Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
816 PM EDT Sat Oct 12 2024

Day 1
Valid 01Z Sun Oct 13 2024 - 12Z Sun Oct 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS & THE FLORIDA GOLD COAST...

Satellite and radar imagery have shown shower and occasional
thunderstorm activity moistening the column in the mid and upper
levels of the troposphere ahead of a shortwave moving through the
eastern Gulf of Mexico/FL Panhandle. This moistening trend is
apparent both of water vapor imagery and trends in the MFL upper
air sounding over the past 24 hours. The overall synoptic pattern
favors slow-moving convective cells along and near the Gold Coast
given easterly low level flow and westerly upper level winds, with
the MFL sounding showing strong veering between the surface and
~600 hPa/15,000 feet. While the mesoscale guidance indicates that
the heaviest rainfall is expected just offshore, VAD wind profiles
from the MIA, FLL, and PBI terminals show slightly stronger flow at
850 hPa than indicated in 22z RAP mass fields to the northeast of
a circulation west of FL Bay and offshore southwest FL. This
appears to indicate a better chance of activity moving ashore,
particularly at or past 06z. After coordinating with MFL/the Miami
FL forecast office, went ahead and raised the threat level to a
Slight.

The ingredients in place support the potential for a localized
higher end flash flood event somewhere within South FL tonight,
particularly coastal Miami-Dade and Broward counties. Precipitable
water values are ~2.25" and effective bulk shear is 30-40 kts. ML
CAPE is mostly 250-500 J/kg except in southern Miami-Dade, but RAP
guidance does show a 500+ J/kg uptick overnight. Both quasi-
stationary thunderstorms and random mesocyclones are the expected
heavy rain threats, with the possibility of 3" an hour totals and
local amounts around 6" should activity move ashore and any areas
see a couple hours of issues before showers and storms reorient, as
they typically do as instability exhausts overland. Weekly
rainfall have been 200-600% of average, so the area is fairly
saturated. While confidence isn't high, raising to a Slight Risk
appears to be the course of least regret.

Roth


Day 2
Valid 12Z Sun Oct 13 2024 - 12Z Mon Oct 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...

The risk on Sunday is a continuation of today's threat over
Southeast Florida. The overall thermodynamic ingredients remain
similar, however there is still spread with the mesoscale to
synoptic setup. The 12/12Z GFS and 12/12Z ECMWF have trended
quicker with the weak wave and suggest the better organized
convective risk will now be offshore and over the Bahamas. However
the GEM and some HREF members are on the slower side and focus
more convection over south Florida.

The uncertainty in these details continues to be too large to go
with anything more than a Marginal risk at this time. Overall the
signals in the models suggest the flood risk on Sunday is trending
downward but lingering amounts of moisture and instability
parameters suggest at least a localized flash flood risk could
persist.

Bann


Day 3
Valid 12Z Mon Oct 14 2024 - 12Z Tue Oct 15 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Bann


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt