MESOSCALE PRECIPITATION DISCUSSION 0189
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1042 PM EDT THU MAY 04 2017

AREAS AFFECTED...SOUTHWEST VA...NORTHWEST NC...EXT SOUTHEAST
WV...EXT EASTERN TN

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

VALID 050245Z - 050800Z

SUMMARY...CONTINUED BROAD SCALE OROGRAPHIC ASCENT ACROSS THE BLUE
RIDGE THROUGHOUT THE NIGHT WITH OCCASIONAL EMBEDDED CONVECTIVE
LINE ELEMENTS WILL LOCALLY INCREASE RATES INDUCING POSSIBLE FLASH
FLOOD CONDITIONS.

DISCUSSION...WV LOOP DENOTES A POTENT SHORTWAVE LIFTING ACROSS THE
EASTERN TN VALLEY TOWARD THE NORTH AT THE INTERSECTION OF THE
SPLIT IN THE WARM CONVEYOR BELT (DEMARCATED BY THE SUBTROPICAL JET
CIRRUS CANOPY) WITH MAIN UPPER LOW CONTINUING ADVANCE ACROSS THE
ALABAMA WITH ALL PIECES PROVIDING LARGE SCALE ASCENT ACROSS THE
SOUTHERN APPALACHIANS AND PIEDMONT REGIONS OF THE CAROLINAS. 
OVERALL THIS SUPPORTS BROAD SWLY FLOW THAT ISENTROPICALLY ASCENDS
ACROSS WEAK SURFACE BOUNDARY IN THE PIEDMONT OF NC/VA...EXPOSING
THE BLUE RIDGE AND FURTHER WEST SW-NE FACING MOUNTAIN RIDGES TO
STEADY MOIST FLOW THAT FOCUSED ON THE MOUNTAINS ALLOWS FOR
OROGRAPHIC ASCENT AND INCREASED RAINFALL OVER THE NEXT FEW HOURS. 
OCCASIONAL NNW-SSE ORIENTED CONFLUENCE BANDS IN THE BROAD FLOW
WILL FOCUS ENOUGH FOR INCREASED SHORT-TERM RATES UP TO
.5"/HR...OVERALL THIS WILL SUPPORT 1-2" TOTALS OVER THE NEXT 6 HRS
(HIGHER FURTHER SW ALONG THE BLUE RIDGE WHERE FLOW IS INCREASED
DUE TO PROXIMITY TO THE UPPER LOW.  THIS WOULD LEAD TO A MARGINAL
THREAT FOR HIGH SUSCEPTIBLE HYDROLOGIC CHANNELS FOR FLOODING.

OF GREATER CONCERNS...THOUGH DIFFICULT TO DISCERN IN CONVENTIONAL
SATELLITE...THE BROAD DIFFLUENCE IN THIS SPLIT SHOULD SUPPORT A
NEW AMPLIFYING LOW LEVEL SPIN UP ACROSS CENTRAL SC...WHICH APPEARS
BEST IN REGION RADARS WITH A WEAK SURFACE REFLECTION WEST OF DCM
AT 02Z.  THESE LOCALIZED HEIGHT/PRESSURE FALLS SUPPORT INCREASED
LOW LEVEL FLOW AND CONFLUENCE OF WEAKLY UNSTABLE (100-400 J/KG PER
RAP FCST) AND DEEPLY MOIST (UP TO 1.5) ENVIRONMENTAL PARAMETERS TO
ALLOW FOR EFFICIENT RAINFALL UP TO 1.25"/HR RATES.  RECENT HI-RES
CAMS HRRR RUNS SEEM TO CAPTURE THIS WEAK SURFACE REFLECTION TOWARD
NW NC AND SW VA...AND WHILE THERE IS SOME REMAINING UNCERTAINTY ON
PRECISE LOCATION DIFFERENCES...THE RECENT 00Z-01Z HRRR APPEAR TO
BE BEST WITH INITIAL PLACEMENT AND TRACK TO HAVE SOME INCREASED
CONFIDENCE IN FOCUSING ALONG THE MAIN TERRAIN SW OF ROANOKE VA AND
TRACK N ALONG THE SPINE...INCREASING LOCALIZED VALUES TO 2-3"
WHICH SIGNIFICANTLY INCREASES THE POSSIBILITY FOR FLASH FLOODING
CONDITIONS PARTICULARITY GIVEN THE LOCALIZED MINIMUM IN THE FFG
FIELDS IN THIS REGION. 

GALLINA

ATTN...WFO...GSP...LWX...MRX...RNK...

ATTN...RFC...LMRFC...MARFC...OHRFC...SERFC...

LAT...LON   38977805 38057845 37357911 36857982 36298058
            36008077 35698114 35598150 35898232 36358210
            36698193 37008168 37718073 37987999 38447951
            38907881