Graphic for MPD #0796

MESOSCALE PRECIPITATION DISCUSSION 0796
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
548 AM EDT SUN SEP 10 2017

AREAS AFFECTED...SOUTHERN FLORIDA...FLORIDA KEYS

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

VALID 100948Z - 101548Z

SUMMARY...HURRICANE IRMA WILL CONTINUE TO APPROACH SOUTH
FLORIDA...SPREADING HEAVY RAIN INTO THE AREA. FLASH FLOODING IS
LIKELY...ESPECIALLY IN FAVORED AREAS OF LOW-LEVEL CONVERGENCE SUCH
AS THE EAST COAST OF FLORIDA THROUGH 16Z.

DISCUSSION...HURRICANE IRMA WAS NEARING KEY WEST AS OF 0930Z AND
THE CENTRAL DENSE OVERCAST (CDO) ON IR SATELLITE WAS BEGINNING TO
ADVANCE INTO THE FLORIDA PENINSULA JUST AHEAD OF THE CENTER OF
CIRCULATION. AS THIS CONTINUES TO OCCUR DURING THE
MORNING...MODERATE TO HEAVY RAIN WILL SPREAD OVER THE ENTIRE
OUTLINED AREA. THIS MAY EVENTUALLY LEAD TO WIDESPREAD FLOODING
ISSUES...BUT MORE FOCUSED HEAVY RAIN BANDS WILL HAVE THE POTENTIAL
TO CAUSE FLASH FLOODING.

SOME OF THESE SPIRAL RAIN BANDS HAVE BEEN QUITE ORGANIZED AND
MANAGED TO PERSIST FOR AN HOUR OR TWO AS THEY WRAP AROUND THE
NORTH SIDE OF IRMA'S CIRCULATION AND FEED INTO THE EAST COAST OF
FLORIDA. EXTREMELY HEAVY RAIN RATES HAVE ALREADY BEEN OBSERVED
WITH THESE BANDS WITH NUMEROUS LOCAL OBSERVING SITES SHOWING UP TO
0.5 INCHES OF RAIN IN 5 MINUTE INCREMENTS WITH THE STRONGEST
CONVECTION. THESE HAVE GENERALLY COINCIDED WITH SPECIFIC
DIFFERENTIAL PHASE (KDP) VALUES FROM LOCAL RADAR SITES ON THE
ORDER OF 2.5 TO 3.0 OR HIGHER...SO SUCH VALUES ON RADAR THROUGH
THE EVENT MAY HELP PINPOINT EXTREME RAIN RATES. LOCALIZED TOTALS
IN PORTIONS OF MONROE...MIAMI-DADE...BROWARD...AND PALM BEACH
COUNTIES LIKELY ALREADY EXCEED 4 INCHES ON A FAIRLY WIDESPREAD
BASIS.

FLASH FLOODING SHOULD BE MOST LIKELY ALONG THE EAST COAST OF
FLORIDA WHERE LOW-LEVEL CONVERGENCE WILL BE MAXIMIZED DUE TO
FRICTIONAL AFFECTS RIGHT ALONG THE SHORELINE. GIVEN THE EXTREME
RATES ALREADY OBSERVED...ANY SUBSTANTIAL RAIN BANDS THAT CAN
PERSIST FOR EVEN AN HOUR IN ANY ONE LOCATION MAY LEAD TO FLASH
FLOODING...PARTICULARLY IF IT COINCIDES WITH PORTIONS OF THE URBAN
CORRIDOR FROM MIAMI TO PALM BEACH. DUAL POL RAINFALL AND MRMS QPE
HAVE GENERALLY BEEN UNDERESTIMATING RAIN RATES IN THE TROPICAL
ENVIRONMENT...AND THE WARM RAIN PROCESSES AT PLAY HAVE BEEN VERY
EFFICIENT THUS FAR.

LAMERS

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

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

LAT...LON   27688209 27598107 27178009 26467991 25518008
            24788055 24508153 24488235 24868232 24998169
            25248130 25768157 25898195 26288205 26508244
            27028270 27368259


Last Updated: 548 AM EDT SUN SEP 10 2017