MESOSCALE PRECIPITATION DISCUSSION 0488
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
227 AM EDT FRI JUL 20 2018

AREAS AFFECTED...COASTAL PLAIN OF SC

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

VALID 200626Z - 201226Z

SUMMARY...AN 850 HPA LOW IN COMBINATION WITH AN INCOMING SHORTWAVE
ALOFT IS LEADING TO A SLOW INCREASE IN CONVECTIVE ORGANIZATION
OVER THE SC COASTAL PLAIN.  HOURLY RAIN TOTALS TO 2.5" WITH LOCAL
AMOUNTS TO 5" ARE POSSIBLE THROUGH 12Z.

DISCUSSION...VAD WIND PROFILES ALONG WITH SATELLITE AND RADAR
IMAGERY IMPLY THE FORMATION OF A NEW 850 HPA LOW CENTER NEAR
VIDALIA GA WITHIN AN ELONGATED SHEAR AXIS AT THAT LEVEL AND IN THE
VICINITY OF A WEAK FRONTAL BOUNDARY.  WATER VAPOR IMAGERY SHOWS A
SHORTWAVE MOVING ACROSS WESTERN GA INTO THE REGION, ENHANCING
UPPER LEVEL DIVERGENCE.  CONVECTION IN THE AREA IS SHOWING COOLING
CLOUD TOPS, WITH RADAR SHOWING OCCASIONAL CURVED BANDS MOVING IN
FROM THE ATLANTIC WITH PEAK HOURLY RAIN RATES OF 1" THUS FAR. 
INFLOW AT 850 HPA IS CONVERGENT OUT OF THE SOUTHEAST AT 15-25 KTS
PER VAD WIND PROFILES, DOUBLE THE MEAN 850-400 HPA WIND, ADVECTING
IN ML CAPE VALUES OF 1000-2000 J/KG FROM OFFSHORE; INSTABILITY
TRENDS HAVE BEEN UPWARD DURING THE PAST COUPLE HOURS.  EFFECTIVE
BULK SHEAR IS 25 KTS JUST OFFSHORE PER VAD WIND PROFILES. 
PRECIPITABLE WATER VALUES ARE 2.1-2.4" PER GPS DATA.  THERE DOES
NOT APPEAR TO BE ANY CIN PRESENT.

THE WET BULB ZERO HEIGHT ON THE CHARLESTON SC SOUNDING WAS 14,600
FEET, WHICH SUPPORTS THE IDEA OF WARM RAIN PROCESSES BEING AT WORK
OVERNIGHT INTO THE MORNING HOURS.  THE MESOSCALE GUIDANCE IS
INSISTENT THAT RAINFALL ACROSS THE REGION SHOULD EXPAND IN
COVERAGE AND INTENSITY THROUGH THE EARLY MORNING HOURS, AND THAT
THE RELATED 850 HPA LOW STRENGTHENS AS IT DRIFTS NORTHEASTWARD UP
THE FALL LINE NEAR AND WEST OF I-95.  THE STATIONARY FRONT
OFFSHORE THE CAROLINAS IS EXPECTED TO MIGRATE NORTHWEST INTO THE
COASTAL TROUGH CURRENTLY LOCATED JUST OFFSHORE THE NORTHERN SC
COAST PER RAP FORECASTS.  THE COMBINATION OF STRENGTHENING
LOW-LEVEL INFLOW AND BULK SHEAR SHOULD CONTINUE THE IDEA OF CURVED
BANDS DEVELOPING OFF THE ATLANTIC INTO THE COASTAL PLAIN OF SC,
WHERE THE HEAVIEST RAINFALL WOULD BE FOCUSED.  THERE IS ALSO THE
POSSIBILITY OF CELL MERGERS OCCURRING FROM ANY LESS ORGANIZED
ACTIVITY MOVING WESTWARD ALONG THE WEAK FRONTAL ZONE IN THE AREA
INTO ANY EXISTING ORGANIZED CONVECTIVE STRUCTURES.  HOURLY RAIN
TOTALS UP TO 2.5" SHOULD BE POSSIBLE IN ANY CURVED BANDS THAT
DEVELOP.  THE MESOSCALE GUIDANCE HAS A UNIFIED SIGNAL FOR LOCAL
AMOUNTS IN THE 5" RANGE THROUGH 12Z.  DESPITE THE MODEL AGREEMENT,
UNCERTAINTY IN THE SCENARIO IS HIGHER THAN USUAL AS COASTAL HEAVY
RAIN EVENTS SOMETIMES MATERIALIZE JUST OFFSHORE.

ROTH

ATTN...WFO...CAE...CHS...ILM...

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

LAT...LON   34277825 34177789 34027790 33837801 33777820
            33727876 33417904 33067920 32687971 32517994
            32168053 32238076 32608094 33038083 33817973
            34187888