MESOSCALE PRECIPITATION DISCUSSION 0918
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1128 PM EDT SAT OCT 28 2017

AREAS AFFECTED...SOUTHERN FLORIDA

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

VALID 290330Z - 290930Z

SUMMARY...GROWING CONCERN FOR ANCHORED CONVECTION WITHIN
SHARPENING DEFORMATION ZONE ACROSS S FL POSING FLOODING CONCERN
THROUGH EARLY MORNING.

DISCUSSION...03Z SFC OBS/RADAR MOSAIC AND 3.9UM SWIR IMAGERY FROM
GOES-16 MESO WINDOW PRESENT A MUDDLED LOW LEVEL PATTERN AT THIS
TIME THOUGH ELEMENTS APPEAR TO BE CONSOLIDATING TO POSE FLOODING
RISK.  A WEAK SURFACE INFLECTION AND DOMINANT WIND CIRCULATION
EXISTS NEAR 25N82.7W WITH STRENGTHENING EFFECTIVE COLD FRONT DUE
SOUTH (STRONG WESTERLY SURFACE FLOW WEST OF THE BOUNDARY) THIS
WHILE LOWEST PRESSURES REMAIN OVER THE WESTERN FL STRAITS DRIVEN
BY BEST LATENT HEAT RELEASE/PRESSURE FALLS WITH CONVECTION OVER
CUBA TOWARD ANDROS ISLAND.  SFC ANALYSIS ALSO DEPICTS A SURGE OF
DEEP TROPICAL AIR MASS WITH PSEUDO WARM FRONT/TROPICAL BAND
LIFTING NORTH ACROSS SOUTHERN TIP OF THE FL PENINSULA (WITH
35-40KT POST BNDY SOUTHERLY FLOW).  RAP ANALYSIS SUPPORTED BY
LOCAL VWP SUPPORT A STRENGTHENING LOW LEVEL CONVERGENCE/
DEFORMATION ZONE EXTENDING FROM THE WIND CIRCULATION TOWARD LAKE
OKEECHOBEE. 

AS THE WIND CIRCULATION DROPS EAST-SOUTHEAST AND BEGINS TO ALIGN
WITH LOW PRESSURE BUBBLE (CENTER OF TS PHILIPPE)...BEST MOISTURE
CONVERGENCE WILL BE A THE INTERFACE OF THE SPLIT IN THE WARM
CONVEYOR BELT NORTH AND EAST OF THE DEEPENING CIRCULATION. 
ASSISTED BY SFC FRICTIONAL CONVERGENCE FROM EASTERLY ONSHORE FLOW
AND INSTABILITY ADVECTION (LOW 80S TEMPS AND MID 70S TDS) ALLOWS
FOR INCREASED INSTABILITY AND SATURATED LOW LEVEL PROFILES.  THIS
COMBINATION WILL SUPPORT SHALLOW BUT HIGHLY EFFICIENT RAINFALL
PRODUCING CONVECTION WITH RATES OF 2.5-3"/HR. AND WILL LIKELY TAKE
A SIMILAR THOUGH EXPANDED FORM OF CURRENT ACTIVITY SEEN IN 10.3UM
CHANNEL SOUTHWEST OF FMY.  LINGERING QUESTION IS LATITUDE OF THE
DEFORMATION ZONE TOWARD 06-09Z WHEN ASCENT/MST FLUX SHOULD BE
MAXIMIZED AS THE SURFACE LOW ROUNDS THE BASE OF SOUTH FL...BETWEEN
MARTIN COUNTY AND N MIAMI-DADE COUNTY WITH CURRENT TRENDS FAVORING
BROWARD/DADE.  THIS POSES THE GREATEST RISK OF FLOODING GIVEN
URBAN ENVIRONMENT WHICH MAY BE ENHANCED GIVEN STRONG ONSHORE FLOW
LIMITING DISCHARGE COMPOUNDING FLOODING CONCERNS.  HI-RES
HRRR/ESRL HRRR V3/ARW AND NSSL-WRF SUPPORT THIS EVOLUTION WITH
RISK OF 4-6" LIKELY THROUGH 09Z WITH POTENTIAL FOR ISOLATED 8-9"
CERTAINLY POSSIBLE.  AS SUCH FLOODING/FLASH FLOODING IS CONSIDERED
LIKELY.

GALLINA

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

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

LAT...LON   27148007 26427991 25288020 24988096 25308125
            25708141 25868162 26228136 26718079 27018049