MESOSCALE PRECIPITATION DISCUSSION 0916
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1206 PM EDT SAT OCT 28 2017

AREAS AFFECTED...SOUTH FLORIDA AND THE FLORIDA KEYS

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

VALID 281604Z - 282200Z

SUMMARY...SHALLOW CONVECTIVE SHOWERS AND RAIN BANDS WILL DEVELOP
THIS AFTERNOON WITH INCREASING RAIN RATES OVER TIME. THIS MAY POSE
THE RISK FOR LOCALIZED FLASH FLOODING...PARTICULARLY IN THE I-95
CORRIDOR ALONG THE SOUTHEAST FLORIDA COAST.

DISCUSSION...THE DEEP TROPICAL MOISTURE PLUME SURROUNDING
NEWLY-MINTED TROPICAL DEPRESSION 18 WAS BEGINNING TO ADVANCE INTO
SOUTH FLORIDA. THIS IS CLEARLY SHOWN IN THE CIRA BLENDED TPW AND
LAYER PW PRODUCTS...BUT CAN ALSO BE INFERRED BY COMPARING THE 12Z
SOUNDINGS FROM KEY WEST (2.3 INCH PW) AND MIAMI (1.68 INCH PW).
THE GRADIENT IS FURTHER NORTH NOW THAN IT WAS AT 12Z AND SHOULD
REACH THE INTERSTATE-4 CORRIDOR IN CENTRAL FLORIDA BY
MID-AFTERNOON. LATEST RAP OBJECTIVE ANALYSIS ALSO SHOWS INCREASING
(BUT STILL WEAK TO MODERATE) INSTABILITY ACROSS PORTIONS OF SOUTH
FLORIDA...WITH HIGHEST VALUES FOCUSED ALONG THE SOUTHEAST FLORIDA
COAST. THE COMBINATION OF THESE FACTORS (VERY MOIST PROFILE WITH
NARROW CAPE) SHOULD SUPPORT SHALLOW CONVECTION WITH DOMINANT WARM
RAIN PROCESSES WITH THE POTENTIAL FOR EFFICIENT RAINFALL
PRODUCTION AND HIGH INSTANTANEOUS/HOURLY RAIN RATES. THE PAST
SEVERAL HRRR RUNS SHOW RAIN RATES PEAKING AS HIGH AS 3 IN/HR IN
THE 18-22Z TIME FRAME AND THAT DOES SEEM ACHIEVABLE.

WHILE THE LARGE SCALE ENVIRONMENT WILL FAVOR HEAVY RAIN RATES IN
ANY CONVECTION THAT CAN DEVELOP...ANY FLASH FLOOD THREAT WOULD
LIKELY REQUIRE MORE FOCUSED AND SUSTAINED LOW-LEVEL CONVERGENCE.
THE PRIMARY AREA WHERE THIS SEEMS MOST LIKELY WOULD BE WITH AN
ALREADY ESTABLISHED CONVERGENT RAIN BAND CURRENTLY CROSSING THE
MIDDLE KEYS (AND MOVING NORTH AROUND 10-15 KNOTS). THE RAP AND
HRRR TRACK THIS FEATURE NORTH THROUGH THE AFTERNOON...AND AN
ASSOCIATED CONVERGENCE MAX RIGHT ALONG THE COAST WHERE THE
MESOSCALE BOUNDARY COMBINES FAVORABLY WITH FRICTIONAL CONVERGENCE.
EXTRAPOLATION AND RAP FORECASTS WOULD BRING THIS ENHANCED
CONVERGENCE INTO MIAMI-DADE AND BROWARD COUNTIES BETWEEN 18Z AND
22Z. ADDITIONALLY...KAMX RADAR SHOWS NEW CONVECTIVE GROWTH NORTH
OF THE AFOREMENTIONED BAND...JUST OFFSHORE OF MIAMI AS OF 16Z. IF
THIS CAN FILL IN A BIT MORE (PARTICULARLY AS INSTABILITY INCREASES
IN THE AFTERNOON)...THE DURATION OF HEAVY RAINFALL COULD BE
INCREASED ALONG THE COAST WITH AN ASSOCIATED INCREASE IN THE FLASH
FLOOD THREAT.

ANOTHER AXIS OF LOW-LEVEL CONVERGENCE EXISTS FURTHER
NORTH...ROUGHLY FROM FORT MYERS TO PORT ST. LUCIE. THIS BOUNDARY
IS NOT EXPECTED TO MOVE MUCH THIS AFTERNOON...AND THE LOW-LEVEL
CONVERGENCE IS FAIRLY WEAK. THE HRRR TRIES TO DEVELOP SOME
ISOLATED TO SCATTERED CONVECTIVE SHOWERS NEAR THIS
BOUNDARY...PARTICULARLY AFTER 18Z...BUT MAY BE OVERESTIMATING
INSTABILITY. THEREFORE...THE THREAT OF HEAVIER RAIN RATES IN THIS
AREA FURTHER NORTH IS MORE CONDITIONAL...AT LEAST THROUGH 22Z.

LAMERS

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

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

LAT...LON   28108057 27808029 27348010 26997995 26617990
            25997993 25468011 25078029 24628090 24458148
            24408223 24748235 24918161 25178132 25638137
            25788177 26238204 26608236 26928246 27238237
            27678185 28018116