Area Forecast Discussion
Issued by NWS Key West, FL

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000
FXUS62 KKEY 250941
AFDKEY

Area Forecast Discussion
National Weather Service Key West FL
541 AM EDT Mon Oct 25 2021

.DISCUSSION...
Currently - A complicated negatively tilted mid latitude trough
is swinging into the eastern United States and beginning to
interact with a west southwest to east northeast oriented southern
stream aligned along the gulf coast and northern Florida. An
associated 700 mb trough is fairly prominent down to central
Florida. The 850 mb and surface trough are somewhat weaker and
aligned across central and northern Florida. Meanwhile, a broad
Atlantic ridge extends west southwestward across the Bahamas and
Cuba, resulting in a gentle south southeasterly flow across the
Keys. A moist confluent zone with active convection has setup
across the far southeastern Gulf of Mexico. However this activity
has only affected our westernmost and northwesternmost marine
zones. Deep layered moisture remains quite high with PW around 2.1
inches with a nearly saturated column and considerable cloud
cover.

Forecast - As a result of the trough digging through the eastern
United States, pressures will fall off the Carolina coast
resulting in cyclogenesis. As the low expands, the nearby ridge
will sharpen to our south. As a result, expect light to gentle
winds to shift from southerly to westerly through the next 12 to
18 hours. The previously mentioned moist confluent zone will
linger and remain near our area. Guidance suggests that this band
may sag southward today. In addition, PW and dew points will
remain high with some low level veering. The combination of the
above would result in considerably higher shower and thunderstorm
activity across the Keys today into this evening.

Heading into mid week, the flow will be zonal westerly through the
lower levels with modest and transitory ridging building aloft.
Precipitable water is expected to fall to near seasonal norms. The
loss of both lower level lift and confluent flow, along with
less deep layered moisture will allow pops to come down to near
or slightly below normal.

The medium term fall in pops will be short lived as a powerful
mid latitude trough, currently working its way through the west
coast, will spawn cyclogenesis in the Plains in the Tuesday
Wednesday time frame. This low will deepen into a sprawling 990 to
995 mb cyclone as it lumbers eastward through the eastern United
States through much of the remainder of the work week. It will
then pinwheel northeastwards into another large low over Canada
through the weekend. The old ribbon of moisture that, by now would
be mostly south of the Keys, will morph into a weak warm front.
This will lift northeastwards across the Keys late Wednesday and
consequently higher rain chances. Then, the first of a number of
pressure troughs rotating around the gyre is expected to push
through the Keys Thursday night. Expect winds to freshen
considerably out of the southwest to west. Shower and thunderstorm
chances are expected to peak late Thursday. While large scale
cyclonic flow will remain well into the weekend, the combination
of increasing dry air aloft, modestly lower theta-e, and a
relaxing surface pressure gradient as the parent low lifts out
will result in pops falling to near to slightly above normal. Only
modest cooling is expected as there is no significant cold air
advection. Temperatures should range near to slightly below
normal.

&&

.MARINE...
A low deepening off the Carolina coast will sharpen a ridge
across Cuba. As a result, winds will shift from southerly to
westerly today and tonight. Shower and thunderstorm coverage
should increase through the day then diminish overnight. Light to
gentle west to northwesterlies will then prevail into mid week.
With that said, a somewhat uncommon phenomena seen in synoptic
setups, like the one we are in, is an afternoon southwest to west
wind surge off the southeast coast of South Florida. This effect
bleeds slightly into our waters surrounding North Key Largo. This
will be most likely Tuesday afternoon. The ridge to our south will
weaken on Wednesday turning our winds light and variable. This
will be short lived as freshening breezes are expected Wednesday
night as a large low in the Plains begins spreading across the
Gulf of Mexico. This low will move eastwards through the eastern
United States during the remainder of the week resulting in fresh
breezes shifting west. Cautions and Advisories will likely be
required for portions of Keys waters beginning Wednesday night or
Thursday.

&&

.AVIATION...
VFR conditions will prevail at both EYW and MTH island terminals
through Monday. Scattered showers are possible during the day on
Monday, however uncertainty in timing and location along with model
inconsistency has resulted in VCSH being omitted at this time. Will
continue monitoring future TAF updates as details become more clear.
Otherwise primarily southerly winds from 6 to 9 knots will turn more
southwest through the day.

&&

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

&&

$$

Public/Marine/Fire...11
Aviation/Nowcasts....Williams

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