MESOSCALE PRECIPITATION DISCUSSION 0694
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
921 PM EST THU DEC 24 2015
 
AREAS AFFECTED...CENTRAL/SOUTHEAST AL...WESTERN GA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 250210Z - 250700Z
 
SUMMARY...VERY INTENSE RAINFALL CONTINUES IN ASSOCIATION WITH
BACK-BUILDING AND TRAINING CONVECTION OVER SOUTHEAST AL AND PARTS
OF WESTERN GA. SIGNIFICANT FLASH FLOODING IS LIKELY TO CONTINUE
THROUGH THE NIGHT.

DISCUSSION...COLD-TOPPED CONVECTION WITH INTENSE RAINFALL RATES
CONTINUES OVER SOUTHEAST AL AND WESTERN GA AS PERSISTENT DEEP
LAYER AND VERY MOIST SOUTHWESTERLY FLOW FEEDS NORTH AND INTERACTS
WITH A CONVECTIVELY REINFORCED COOL POOL/SURFACE BOUNDARY.
GPS-DERIVED PWATS ARE AS HIGH AS 1.7 TO 1.8 INCHES IN THIS REGION.
ADDITIONALLY...THE CONVECTION IS CONTINUING TO BE AIDED BY A
BROADLY DIVERGENT FLOW PATTERN ALOFT WHICH IS BEING FACILITATED
ESPECIALLY OVER SOUTHEAST AL BY A 250 MB UPPER JET COUPLET
ASSOCIATED WITH THE LEFT-EXIT REGION OF THE SUBTROPICAL JET
CROSSING THE NORTHEAST GULF OF MEXICO AND SOUTHEAST COASTAL
PLAIN...AND THE RIGHT-ENTRANCE REGION OF THE POLAR JET OVER THE TN
VALLEY.

THE COMBINATION OF DEEP LAYER ASCENT/DIVERGENCE ALOFT AND LOW
LEVEL CONVERGENCE NEAR A LOW LEVEL COLD POOL BOUNDARY WILL
CONTINUE TO FOCUS INTENSE CONVECTIVE RAINS IN THE SHORT-TERM AS
THE LATEST RAP SHOWS ALSO A WELL-DEFINED POOL OF MODERATE
INSTABILITY...2000 J/KG...AS PER LATEST RAP SOUNDINGS AND 00Z RAOB
DATA OVER THE CENTRAL GULF COAST WHICH IS FEEDING NORTHEAST INTO
THE AREA OF ACTIVE CONVECTION WITH THE AID OF A 30 KT
SOUTHWESTERLY LOW LEVEL JET.

CAM GUIDANCE OVERALL IS NOT HANDLING THE ACTIVITY PARTICULARLY
WELL AS THE HRRR CONTINUES TO BE TOO QUICK TO WANT TO WEAKEN THE
CONVECTION WHICH SEEMS TO BE PROBLEM IT HAS HAD MUCH OF THE DAY
OVER THE REGION. THE 12Z ARW AND 12Z NMMB ARE MORE ROBUST IN
MAINTAINING CONVECTION AND TEND TO HAVE A BETTER HANDLE ON THE
CURRENT ACTIVITY. GIVEN THE CURRENT THERMODYNAMIC ENVIRONMENT AND
LARGER SCALE ASCENT OVER THE REGION...THESE MODELS WILL BE FAVORED
OVERALL.

EXPECT ADDITIONAL RAINFALL AMOUNTS GOING PAST 06Z OF AS MUCH AS 3
TO 5 INCHES OF RAIN. SOME ISOLATED HEAVIER TOTALS ARE POSSIBLE
GIVEN THE EXCELLENT SET-UP FOR BACK-BUILDING AND TRAINING
CONVECTION.

ORRISON

ATTN...WFO...BMX...FFC...MOB...TAE...

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

LAT...LON   33088468 32918375 32158368 31918468 31598579 
            31518681 31218755 31378775 31948758 32508679 
            32768573