MESOSCALE PRECIPITATION DISCUSSION 0793
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
608 PM EDT SAT SEP 09 2017

AREAS AFFECTED...CENTRAL/SOUTHERN FL

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

VALID 092205Z - 100405Z

SUMMARY...STRONG CONVECTIVE BANDS ARE EXPECTED TO EXPAND IN
COVERAGE OVER THE NEXT SEVERAL HOURS WITH VERY HEAVY RAINFALL
RATES THAT BEGIN TO ENCOURAGE SUBSTANTIAL RUNOFF AND POSSIBLE
FLASH FLOODING.

DISCUSSION...HURRICANE IRMA CONTINUES TO ADVANCE OFF TO THE
WEST-NORTHWEST WHILE APPROACHING THE FLORIDA STRAITS. OVER THE
LAST COUPLE OF HOURS THERE HAS BEEN A GENERAL INCREASE IN THE
INTENSITY AND ORGANIZATION OF CONVECTIVE BANDS AROUND THE NORTHERN
SEMICIRCLE OF IRMA...WITH THE STRONGEST ACTIVITY SOUTH OF LAKE
OKEECHOBEE AND GENERALLY IN BETWEEN WEST PALM BEACH AND MIAMI.
NON-OPERATIONAL GOES-16 10.3 MICRON/IR DATA HAS SHOWN A GENERAL
COOLING OF THESE CLOUD TOPS AS THE BANDS ADVANCE RAPIDLY WEST
ACROSS THE SOUTHERN PART OF THE FL PENINSULA.

THESE CONVECTIVE BANDS AND ASSOCIATED SQUALLS ARE PRODUCING
RAINFALL RATES OF UP TO 2.5 TO 3 INCHES/HR AND ARE LOCALLY
BEGINNING TO TRAIN OVER THE SAME AREA AND WILL CONTINUE TO ROTATE
IN OFF THE ATLANTIC OCEAN THROUGH THE EVENING HOURS. GIVEN THE
STRENGTHENING OF THE WIND FIELD WITH IRMA OVER SOUTH FL AND
SUBSEQUENT INCREASE IN LOW LEVEL FORCING/CONVERGENCE...ALONG WITH
ENHANCED MOISTURE/INSTABILITY TRANSPORT...EXPECT THERE TO BE AN
INCREASED CONCENTRATION OF VERY HEAVY RAINFALL. THESE BANDS WILL
BE CAPABLE OF PRODUCING RAINFALL TOTALS OF 3 TO 6+ INCHES THROUGH
THE LATE-EVENING HOURS.

MEANWHILE...THE CENTRAL CORE OF IRMA WILL BE APPROACHING THE FL
KEYS WITH TIME...AND THE NORTHWEST FLANK OF IRMA'S CDO WILL BEGIN
TO OVERSPREAD THE KEYS THROUGH THE EVENING HOURS...WITH EXTREMELY
HEAVY AND PERSISTENT RAINFALL. THE LATEST HRRR AND HRRRX GUIDANCE
SUPPORT RAINFALL TOTALS LOCALLY EXCEEDING 6 INCHES BY MIDNIGHT
ACROSS THE FL KEYS.

ALL OF THIS HEAVY RAIN WILL ENCOURAGE THE ONSET OF LOCALLY
SIGNIFICANT RUNOFF PROBLEMS AND POSSIBLE FLASH FLOODING. WILL
CONTINUE TO MONITOR.

ORRISON

ATTN...WFO...KEY...MFL...MLB...TBW...

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

LAT...LON   27888072 27468011 26507983 25917991 25368015
            24778059 24408142 24428179 24548204 24748207
            24888179 25028151 25358136 25678141 25898184
            26398216 26888233 27438198