MESOSCALE PRECIPITATION DISCUSSION 0415
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1155 AM EDT SAT JUL 07 2018

AREAS AFFECTED...ERN TN...NERN AL...NRN GA...SWRN NC...FAR UPSTATE
SC

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

VALID 071555Z - 072155Z

SUMMARY...THE POTENTIAL FOR FLASH FLOODING WILL BE ON THE INCREASE
THIS AFTERNOON NEAR THE SOUTHERN TIP OF THE APPALACHIAN MOUNTAINS
AS SCATTERED THUNDERSTORMS DEVELOP ACROSS THE REGION. RAIN RATES
MAY REACH 1-2 IN/HR WHICH COULD LEAD TO LOCALIZED FLASH FLOODING.
THE GREATEST CHANCES FOR HEAVY RAINFALL APPEAR TO EXIST IN SE
TENNESSEE... SW NORTH CAROLINA... AND NORTH GEORGIA.

DISCUSSION...KGSP RADAR AND GOES-16 VISIBLE SATELLITE SHOWED THE
FIRST INDICATIONS OF CONVECTIVE INITIATION NEAR THE SOUTHERN TIP
OF THE APPALACHIANS AROUND 16Z. THIS IS EXPECTED TO BE A MORE
FOCUSED AREA FOR HEAVY RAINFALL POTENTIAL GIVEN INCREASING
LOW-LEVEL EASTERLY FLOW IMPINGING ON THE HIGHER TERRAIN THIS
AFTERNOON IN THE PRESENCE OF ELEVATED ATMOSPHERIC MOISTURE LEVELS.
THE 12Z SOUNDINGS FROM GSO AND FFC SHOWED PRECIPITABLE WATER
VALUES AROUND 2 INCHES WITH TALL NARROW CAPE PROFILES...A
FAVORABLE SOUNDING CONFIGURATION FOR HIGHLY EFFICIENT RAINFALL
PRODUCTION. ALTHOUGH DRIER AIR MAY BEGIN TO WORK SOUTH INTO THE
CAROLINAS DOWN THE PIEDMONT IN A BELT OF ENHANCED NORTHEASTERLY
FLOW...SUFFICIENT MOISTURE SHOULD CONTINUE TO EXIST IN THE
OUTLINED AREA TO SUPPORT A THREAT OF HEAVY RAINFALL.

THE 12Z HREF SHOWS THE GREATEST NEIGHBORHOOD PROBABILITIES (80-90
PERCENT) OF OVER 2 INCHES OF RAIN BY 21Z FOCUSED NEAR WHERE THE
TN-GA-NC BORDERS MEET AT THE SOUTHERN TIP OF THE APPALACHIANS.
THIS CLOSELY MATCHES THE EXPECTATIONS BASED ON MESOSCALE
ANALYSIS...WITH THE HEAVIEST RAINFALL LIKELY NEAR AND JUST
DOWNSTREAM OF THE TERRAIN FEATURE AS LOW-LEVEL FLOW INCREASES.
HI-RES MODELS ALSO SHOW OTHER CONVECTION FOCUSING NEAR THE
CUMBERLAND PLATEAU THIS AFTERNOON IN EAST-CENTRAL TENNESSEE...AND
PERHAPS LATER IN THE AFTERNOON IN A MORE UNSTABLE AIR MASS IN
NRN-CTRL GEORGIA TO THE SOUTH OF THE MOUNTAINS. A NUMBER OF THE
NEW 12Z HI-RES MODELS SHOW A BAND OF ENHANCED QPF IN THE VICINITY
OF THE ATLANTA METRO AREA...ALTHOUGH THE TIMING MAY BE JUST BEYOND
THE SCOPE OF THIS DISCUSSION (CLOSER TO 00Z).

IN ADDITION TO THE FAVORABLE ATMOSPHERIC CONDITIONS...EASTERN
TENNESSEE AND FAR NORTHERN GEORGIA HAS BEEN IN A WET PATTERN OVER
THE PAST COUPLE WEEKS...WITH 14-DAY RAINFALL OVER 200 PERCENT OF
NORMAL. FLASH FLOOD GUIDANCE IS REDUCED IN THIS AREA WITH 3-HOUR
FFG AROUND 2 INCHES...WELL WITHIN THE REALM OF WHAT IS ACHIEVABLE
GIVEN THE ENVIRONMENT AND POTENTIAL FOR OROGRAPHIC FOCUSING
MECHANISMS TO PROLONG HEAVY RAIN DURATION.

THE REGION CENTERED NEAR THE CONFLUENCE OF THE TN-NC-GA BORDER HAS
BEEN UPGRADED TO A SLIGHT RISK ON THE DAY 1 EXCESSIVE RAINFALL
OUTLOOK.

LAMERS

ATTN...WFO...BMX...FFC...GSP...HUN...JKL...MRX...OHX...

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

LAT...LON   36638435 36428386 35908376 35638271 35138254
            34438317 33848294 33248252 32878301 32958404
            33268464 33768534 34118597 34558706 35308696
            35828622 36408530