311
AWUS01 KWNH 131459
FFGMPD
FLZ000-132100-

Mesoscale Precipitation Discussion 0431
NWS Weather Prediction Center College Park MD
1057 AM EDT Thu Jun 13 2024

Areas affected...South FL

Concerning...Heavy rainfall...Flash flooding likely

Valid 131500Z - 132100Z

Summary...Renewed flash flooding is likely over areas that already
received 5-10"+ of rainfall over the past 24 hours. Significant
flash flooding is expected over urban corridors, and localized
catastrophic flash flooding is possible.

Discussion...Skies are partly cloudy this morning across South FL,
after tremendous rainfall over the past 24 hours (widespread
totals of 5-10"+, per gauge corrected QPE via NSSL). Unfortunately
this lull looks to be short-lived, as convective coverage is
already expanding upstream over the Florida Shelf/Bay side Gulf
waters of the FL Keys. The environment is remarkably similar to
yesterday, with plenty of instability (ML CAPE already 1000-2000
J/kg), near record tropospheric moisture (PWATs 2.2-2.5"), and
effective bulk shear of 20-30 kts (largely from the influence of a
500 mb trough over the central Gulf). This environment is likely
to result in deep convective activity again today, and there is
potential for another organized convective complex (should
convective grow sufficiently upscale). The deep warm layer
(freezing levels ~16-17k ft) will support highly efficient warm
rain processes (i.e. tropical) with rainfall rates of 2-4"/hr.

Hi-res guidance is in fairly good agreement, though the exact
convective evolution is still rather uncertain (as it typically is
with complex events in this region). Taking a look at the hi-res
ensembles (both the 06z HREF suite and the experimental 00z REFS
suite) suggests relatively widespread totals of 2-5" from near
Fort Myers/Naples eastward (across Alligator Alley/Tamiami Trail)
to the greater Miami metro region (per the mean QPF fields and
relatively high ensemble agreement scale, EAS, probabilities).
When taking into account higher localized totals, there is also
good agreement on the potential for 5-10" totals (per the
probability matched mean, PMM, and local probability matched mean,
LPMM, QPF depictions). 06z HREF 40-km neighborhood exceedance
probabilities for 5" through 21z are between 40-50%, and the
experimental REFS probabilities are even higher (50-70%). These
depictions are consistent with a scenario where deep convective
activity is realized, possibly organizing into a complex to result
in more widespread 4-5" totals. Conversely, the 12z HRRR
represents a more "best case" scenario, where the convective line
remains relatively narrow and shallow (which still suggests
relatively widespread 2-3" totals with localized 3-6" amounts).
Should convection not organize into a complex, this may also
result in a later arrival of the line to the Miami metro (possibly
delaying the heaviest rainfall until after 21z).

Given the aforementioned antecedent conditions (5-10"+ totals over
the past 24 hours alone), numerous to widespread instances of
flash flooding are considered to be likely (particulary across the
urban corridors, where all the water has yet to even drain).
Isolated to scattered instances of significant flash flooding are
also considered likely, and localized catastrophic flash flooding
is possible (conditionally dependent on the highest totals
overlapping some of the already hardest hit areas of the Miami
metro).

Churchill

...Please see www.wpc.ncep.noaa.gov for graphic product...

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

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

LAT...LON   27258052 26968000 26137995 25378035 25398146
            25748189 26718251 27158179