422
FXUS62 KKEY 300751
AFDKEY

Area Forecast Discussion
National Weather Service Key West FL
351 AM EDT Sat Aug 30 2025

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

.DISCUSSION...
Issued at 350 AM EDT Sat Aug 30 2025

A very active first half to the overnight for the Florida Keys and
surrounding coastal waters. Multiple boundaries from seemingly all
directions collided across our area, leading to quite the
spectacular light show. Most of the activity did somehow manage to
stay offshore, only occasionally grazing the island chain. The
"winner" tonight were the Lower Keys, specifically Key West and
Stock Island. On the low end we measured just shy of a quarter of
an inch to as much as nearly one inch of rainfall. That being
said, most of the rain fell after midnight and does not count
towards Friday`s daily accumulation. Most of the convection has
diminished to stratiform rain this hour. The rain did manage to
cool down some island communities with a few locations dipping
below 80 degrees briefly.

.FORECAST...

BLUF: The forecast remains riddled with high uncertainty due to an
usual, early season weather system.

A large, complex upper level low is parked across the eastern
half of the U.S. and Canada. The associated trough digs as far
south as the Florida Panhandle. Upstream of this large system is a
broad high pressure moving across northern Canada. As this high
moves east southeast over the coming days it will set up what`s
called a Rex Block, something we don`t see in the summer! Let`s
discuss. As the high moves into eastern Canada, it will push one
portion of the upper level low to the east. However, at the base
of the trough will be a potent shortwave that will become trapped
and eventually develop in a closed, cut-off upper level low. The
high will then sit poleward of this newly formed low, creating the
Rex Block. Given that this is an usual occurence this time of the
year, the Rex Block will be short lived as the flow tries to
immediately return to a more progressive nature.

When this occurs, the cut-off upper level will absorbed the
numerous shortwaves that are initially backed up by the Rex
Block. This will allow the low to sit in place and expand.
Finally, a new large upper level low/trough will dive southeast
out of Canada and merge with the aforementioned trough. This will
lead to a prolonged period of troughiness for the eastern half of
the U.S.

Ok, so what does this mean for the Florida Keys? Depending on how
this intricate dance of physics plays out will ultimately
determine how active of a weather pattern we can expect locally.
This is where most of the uncertainly lies. If the trough is
allowed to expand more equatorward, it will place the base closer
to the Keys, allowing for some upper level support. At the same
time, depending on how far south the trough makes it, will also
determine the placement of a low level ridge that is currently
pivoting northward from the western Caribbean. Lastly, the
placement of a quasi-stalled frontal boundary will be the final
key piece. It currently resides across northern Florida but will
shift south depending on the other factors listed above.

The GFS is more "optimistic" in keeping the trough just far enough
north that the low level ridge is allowed to settle across Cuba.
This would place the Florida Keys in generally a west-southwest to
at times northwest flow. This in turn would not be very favorable
for rain chances. The ECMWF on the other hand is more "pessimistic"
in that it pushes the trough far enough south that the ridge is
not allowed to move up from the Caribbean. This would place us in
a convergence zone of not only some (not great) upper level
support but moisture that will continue to pool across the Keys.
As a result, a more active, wetter period would be expected.

For now, will maintain a middle of the road approach with
rain/thunder chances slightly above normal. There will be days
that ultimately may be wetter or drier than some, but where and
when that may occur too uncertain at this time. Given the lack of
appreciable synoptic flow, our rain chances in the short term will
be heavily dependent on mesoscale forces.

&&

.MARINE...
Issued at 350 AM EDT Sat Aug 30 2025

An early season Canadian high pressure system will
continue to traverse across the eastern United States and move
into the Atlantic over the next few days. This will allow for
breezes to remain light through the weekend. Pressure gradient
across the Gulf including Florida and the Florida Keys will
increase early next week, resulting in freshening breezes.
Moisture will increase as well, supporting higher shower and
thunderstorm coverage through early next week.

&&

.AVIATION...
(06Z TAFS)
Issued at 350 AM EDT Sat Aug 30 2025

After a lull in activity from earlier this morning, scattered
showers and thunderstorms are expected to develop once again across
Florida Keys waters and near the island chain. VCSH is reintroduced
when convection is expected to start developing nearby again,
however this is an early guess and will be adjusted as needed. Near
surface winds will be light and generally out of the southwest,
becoming west to northwest by tonight.

&&

.PRELIMINARY POINT TEMPS/POPS...
Key West  92  81  92  81 /  50  50  40  50
Marathon  91  81  91  81 /  50  50  50  50

&&

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

$$

Public/Marine/Fire...LIW
Aviation/Nowcasts....JAM
Data Acquisition.....JAM

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