MESOSCALE PRECIPITATION DISCUSSION 0337
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1127 AM EDT SUN JUN 18 2017

AREAS AFFECTED...SOUTH-CENTRAL AL

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

VALID 181526Z - 181826Z

SUMMARY...THUNDERSTORMS ARE FORMING DUE TO DAYTIME HEATING IN THE
SOUTHWEST FLANK OF A STATIONARY MCV.  HOURLY RAIN RATES COULD RISE
BACK TO 2.5", WITH LOCAL AMOUNTS UP TO 5" POSSIBLE OVER SATURATED
SOILS.

DISCUSSION...AN MCV HAS BEEN STALLED IN EAST-CENTRAL AL NEAR
TALLASSEE FOR QUITE SOME TIME, REINFORCED BY EARLY MORNING
CONVECTION.  THE SYSTEM LIES IN A COL IN THE MID-LEVEL FLOW, WITH
GUIDANCE SUGGESTING IT SHOULD REMAIN STATIONARY THROUGH THE DAY
TODAY.  THE ACTIVITY INITIALLY FORMED NEAR A WEAK DEW POINT
GRADIENT STRETCHING NORTH TO SOUTH ACROSS AL, WHICH HAS BEEN
REINFORCED BY A COLD POOL FROM MORNING CONVECTION.  MLCAPE VALUES
ARE RISING SLOWLY DUE TO DAYTIME HEATING, IN THE 500-1000 J/KG
RANGE, WHICH IS ALLOWING NEW CONVECTION TO FORM WITHIN ITS
SOUTHWEST QUADRANT.  INFLOW AT 850 HPA IN THAT AREA IS 15-20 KTS
AND CONVERGENT FROM THE SOUTHWEST.  BULK SHEAR IS LESS THAN 25
KTS, SUGGESTING THAT CONVECTION WOULD BE PULSE IN NATURE.

WITH SUNSHINE FILTERED BY HIGH CLOUDS, BELIEVE 500-1000 J/KG OF
MLCAPE COULD BUILD TO THE WEST AND SOUTH OF THE RENEWED CONVECTION
TODAY, WHICH WOULD KEEP THUNDERSTORM DEVELOPMENT GOING.  IF THE
CONVECTION GROWS IN SCALE, A SOUTHWEST DRIFT IS POSSIBLE PER THE
FORWARD PROPAGATING CONVECTIVE VECTORS.  HOWEVER, IF CONVECTION
DOES NOT GROW IN SCALE, THE COLD POOL WOULD WEAKEN WHICH WOULD
KEEP CONVECTION LOOSELY ORGANIZED WITH LESS OF A DRIFT.  THE
MESOSCALE GUIDANCE DOES NOT BELIEVE IT WILL PERSIST, BUT RECENT
RADAR TRENDS ARE HARD TO IGNORE.  SOILS ARE SATURATED IN THIS
REGION FROM THE PAST FEW DAYS OF RAINFALL, AND FLASH FLOOD
GUIDANCE VALUES HAVE FALLEN QUITE A BIT DUE TO THE EARLY MORNING
RAINS.  HOURLY RAIN RATES COULD RISE BACK TO 2.5", WHICH COULD
ALLOW FOR LOCAL AMOUNTS UP TO 5" (SHOULD IT REORGANIZE) OVER THE
NEXT SEVERAL HOURS.  UNCERTAINTY IS HIGH IN THIS SCENARIO SO KEPT
THE CATEGORY AS POSSIBLE AND KEPT THE HORIZON AT THREE HOURS.

ROTH

ATTN...WFO...BMX...MOB...

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

LAT...LON   32958629 32718603 32578586 32238581 32078596
            31808629 31748665 32028697 32318708 32638704
            32888678