Flash Flood Guidance
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000
AWUS01 KWNH 191006
FFGMPD
MSZ000-LAZ000-191535-

MESOSCALE PRECIPITATION DISCUSSION 0026
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
505 AM EST THU JAN 19 2017

AREAS AFFECTED...SOUTHERN LA...SOUTHWEST MS

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

VALID 191005Z - 191535Z

SUMMARY...REPEATED CONVECTION AND A FEW CELL MERGERS WITHIN A RIPE
ENVIRONMENT WILL LEAD TO 3-PLUS INCHES OF RAIN IN SOME LOCATIONS
THROUGH 15Z...WITH FLASH FLOODING POSSIBLE.

DISCUSSION...AT 0930Z RADAR SHOWED CONVECTION AND HEAVY RAIN
EXPANDING IN COVERAGE PER REFLECTIVITY AND HYDROMETEOR
CLASSIFICATION. THE FEW GAUGES THAT WERE AVAILABLE...MAINLY
CITIZEN GAUGES...INDICATED A QUICK 1.5 TO 1.75 INCHES OF RAIN
NORTH OF LAFAYETTE. ONE WOULD EXPECT SYNOPTIC ASCENT TO ONLY
INCREASE OVER THIS REGION THE NEXT FEW HOURS...WITH WATER VAPOR
IMAGERY DEPICTING A WELL DEFINED SHORTWAVE TROUGH EMANATING FROM
THE SUB-TROPICAL MOISTURE FEED OVER MEXICO. THE INFLECTION IN THE
FLOW AHEAD OF THIS FEATURE WAS VERY BROAD SCALE...LOW
AMPLITUDE...SUCH THAT THERE WILL BE SEVERAL HOUR PERIOD OF
ENHANCED DEEP LAYER LIFT AND RELATIVELY UNIDIRECTIONAL DEEP LAYER
FLOW PRIOR TO ITS PASSING. THIS IS BORN OUT IN RAP MODEL FORECASTS
OF PERSISTENT 700 MB VERTICAL VELOCITY OVER THE DISCUSSION AREA.

OF THE HI-RES MODELS...THE HRRR WAS BEST CAPTURING THE ONGOING
ACTIVITY AT 0930Z...AND FOR THREE CYCLES THE HRRR HAD HELD ONTO
INCREASED RAINFALL AMOUNTS...LOCALLY GREATER THAN 3
INCHES...WITHIN THE SAME SWATH OF PERSISTENT VERTICAL MOTION
FORECAST BY THE RAP. THIS REGION FROM LAFAYETTE LA TO JACKSON MS
WAS ALONG A GRADIENT IN THE GRIDDED FLASH FLOOD GUIDANCE...BUT
RAINFALL OF THIS INTENSITY SHOULD NOSE ABOVE THE 1-HR AND 3-HR FFG
IN SPOTS. THE ONLY REAL LIMITING FACTOR IS A LACK OF LOW LEVEL
FOCUS...WITH THE CONVECTION BEING ROOTED JUST ABOVE THE SURFACE
AND OUT AHEAD OF A COLD FRONT. MOISTURE FLUX AND DEEP LAYER
ASCENT... HOWEVER...WILL INCREASE...WITHIN AN ENVIRONMENT MARKED
BY NEARLY 1.75 INCH PRECIPITABLE WATER...SUGGESTING ISOLATED
TOTALS MAY GO EVEN HIGHER THAN 4 INCHES...PARTICULARLY AS
SOUNDINGS SHOW LITTLE COLD POOL POTENTIAL TO SUPPORT EASTWARD
PROPAGATION UNTIL THE WAVE PASSES AND 700 MB WINDS VEER TOWARD
14-15Z.

BURKE

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

ATTN...WFO...JAN...LCH...LIX...SHV...

ATTN...RFC...LMRFC...

LAT...LON   32909071 32558991 31069046 29569251 29569263
            29589277 29779342 30689287 32009181



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