MESOSCALE PRECIPITATION DISCUSSION 0659
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
404 AM EDT WED AUG 09 2017

AREAS AFFECTED...CENTRAL GA & SOUTH-CENTRAL SC

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

VALID 090803Z - 091203Z

SUMMARY...SHOWERS ARE DEVELOPING WITHIN A MOIST AIRMASS NEAR AN
850 HPA BOUNDARY.  HOURLY RAIN RATES TO 2" WITH LOCAL AMOUNTS TO
4" ARE POSSIBLE.

DISCUSSION...RADAR AND NON-OPERATIONAL GOES-16 INFRARED SATELLITE
IMAGERY SHOW A SLOWLY GROWING AREA OF SHOWERS WITH COOLING CLOUD
TOPS ACROSS THE REGION.  THIS IS OCCURRING JUST AHEAD OF A
SHORTWAVE ENTERING NORTHWEST GA NEAR THE ENTRANCE REGION OF A JET
STREAK ALOFT.  PRECIPITABLE WATER VALUES OF 2-2.2" EXIST HERE PER
GPS DATA.  THE 00Z FFC/PEACHTREE CITY GA SOUNDING WAS SATURATED UP
TO 250 HPA, WHICH SHOULD MAKE EFFICIENT USE OF THE 500-1000 J/KG
MU CAPE VALUES ACROSS THE AREA.  INFLOW AT 850 HPA WAS WESTERLY AT
10-15 KTS PER VAD WIND PROFILES, NEAR THE MAGNITUDE OF THE 850-400
HPA MEAN WIND.

THE MESOSCALE GUIDANCE IS BULLISH ON LOCAL AMOUNTS IN THIS REGION,
SHOWING A CONSISTENT SIGNAL FOR AMOUNTS TO 5" THROUGH 13Z --
BELIEVE THE 00Z ARW HAS THE BEST GRASP ON WHAT IS DEVELOPING. 
HOWEVER, RAP FORECASTS INDICATE DECREASING INSTABILITY WITH TIME,
LIKELY DUE TO THE DEVELOPING SHOWER AND THUNDERSTORM ACTIVITY. 
THE 00Z SSEO NEIGHBORHOOD PROBABILITIES OF EXCEEDING 3" OF RAIN IN
THREE HOURS RAMP UP SLOWLY WITH TIME, PEAKING AFTER 15Z.  HOURLY
RAIN RATES UP TO 2" WITH LOCAL AMOUNTS TO 4" ARE EXPECTED, WHICH
COULD THREATEN THE THREE HOUR FLASH FLOOD GUIDANCE VALUES, WHICH
IS MORE LIKELY WITHIN THE PIEDMONT AND URBAN AREAS.  USED A FOUR
HOUR WINDOW DUE TO UNCERTAINTY INTRODUCED WITH THE EXPECTED
DECREASING INSTABILITY WITH TIME.

ROTH

ATTN...WFO...CAE...CHS...FFC...

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

LAT...LON   33878216 33738060 33287978 32758077 32628230
            32798442 33108490 33288490 33808419