLOWED THE TRANSPORT OF LOW LEVEL MOISTURE INTO THE LOWER TO
MID-MISSISSIPPI VALLEY WITH SURFACE OBSERVATIONS SHOWING DEWPOINTS
WELL INTO THE 60S. 

STORMS HAVE BEEN AVERAGING MOTIONS TOWARD THE NORTHEAST AT 40-50
KTS AND AS AN UPPER TROUGH APPROACHES FROM THE WEST OVER THE NEXT
3-6 HOURS...TWO THINGS ARE EXPECTED TO HAPPEN DOWNSTREAM.
FIRST...THE EXPANSION OF A 150-170 KT JET EXTENDING NEWD FROM THE
MID-MISSISSIPPI VALLEY WILL ACT TO INCREASE UPPER LEVEL DIVERGENCE
AND DIFFLUENCE OVER THE AR/TN/MS TRI-STATE REGION. SECOND...850 MB
WINDS ARE FORECAST TO INCREASE AN ADDITIONAL 10-15 KTS AHEAD OF
THE COLD FRONT...WHICH WILL STEADILY DROP TOWARD THE SOUTHEAST AT
15-20 KTS. THESE FACTORS SHOULD ACT TO INCREASE COVERAGE OF STORMS
AHEAD OF THE COLD FRONT WITHIN THE UNIDIRECTIONAL FLOW PATTERN IN
PLACE BETWEEN THE LFC AND EL. 

FLASH FLOOD GUIDANCE VALUES ARE 2-3 INCHES IN 3 HOURS WHICH MAY BE
EXCEEDED...BUT GIVEN RELATIVELY DRY CONDITIONS OVER THE PAST FEW
WEEKS AND USGS INDICATED STREAMFLOWS ARE NORMAL TO MUCH BELOW
NORMAL AREA WIDE...FLASH FLOODING IS ONLY CONSIDERED TO BE
POSSIBLE. 

OTTO

ATTN...WFO...JAN...LMK...LZK...MEG...OHX...PAH...

ATTN...RFC...ABRFC...LMRFC...OHRFC...

LAT...LON   37758700 37268611 36428637 35038841 34089012 
            33769150 34089229 35159199 36489024 37358853