338
AWUS01 KWNH 141523
FFGMPD
FLZ000-142100-

Mesoscale Precipitation Discussion 0436
NWS Weather Prediction Center College Park MD
1123 AM EDT Fri Jun 14 2024

Areas affected...South Florida and the Florida Keys

Concerning...Heavy rainfall...Flash flooding possible

Valid 141522Z - 142100Z

Summary...Showers and thunderstorms are expected to redevelop
across South Florida and train to the east through the afternoon.
Rainfall rates of 2-3"/hr are expected, producing 1-3" of
additional rain with locally higher amounts. Flash flooding is
possible.

Discussion...The regional radar mosaic this morning indicates
widespread showers and thunderstorms bracketing the southern
Florida Peninsula, but so far activity has struggled to develop
onshore. During the past hour, a few showers have begun to shift
northeast onto the southern Lee Island Coast and the Everglades in
response to increasing instability over the land. Recent local
VWPs indicate a subtle veering to the 850mb winds, and although
wind speeds remain light, this is sufficient to draw the higher
instability and moisture northward to erode the morning CIN.
Rainfall rates offshore have been estimated via local radars to be
1.5-2.5"/hr. This convection is developing within the increasing
thermodynamics thanks to strengthening ascent through modest
low-level convergence along a trough extending from a wave of low
pressure off the NC coast, enhanced by convergence at the nose of
this modest LLJ and modest upper level diffluence.

The morning U/A soundings from both KEY and MFL had PWs that were
much lower than 24-hrs ago, primarily due to drier air noted above
700mb, but still above the 90th percentile for the date.
Additionally, freezing levels and warm cloud depths were still
around 16,000 ft and 14,000 ft, respectively, supporting efficient
warm-rain collision processes. This will support heavy rainfall
rates in any convection that develops today, for which HREF
neighborhood probabilities indicate a 40-50% chance for exceeding
2"/hr, and the 15-min HRRR accumulations support 3"/hr at times.
Although there remains uncertainty in the coverage of
thunderstorms this aftn, the CAMs are in relatively good agreement
in a stripe of heavier rainfall from SW FL through Biscayne Bay
and points south, where 1-3" of rainfall is likely and the HREF
neighborhood probabilities indicate a 10-30% chance for an
additional 5 inches in some areas. These higher rainfall amounts
appear most likely in portions of SW FL where propagation vectors
align anti-parallel to the mean wind and collapse to less than 5
kts, suggesting more prolonged backbuilding into the higher
instability just offshore.

The greatest risk of these higher rainfall totals appear to focus
in SW FL and across the Everglades, somewhat limiting the flash
flood risk, despite soils that are fully saturated from recent
heavy rainfall. The NASA SPoRT 0-10cm RSM is above 80% across this
region, producing an environment more susceptible to rapid runoff
due to limited infiltration potential. Where any of these
impressive rates can train, especially atop the most sensitive
soils or in any urban areas, renewed instances of flash flooding
will be possible.

Weiss

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

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

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

LAT...LON   26348053 26348007 26008014 25768011 25428017
            25078033 24838062 24578122 24468162 24448198
            24468219 24518231 24658208 24758173 24938147
            25078135 25198130 25498133 25728154 25848171
            25998166 26188123