MESOSCALE PRECIPITATION DISCUSSION 0804
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
437 AM EDT MON SEP 11 2017

AREAS AFFECTED...NORTHEAST FL...SOUTHEAST GA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 110834Z - 111430Z

SUMMARY...FLASH FLOODING WILL BE POSSIBLE GENERALLY TO THE NORTH
OF HURRICANE IRMA'S CENTER...WITH THE GREATEST RISK SHIFTING TO
SOUTHEAST GEORGIA AND FAR NORTHEAST FLORIDA.

DISCUSSION...DRY AIR IN THE MID-UPPER LEVELS OF THE TROPOSPHERE
WAS CONTINUING TO WRAP AROUND THE SOUTHERN SIDE OF IRMA'S
CIRCULATION...EVIDENT ON CIRA LAYER TPW PRODUCTS IN THE 700-500MB
AND 500-300MB LAYERS. A NARROW CHANNEL OF THIS DRY AIR HAD EVEN
WRAPPED AROUND TO THE NORTHEAST SIDE OF IRMA...IMMEDIATELY BEHIND
A PRIMARY SPIRAL RAIN BAND. THIS SHOULD LIMIT THE RAIN RATES IN
THE SOUTHERN SEMICIRCLE OF IRMA DESPITE CONTINUED ABUNDANT
LOW-LEVEL MOISTURE...RAIN SOUTH OF THE I-4 CORRIDOR WILL TREND
INCREASINGLY TOWARD SHALLOW TROPICAL RAIN SHOWERS.

TO THE NORTH...FLASH FLOODING WILL BE POSSIBLE ACROSS THE
HIGHLIGHTED AREA...BUT THE GREATEST RISK WILL BE SHIFTING INTO FAR
NORTHEAST FLORIDA AND SOUTHEAST GEORGIA. A FRONT WAS BECOMING
BETTER DEFINED IN SURFACE OBSERVATIONS FROM NEAR CROSS CITY (KCTY)
TO NEAR ST. SIMONS ISLAND (KSSI). LOW-LEVEL (1000-850MB)
FRONTOGENESIS WAS MARKEDLY INCREASING ALONG AND JUST TO THE
NORTHWEST OF THE SURFACE THETAE GRADIENT AND THIS SHOULD BECOME A
FOCUS FOR HEAVY RAIN OVER THE NEXT SEVERAL HOURS. HI-RES MODELS
APPEAR TO HAVE LATCHED ONTO THIS IDEA AS THEY UNANIMOUSLY SHOW A
HEAVY RAIN SIGNAL IN THE SAME AREA. ALMOST ALL SHOW BROAD 4 INCH
RAINFALL TOTALS IN THE SIX HOURS FROM 09-15Z...WITH MAXIMUM
AMOUNTS IN EXCESS OF 7 INCHES.

HOWEVER...THE HI-RES MODELS MAY BE PLACING THE AXIS OF HIGHEST QPF
TOO FAR INLAND. RAP ANALYSIS SHOWS THE HIGHEST INSTABILITY IS
WITHIN 20-30 MILES OF THE COAST AND PARTICULARLY OUT OVER THE
ATLANTIC OCEAN. CONVERGENT RAIN BANDS DEVELOPING IN THIS
ENVIRONMENT HAVE BEEN THE MOST PROLIFIC RAIN PRODUCERS THROUGHOUT
IRMA'S TREK THROUGH FLORIDA...AND THIS IS LIKELY BECAUSE THE
INSTABILITY IS ENABLING DEEPER CONVECTION. RAIN RATES AND
REFLECTIVITY LEVELS TEND TO DIMINISH AS THE RAIN BANDS PROPAGATE
FURTHER INLAND. THEREFORE...THE GREATEST RISK OF FLASH FLOODING IS
LIKELY TO BE VERY NEAR THE I-95 CORRIDOR FROM JACKSONVILLE METRO
TO NEAR SAVANNAH. OBSERVING SITES THAT HAVE RECEIVED THESE
SIGNIFICANT RAIN BANDS OVERNIGHT HAVE OBSERVED AN INCH OF RAIN
FALLING IN AS LITTLE AS 10-20 MINUTES...AND HOURLY RAIN RATES
EXCEEDING 3 IN/HR WILL CONTINUE TO BE POSSIBLE...PARTICULARLY
WHERE THE CONVERGENT RAIN BANDS INTERSECT WITH THE DEVELOPING
FRONT.

AS IRMA BEGINS TO HAVE MORE OF A WESTERLY COMPONENT TO ITS
MOTION...THE RAIN BANDS FEEDING IN OFF THE ATLANTIC MOTION SHOULD
GAIN LATITUDE SLOWER THAN HAS BEEN OBSERVED OVERNIGHT...AND SOME
STALLING IS POSSIBLE. IN FACT...AS OF 0830Z...ONE SUCH BAND WAS
RELATIVELY STATIONARY FROM BRUNSWICK EAST ROUGHLY ALONG 31N. THIS
IS A SIGN THAT PERSISTENT RAIN BANDS COULD FEED INTO COASTAL
GEORGIA THROUGH THE MORNING HOURS.

FURTHER TO THE WEST AND NORTHWEST...RAIN RATES SHOULD BE
LOWER...WITH A LACK OF INSTABILITY AND PRIMARY FORCING GENERALLY
ASSOCIATED WITH AN EVOLVING LOW-MID LEVEL DEFORMATION ZONE AS IRMA
CONTINUES TO INTERACT WITH A JET STREAK OVER THE MID SOUTH.
HOWEVER...GIVEN THE TROPICAL ENVIRONMENT...A FLOOD THREAT MAY
BEGIN TO GRADUALLY DEVELOP UNTIL IRMA'S CENTER DRAWS CLOSER.

LAMERS

ATTN...WFO...CHS...FFC...JAX...MLB...TAE...TBW...

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

LAT...LON   32378086 32048054 31548097 31068127 30418128
            29678105 29218078 29028141 28988214 28888280
            29168315 29508346 29748361 29978397 29818443
            30198472 30828457 31638393 32138309 32368186