Mesoscale Precipitation Discussion 1033
NWS Weather Prediction Center College Park MD
234 PM EDT Thu Sep 19 2024
Areas affected...Southeast Florida
Concerning...Heavy rainfall...Flash flooding possible
Valid 191833Z - 200000Z
Summary...Slow moving showers and thunderstorms will expand across
Southern Florida through the afternoon. Rainfall rates of 2-3"/hr
are likely, which could result in 2-4" of rain with locally higher
amounts. This may result in flash flooding, primarily within urban
areas.
Discussion...The regional radar mosaic this afternoon depicts a
slow increase in coverage of reflectivity across the southern and
central portions of the Florida Peninsula. This coverage is
increasing thanks to rapid destabilization characterized by 3-hr
MLCAPE change as high as +1000 J/kg according to the SPC RAP
analysis, in the presence of PWs around 2.1 inches, or about the
90th percentile for the date. Into these impressive
thermodynamics, ascent is occurring in the vicinity of a weakening
surface boundary draped from near Cape Canaveral to Port
Charlotte, with additional lift provided through a weak shortwave
lifting eastward near the Everglades and a sharpening sea breeze
boundary along the Gold Coast. Storms which have developed so far
have been pulse thanks to weak shear, but have contained at least
brief rainfall rates estimated by KAMX to be as high as 2.5"/hr.
There is good agreement among the various high-res members that
the coverage of convection will continue to expand the next few
hours as instability maximizes and the different forcing
mechanisms continue to impinge on the area. Although bulk shear
will remain weak to limit much organization, widespread cell
development, especially as the shortwave lifts northeast, should
result in numerous outflow boundaries, suggesting disorganized
clusters through storm mergers and boundary interactions. With the
anomalous PWs in place, this should result in rainfall rates that
have a 20-40% chance of exceeding 2"/hr according to the HREF,
with short-duration rainfall rates potentially reaching 4"/hr as
suggested by HRRR 15-min rainfall reaching 1" in some areas.
Mean 850-300mb wind speeds are expected to remain weak at just
5-10 kts, and Corfidi vectors collapse to 5 kts or less into the
evening. This indicates slow and chaotic cell motions, especially
during collisions/mergers, to lengthen residence times of these
heavy rain rates. The most likely location for any persistent
rainfall rates will be along the westward advancing sea breeze
however, as anti-parallel mean flow to this boundary should result
in regeneration of cells from west to east. This could create
rainfall amounts of 2-4", with locally higher amounts possible
across the urban Gold Coast. This will create a 40-60% chance of
exceeding FFG according to the HREF, and if these intense rates
linger across any urban areas this afternoon, instances of flash
flooding could result.
Weiss
ATTN...WFO...MFL...MLB...
ATTN...RFC...SERFC...NWC...
LAT...LON 27478032 27178010 26928001 26498001 26128003
25768013 25438027 25308036 25408051 25768074
26178092 26638083 26888062 27308073 27418058
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