228
FXUS62 KKEY 210918
AFDKEY

Area Forecast Discussion
National Weather Service Key West FL
518 AM EDT Mon Sep 21 2026

...New DISCUSSION, MARINE, AVIATION...

.KEY MESSAGES...

- Heavy rainfall is expected at times today. There is a marginal
  risk for excessive rainfall through Tuesday morning.

- Rain chances will increase further by the middle of the week as
  tropical moisture spreads into the area.

- Drier air associated with a cold front will arrive late week and
  into the weekend will bring a lower risk of excessive rainfall,
  though some rain chances will linger.

&&

.DISCUSSION...
Issued at 428 AM EDT Mon Sep 21 2026
The upper level low that has dominated our weather pattern over the
past several days still remains stalled just off the coast of west
central Florida near the Tampa Bay area, though the influence of
this feature will finally start to wane by the end of the day. For
now, its presence is still felt as GOES-19 satellite imagery
continues to reveal a broad cyclonic flow to the cloud pattern
across the Sunshine State. The 00Z KEY RAOB measured a deep
southwesterly flow regime throughout the vertical column with little
inhibition and abundant moisture with PWATs of 2.11 inches, though
mid level temps were notably warmer than recent days at -4.7 C.
Given that the proximity of the upper low is still close enough to
our region to supply continued forcing for ascent, another active
day of convection is generally expected to occur across the Florida
Keys with today`s setup possibly favoring a reverse cloud line given
a somewhat more southwest flow orientation. However, this depends
on if enough low level veering ultimately occurs as there might
remain just enough of a southerly component to prevent an ideal
alignment with the Island Chain. Should a more organized cloud
line develop later today, rain rates over land communities will
need to be monitored closely as forecast soundings show corfidi
upshear values generally less than 10 knots, possibly enhancing
the potential for back building cells and quick accumulations. It
should also be noted that several recent CAM solutions are
highlighting some maximum QPF bullseye values of 4-6" through
tonight and while it remains to be seen if these totals will be
more favored over land or offshore, there is a reasonable threat
for potential hydro impacts, especially in locations that received
multiple rounds of heavy rainfall in short durations. As a
result, WPC maintains a marginal risk of excessive rainfall
through early Tuesday morning across the entire Island Chain.

The aforementioned upper level low will open into a trough by
tonight and lift away to the northeast, eventually getting absorbed
into a mid latitude trough swinging across the Ohio Valley. As this
occurs, remnant energy associated with this departing disturbance
will lead to the development of a coastal low offshore of the
Southeast US on Tuesday with the low pressure expected to then lift
northeastward, deepening as it gains latitude. Eventually, it will
stall and retrograde slightly as it meanders offshore of the mid-Atlantic
and becoming increasingly occluded towards the end of the week. A
cold front attached to this system is expected to drop southward
across the central Florida peninsula by mid week with and will
continue advancing further southward into late week across the
south FL and into the Florida Keys. Meanwhile, ahead of this
boundary, tropical moisture is forecast to surge northward across
western Cuba and into the Florida Keys with peak moisture values
expected to occur late Tuesday into Thursday as PWATs possibly
reach as high as 2.5 inches. This moisture has its source region
in the southwestern Caribbean Sea as it appears that an inverted
trough feature pinches off from a larger Central American Gyre
counter-clockwise circulation. This disturbance is generally
expected to move westward after it crosses the Yucatan
Channel as it encounters stout mid-level ridging in the Ark-La-Tex
region, but the substantial increase in moisture advection
expected across the Keys should yield several days of a heavy
rainfall threat with likely to categorical PoPs mid week before
the arrival of the aforementioned frontal boundary brings a drier
air mass into the area by end of the week. However, this is not
expected to be a very clean frontal passage at this time as
deeper moisture doesn`t completely scour out of our area so PoPs
should linger in the high chance to likely range through the end
of the period. While it still remains a bit early in the season
for the cooler air mass changes and corresponding cold air
advection that typically follow cold frontal passages, the
northerly flow late in the week should help shave off a degree or
two from highs and also bring dewpoints a few degrees lower, which
should still be noticeable after an extended period of the
typical Florida summer humidity.

&&

.MARINE...
Issued at 428 AM EDT Mon Sep 21 2026
An upper-level low lingers in the eastern Gulf waters through
tonight but will finally lift away from the area on Tuesday.
However, weather conditions will continue to remain unsettled
throughout most of the week as tropical moisture from the
Caribbean spreads northward into the Keys into the middle of the
week. As a result, rain chances will remain elevated with daily
scattered to numerous showers and thunderstorms. The main hazards
that mariners can expect with this activity will be a daily
occurence with gusty winds, lightning strikes, blinding downpours,
and locally hazardous seas. Breezes will generally remain light
to gentle for most of the week but brief periods of moderate
breezes may occur at times, mainly in the evening and overnight
hours. In addition, winds will be out of the south to southeast
throughout most of the week but winds will start to shift more
northerly by late week and into the weekend as a frontal boundary
gradually pushes southward across coastal waters of the Florida
Keys. Seas will also start to build by the end of the week and
into the weekend as northeasterly swell increases in the wake of
the frontal boundary, especially in the Straits of Florida.

&&

.AVIATION...
(06Z TAFS)
Issued at 428 AM EDT Mon Sep 21 2026
Scattered SHRA/TSRA will be possible across either of the Island
Chain terminals today as an upper level disturbance continues to
linger to the north of the area. As a result, brief IFR/MVFR
restrictions may occur during periods of heavier precipitation,
though confidence in timing this activity is low at this time as
multiple rounds of showers and storms could occur throughout he
day and into this evening. VFR conditions will prevail outside of
any precipitation activity with south- southwest winds today
around 5-10 knots before shifting slightly more southeasterly and
increasing slightly into tonight.

&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...None.
GM...None.
&&

$$

Public/Marine/Fire...RG
Aviation/Nowcasts....RG
Data Acquisition.....MJV

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