MESOSCALE PRECIPITATION DISCUSSION 0186
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
235 AM EDT WED MAY 23 2018

AREAS AFFECTED...EAST-CENTRAL AL...WESTERN/CENTRAL GA

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

VALID 230635Z - 231235Z

SUMMARY...VERY SLOW-MOVING SHOWERS AND THUNDERSTORMS WITH
EXTREMELY HEAVY RAINFALL RATES ARE EXPECTED TO CAUSE FLASH
FLOODING OVER THE NEXT SEVERAL HOURS.

DISCUSSION...A SMALL-SCALE LOW TO MID LEVEL TROUGH AXIS IS ALIGNED
GENERALLY SW/NE FROM AREAS OF EASTERN AL UP ACROSS NORTHWEST GA.
THIS TROUGH HAS SOME CONCENTRATED VORT ENERGY EMBEDDED WITHIN IT
AND PARTICULARLY NEAR THE AL/GA LINE EAST OF THE AUBURN AREA.

THERE HAS BEEN A NOTABLE EXPANSION AND COOLING OF CONVECTIVE TOPS
OVER THE LAST COUPLE OF HOURS AS SEEN IN THE GOES-16 CLEAN IR
SATELLITE IMAGERY. THE ACTIVITY IS FOCUSING JUST EAST OF A MEAN
925/700MB LAYER TROUGH AXIS AND IS BEING AIDED BY AN INSTABILITY
GRADIENT WITH MUCAPE VALUES OF 500 TO 1000 J/KG.

MOISTURE IS ALSO VERY PLENTIFUL AND THE CONVECTION HAS BEEN
EMBEDDED WITHIN A VERY MOIST 850 MB AXIS WITH SOUTHWEST 850 MB
FLOW OF 20 TO 25 KTS SUSTAINING LOW LEVEL MOISTURE TRANSPORT.
PWATS ARE UPWARDS OF 1.8 TO 1.9 INCHES BASED ON GPS-DERIVED DATA
AND THIS IS CONTRIBUTING TO HIGHLY EFFICIENT RAINFALL PROCESSES
FOR EXTREMELY HEAVY RAINFALL RATES TO NEAR 3 INCHES/HR LOCALLY.

THE HIRES MODELS ARE NOT NECESSARILY IN GOOD AGREEMENT ON THE
LOCATION OF THE HEAVIEST RAINFALL POTENTIAL EARLY THIS
MORNING...BUT THEY ALL AGREE IN A VERY HEAVY RAINFALL SIGNAL. THE
EXPERIMENTAL HRRR GUIDANCE APPEARS TO HAVE THE BEST HANDLE ON THE
ACTIVITY AND IS FOLLOWED AT THIS TIME.

EXPECT THE ACTIVITY CURRENTLY TO TEND TO ADVANCE MORE INTO
WEST-CENTRAL GA AND EXPAND A BIT MORE IN COVERAGE OFF TO THE
EAST-NORTHEAST. WILL NEED TO CLOSELY MONITOR THE POTENTIAL CLOSER
TO DAWN ACROSS THE SOUTHERN SUBURBS OF ATLANTA AS CONVECTION MAY
DEVELOP OR ADVECT INTO THESE AREAS. EXPECT LOCALLY OVER 5 INCHES
OF RAIN GIVEN THE EXTREMELY HEAVY RAINFALL RATES AND SLOW MOVEMENT
OF THE CONVECTION. FLASH FLOODING WILL BE LIKELY AS A RESULT AND
HAS ALREADY BEGUN AND WILL CONTINUE FOR SOME AREAS CURRENTLY ALONG
THE AL/GA BORDER.

ORRISON

ATTN...WFO...BMX...FFC...

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

LAT...LON   33888388 33768314 33328285 32628324 32198418
            32168530 32408581 32728578 33078544 33438484