MESOSCALE PRECIPITATION DISCUSSION 0798
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1144 AM EDT SUN SEP 10 2017

AREAS AFFECTED...CENTRAL AND SOUTHERN FL

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

VALID 101543Z - 102200Z

SUMMARY...FLASH FLOODING IS LIKELY THROUGH LATE AFTERNOON AS
SPIRAL BANDS ASSOCIATED WITH HURRICANE IRMA PRODUCE LOCALIZED
RAINFALL RATES AS HIGH AS 3 IN/HR.

DISCUSSION...HURRICANE IRMA WAS POSITIONED NORTH OF THE LOWER
FLORIDA KEYS AS OF 15Z AND THE NATIONAL HURRICANE CENTER IS
FORECASTING IT TO CONTINUE MOVING NORTH-NORTHWEST ALONG THE
WESTERN COAST OF FLORIDA THROUGH TONIGHT. CURRENT RADAR ESTIMATES
SUGGEST RAINFALL RATES OF 1-2 IN/HR IN SOUTHEAST TO NORTHWEST
ORIENTED BANDS ACROSS SOUTHERN FLORIDA, NORTHEAST OF THE STORM'S
EYE. IN THE EYE WALL (CURRENTLY OFFSHORE), RADAR ESTIMATES
INDICATE RATES UP TO 3 IN/HR. THESE ESTIMATES WERE SUPPORTED BY
THE NATIONAL KEY DEER NWR RAWS SITE, WHICH MEASURED 3.16 INCHES IN
ONE HOUR AS THE EYE WALL PASSED THROUGH. GIVEN THE EFFECT OF
EXTREME WINDS ON RAIN GAUGE MEASUREMENTS, ACTUAL AMOUNTS WERE
LIKELY HIGHER, AND RADAR RAINFALL ESTIMATES IN THE EYE WALL ARE
LIKELY UNDERDONE. LATEST SPC/RAP MESOANALYSIS INDICATES PWAT
VALUES OF 2.7-2.9 IN. ACROSS SOUTHERN FL GOES-13 AND EXPERIMENTAL
GOES-16 INFRARED IMAGERY SUGGEST A MODEST IMPROVEMENT IN THE
STORM'S CENTRAL DENSE OVERCAST OVER THE PAST HOUR, WITH AN
EXPANSION OF CLOUD TOPS APPROACHING -80 DEG C. LOW-LEVEL WATER
VAPOR IMAGERY SHOW VERY DRY AIR WRAPPING AROUND THE WESTERN SIDE
OF IRMA, BUT AT THIS TIME THE DRY AIR IS FAILING TO SIGNIFICANTLY
PENETRATE THE STORM'S CORE, KEEPING PWATS HIGH.

AS HURRICANE IRMA CONTINUES TO MOVE NORTH-NORTHWESTWARD ALONG THE
WESTERN FL COAST THROUGH THIS AFTERNOON, SPIRAL BANDS ARE EXPECTED
TO CONTINUE AFFECTING THE CENTRAL AND SOUTHERN FL PENINSULA. A
DIURNAL INCREASE IN INSTABILITY (MLCAPES INCREASING TO 500-1000
J/KG) DURING THE AFTERNOON, ALONG WITH CONTINUED VERY HIGH PWATS,
SHOULD INCREASE PRECIPITATION EFFICIENCY. AS A RESULT, RAINFALL
RATES OF 2-3 IN/HR ARE POSSIBLE. AS OF 15Z THE EYE WALL IS
POSITIONED OFFSHORE, BUT ANY AREAS WHERE THE EYE WALL BRUSHES THE
WESTERN FL COAST WOULD SEE THE POTENTIAL FOR 3+ IN/HR RAINFALL
RATES. ADDITIONAL RAINFALL AMOUNTS OF 4-8 INCHES ARE EXPECTED
THROUGH 22Z, WITH LOCALLY HIGHER AMOUNTS POSSIBLE. THESE
ADDITIONAL AMOUNTS ON TOP OF HEAVY RAINS WHICH HAVE ALREADY FALLEN
ACROSS MUCH OF THE AREA WILL LEAD TO THE LIKELIHOOD OF  FLASH
FLOODING.

RYAN

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

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

LAT...LON   28808138 28698096 28348034 27797999 26977980
            25827977 24888016 24308089 24448135 24748113
            25198127 25678180 26548252 27238276 27888277
            28398260 28788199