MESOSCALE PRECIPITATION DISCUSSION 0808
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
438 AM EDT TUE SEP 12 2017

AREAS AFFECTED...NORTHEAST SC & SOUTHEAST NC

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

VALID 120837Z - 121237Z

SUMMARY...OCCASIONAL BANDS ARE EXPECTED TO ORGANIZE AS THEY MOVE
IN FROM THE ATLANTIC.  HOURLY RAIN TOTALS TO 2.5" WITH LOCAL
AMOUNTS TO 5" ARE EXPECTED.

DISCUSSION...THE OCCLUDED CYCLONE ASSOCIATED WITH POST-TROPICAL
CYCLONE IRMA IS MOVING INTO EAST-CENTRAL AL PER THE 08Z SURFACE
OBSERVATIONS.  A DRY SLOT CONTINUES TO MAKE INROADS THROUGH THE
CAROLINAS INTO SOUTHWEST VA PER WATER VAPOR IMAGERY.  WELL TO ITS
EAST ARE TWO BOUNDARIES WITHIN THE CYCLONE'S WARM SECTOR; ONE WITH
THE LEADING EDGE OF THE THERMAL GRADIENT INSIDE SOUTHWEST SC (THE
LEADING EDGE OF A STRATUS FIELD WHICH IS THE BACKSIDE OF THE
INSTABILITY FIELD) AND THE OTHER SHOWING OFF THE LEADING EDGE OF
THE DEW POINT GRADIENT WITH SOME SURFACE WIND CONVERGENCE
EXTENDING FROM KINGSTREE AND MCCLELLANVILLE OFFSHORE INTO THE
WESTERN SUBTROPICAL ATLANTIC NEAR THE 79TH MERIDIAN (WHICH SHOWS
UP AS A LOW-CLOUD LINE).  TO THE EAST OF THIS CLOUD LINE, ONE
CONVECTIVE BAND DROPPED HEAVY RAINFALL NEAR AND SOUTHWEST OF
BRUNSWICK NC WITH HOURLY RAIN TOTALS TO 2.5", AND ANOTHER WITH
INCREASING RAIN RATES IS POISED TO MOVE ASHORE NEAR CAPE FEAR. 
INFLOW AT 850 HPA IS CONVERGENT AT 35-50 KTS PER RAP MASS FIELDS,
SLIGHTLY IN EXCESS OF THE MEAN 850-400 HPA WIND.  PRECIPITABLE
WATER VALUES ARE 1.5-2" PER GPS DATA.  MU CAPE VALUES OF 500-1000
J/KG LIE NEAR AND JUST OFFSHORE THE COAST.

THE SATURATED ATMOSPHERE SHOULD CONTINUE TO MAKE THE MOST OF THE
LIMITED INSTABILITY AVAILABLE, WITH OCCASIONAL CONVECTIVE BANDS
DROPPING 2.5" IN AN HOUR.  THE MESOSCALE GUIDANCE SHOWS A STRONG
SIGNAL FOR 5" THROUGH 14Z.  HOWEVER, THE BOUNDARY LAYER MOISTURE
CONVERGENCE WEAKENS IN THIS REGION, PARTICULARLY AFTER 12Z WITHIN
RAP MASS FIELDS.  THE 00Z SSEO NEIGHBORHOOD PROBABILITIES AGREE
WITH THE RAP, INDICATING THAT A 50%+ CHANCE OF EXCEEDING THE THREE
HOUR FLASH FLOOD GUIDANCE VIRTUALLY DISAPPEARS AFTER 12Z.  USED A
FOUR HORIZON FOR THE MESOSCALE PRECIPITATION DISCUSSION DUE TO THE
ABOVE.

ROTH

ATTN...WFO...CHS...ILM...MHX...RAH...

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

LAT...LON   35387842 35367767 34717700 34387749 33787794
            33757863 33557892 33347902 32997933 33617961
            34457982 34877958