MESOSCALE PRECIPITATION DISCUSSION 0251
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
818 PM EDT SAT MAY 20 2017

AREAS AFFECTED...CENTRAL GULF COAST

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 210017Z - 210547Z

SUMMARY...A MESOSCALE WAVE ENTERING SOUTHERN MS IS CAUSING A
FAIRLY LONG TRAINING BAND TO FORM INLAND OF THE CENTRAL GULF
COAST.  WITH HOURLY RAIN RATES UP TO 2.5", AND LOCAL AMOUNTS OF
5-7", FLASH FLOODING IS CONSIDERED LIKELY.

DISCUSSION...INSTABILITY ACROSS CENTRAL MS AND CENTRAL AL HAS
ERODED DUE TO HOURS OF CONVECTIVE ACTIVITY.  THIS HAS CAUSED AN
INSTABILITY GRADIENT/OUTFLOW BOUNDARY TO SET UP WITHIN A FEW
COUNTIES OF THE COASTLINE, AND A FAIRLY LONG TRAINING BAND APPEARS
TO BE SETTING UP IN ADVANCE OF A MESOSCALE WAVE ENTERING
SOUTHERNMOST MS.  PRECIPITABLE WATER VALUES OF 2-2.25" LIE ACROSS
THE REGION.  INFLOW AT 850 HPA CONVERGENT AT 20-30 KTS PER VAD
WIND PROFILES, IN EXCESS OF THE MEAN 850-400 HPA WIND.  BULK SHEAR
IN THE 0-6 KM LAYER IS 25-40 KTS.

THE EXPECTATION IS FOR INFLOW TO INCREASE SLIGHTLY MORE OVER THE
NEXT SEVERAL HOURS AS THE WAVE ADVANCES EASTWARD AT ~25 KTS.  IT
IS POSSIBLE THAT A HIGHLY EFFICIENT MCS FORMS IN THIS REGION
WITHIN THE NEXT FEW HOURS.  THE MESOSCALE GUIDANCE THAT APPEARS
BEST PLACED HERE ARE THE 22Z HRRR, 18Z NAM CONEST, AND 18Z HRRR
EXPERIMENTAL RUNS.  DESPITE POOR PLACEMENT, THERE IS NEAR
UNIVERSAL AGREEMENT ON LOCAL AMOUNTS OF 5-7" IN THIS REGION. 
AVAILABLE MOISTURE SUPPORTS HOURLY RAIN RATES UP TO 2.5", WHICH
COULD BE EVEN HIGHER WITHIN A LONG TRAINING BAND OR WITHIN ANY MCS
WHICH EVOLVES FROM THIS SYSTEM.  CONSIDERING THE RAIN THAT HAS
OCCURRED SO FAR AND THE URBAN AREAS POTENTIALLY AFFECTED, FLASH
FLOODING IS CONSIDERED LIKELY. 

ROTH

ATTN...WFO...BMX...JAN...LIX...MOB...TAE...

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

LAT...LON   31988517 30068532 30208638 30158901 30588946
            31068921 31858738