Area Forecast Discussion
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FXUS62 KKEY 220850

Area Forecast Discussion
National Weather Service Key West FL
450 AM EDT Tue Aug 22 2017

An upper-level composite analysis this morning revealed a deep
layer mean anticyclone centered over the western North Atlantic
near 30N/72W. A ridge extended westward over most of the Deep
South. A deep, but rather weak easterly current prevailed across
the Florida Keys and adjacent coastal waters, with a weak low-
level trough over the northern Bahamas approaching South Florida.
This disturbance has tracked west-northwestward across the Tropics
for several days, and has been monitored for potential tropical
cyclogenesis. However, it remains very disorganized this morning,
with little deep cumulus convection, weak momentum, and large
plumes of deep dry air nearby. The approach of the weak surface
trough has backed winds, and current observations at C-MAN and
Weatherflow sites indicate ENE winds 10-15 knots, with occasional
higher gusts. The Key West Monday evening radiosonde balloon
indicated a deep layer of very dry air over the service area.
However, recent satellite microwave data from the University of
Wisconsin CIMSS group indicate the plume of dry air pulling away
from the Keys, with deeper/richer moisture moving in. Local
Doppler radar trends over the last several hours indicate slowly
increasing coverage of rain showers, but very few deep cumulus
cells or lightning strikes. Most of the showers are along and
south of a line from Ocean Reef to Dry Tortugas National Park.
Only isolated Keys communities have received measurable rainfall
overnight. Currently, air temperatures in most Keys island
communities are 80-85F, with dewpoint temperatures of 76-79F. Heat
indices remain in the mid 90s.

Our prognostic reasoning has changed little from the last forecast
cycle. We are maintaining 12-hour measurable rain chances of 40
percent throughout the seven-day forecast. Weather prediction
models indicate a typical presence of rich tropical moisture
across the Florida Keys and adjacent coastal waters, with either
neutral or weak ridging at all levels of the atmosphere. Mesoscale
circulations will be very influential with regard to daily
initiation of deep cumulus convection and rainfall patterns.

No watches, warnings, or headlines. Northeast to east breezes
should be diminishing later this morning, with seas subsiding as
well. A weak trough will linger near South Florida from today
through Saturday. Winds and seas will be higher in and near
scattered showers and thunderstorms.

VFR conditions will prevail through 22/15Z-22/16Z. Thereafter,
cumulus clouds and lines should proliferate, with subsequent
development of scattered showers and thunderstorms, some in the
vicinity of either EYW or MTH. Amendments for mvfr/ifr cigs and/or
vis will be issued as needed.

The low temperature at Key West yesterday was 84 degrees F. This
tied the daily warm minimum temperature record, set in 1990.
Temperature records for Key West date back to 1872.

Key West  90  80  89  80 / 40 40 40 40
Marathon  93  81  94  81 / 40 40 40 40




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