Upper Key Largo, Florida Weather Forecast Discussion

938
FXUS62 KKEY 251904
AFDKEY
Area Forecast Discussion
National Weather Service Key West FL
304 PM EDT Thu Sep 25 2025
...New AVIATION, PREV MARINE, DISCUSSION AND FORECAST...
&&
.AVIATION...
(18Z TAFS)
Issued at 232 PM EDT Thu Sep 25 2025
VFR conditions are expected to prevail at the terminals through
the TAF period. While shower and thunderstorm chances sit at 20%
through this evening, coverage will be limited at best, and so
categorical impacts to the terminals from this activity will be
handled by later amendments as needed. Breezes will be light and
variable through the TAF period.
&&
.MARINE...
Issued at 1045 AM EDT Thu Sep 25 2025
There are currently no watches, warnings, or advisories in effect
across the Florida Keys coastal waters. From synopsis, breezes
will become variable within a weak pressure gradient. An area of
low pressure is expected to develop across the western Atlantic
north and northwest of Hispaniola over the next couple of days. An
approaching upper level trough will help to steer this system
northwestward which will further weaken the pressure gradient
initially. This system will continue moving northwestward in the
vicinity of the Bahamas through the weekend and into early next
week. Therefore, breezes are anticipated to freshen out of the
northwest to north.
&&
.PREV DISCUSSION...
Issued at 1045 AM EDT Thu Sep 25 2025
A quiet and dry morning is unfolding across the Florida Keys. BYX
radar has detected only a few short-lived showers over Keys
coastal waters over the past few hours, and current activity is
about as minimal as we can expect in September. The 12z KKEY
sounding again depicts a somewhat drier layer between 900mb and
450mb, though PWat remains near 2 inches, which is at the 75th
percentile for the date. This is demonstrated by observed shallow
Cu cloud fields on G19 visible satellite imagery, though coverage
is scattered at best. Little indication of convergent flow and few
boundaries seen in this imagery suggest limited shower activity
for the next several hours, so the currently-advertised 20% PoPs
remains sufficient. Observed temperatures in the mid 80s will
continue to rise to near 90F this afternoon, with dew points in
the upper 70s to near 80. Breezes will remain light and variable.
&&
.FORECAST...
Issued at 430 AM EDT Thu Sep 25 2025
The aforementioned high pressure system will maintain control
across the Keys for a couple more days. GOES Total Precipitable
Water (TPW) shows quite a bit of moisture across the area with
PWAT values ranging from 1.9 inches to 2.2 inches. Instability
also remains quite plentiful. We are missing a source of lift such
as a favorable upper level pattern setup or a trigger. Therefore,
we have two out of the three necessary ingredients to get more
widespread and robust showers and thunderstorms. The main concern
will be clouds lines which may develop each afternoon (Thursday
and Friday). Usually light wind flow is a favorable setup for
cloud lines. Rain chance remains at 20% as a result.
The forecast for the weekend is becoming clearer with each
consecutive model run. An upper level trough is expected to move
into the Gulf beginning Friday night. This trough is expected to
remain in place for a few days becoming negatively tilted
sometime Saturday night into Sunday. A tropical wave currently in
the vicinity of Puerto Rico (Invest 94L as designated by the
National Hurricane Center (NHC)) will continue moving to the west
and eventually northwest heading through the weekend. The latest
thinking is this wave will strengthen as it moves northwestward
towards the Bahamas over the weekend. However, whatever this
becomes, it appears the upper trough will act to keep the system
away from the Florida Keys. The NHC currently gives this area a
50% chance of development within the next 48 hours and 80% chance
of development over the next 7 days.
As of now, the forecast calls for a weakening of the pressure
gradient initially, leading to light and variable winds through
approximately Friday night. As this system continues moving
northwestward, whether or not it develops remains to be seen, will
lead to a slight strengthening of the gradient. This will lead to
freshening breezes out of the northwest to north as a result.
Also, moisture will remain low as the system is anticipated to
pull drier air south to southeastward into the Keys. There might
be a slight uptick in moisture for Saturday night and Sunday where
30% is currently advertised before dropping back to slight chances
of 20% for early to mid next week. Also, as the drier air moves
in, model guidance is hinting at our first potential noticeable
drop in dew points for the season. Stay tuned!
Also, Tropical Storm Humberto formed in the Atlantic yesterday and
is currently 505 miles to the east northeast of the Northern
Leeward Islands. It is worth noting here that this system will
remain out to sea and not be a threat to the Florida Keys.
&&
.PRELIMINARY POINT TEMPS/POPS...
Key West  90  82  90  82 /  20  10  10  10
Marathon  90  81  90  81 /  20  20  20  20
&&
.KEY WATCHES/WARNINGS/ADVISORIES...
FL...None.
GM...None.
&&
$$
Public/Marine/Fire...MC
Aviation/Nowcasts....MC
Data Acquisition.....AP
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NWS KEY Office Area Forecast Discussion