Area Forecast Discussion
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FXUS62 KKEY 211620

Area Forecast Discussion
National Weather Service Key West FL
1120 AM EST Mon Jan 21 2019

.SHORT TERM...(Today-Thursday)
The main forecast issue through Wednesday will be a veering of
winds out of the east tonight, with windy conditions developing on
Tuesday and persisting into Wednesday. The next cold front is
expected to pass the Keys on Thursday.

For today, a cool and dry flow of N-NE surface winds will come off
the Florida Peninsula and across the Keys, with clear skies and a
relative lull in wind.

A strong surface high pressure center over Ohio this morning will
move off the NC/VA coast on Tue evening. In response, our low-
level wind flow will become easterly by late tonight. Pressure
gradients on the southern periphery of this polar high will
bring windy conditions to the Keys on Tue and Wed. Land-based wind
gusts of 25-30 mph can be expected as easterly winds peak on Tue

The strong low-level easterly flow will bring a quick return of
low-level moisture starting tonight. Surface dewpoints will rise
through the 60s on Tue, reaching near 70F on Wed. Given increasing
moisture in the boundary layer and subtle mesoscale features
within the strong easterly low-level flow, isolated to scattered
trade wind showers will return as soon as late tonight then
continue through Wed. Best chance for a shower may actually come
on Tue AM, as a little warm advection upglide adds some shallow
lift within the low-level moist layer.

The next cold front will cross the Middle Gulf on Wed evening,
then cross the Keys on Thu PM. This timing is a few hours slower
than what models were depicting yesterday. Moderate instability is
forecast in the pre-frontal environment, and Precipitable Water
values should spike to near 1.8" in advance of the front. So would
expect a broken or solid line of convection to cross the Keys
just in advance of the frontal passage. So the likely PoPs for Thu
appear justified and will continue. With somewhat slower frontal
timing, may need to extend higher PoPs into Thu evening.


.LONG TERM...(Friday-Monday)
The extended forecast period is looking increasingly wet and
unsettled. An increasing number of GFS ensemble members show
precip next weekend. Expect to be adding PoPs to the weekend

Thursday`s cold front will settle to our south on Friday, with our
driest period of post-frontal weather coming early Saturday. Warm
advection may being later Saturday over the top of the cooler air
in place, leading to a return of rain from the south.

In the big picture, next weekend will be marked by a deep and
broad upper trough over the eastern half of the U.S., extending
south from a polar vortex over James Bay, Canada. Within the fast
flow in the base of this deep broad trough, the GFS and EC
ensemble means reveal a significant shortwave that will round the
base and cross the Northern Gulf Coast around Saturday and Sunday.
Significant negative height anomalies and strong jet stream flow
will be present over the entire Gulf next weekend. Cyclogenesis is
expected in the coastal trough off the Texas coast on Saturday.
Not all ensembles are on board, but the deterministic 00z EC
shows rapid deepening of the low as it tracks east across the
northern Gulf and eventually north Florida around Sunday. This is
at least consistent with its 12z solution from yesterday. This
solution would push a strong cold front and possibly a squall line
across the Keys next Sunday.

Again, look for a major boost in PoPs for next weekend.


.MARINE...The waters are currently in a relative lull in wind
which will continue through early this afternoon. This lull will
be fairly on.

A strong 1039 mb surface high is centered over Ohio this morning.
This high pressure cell will move eastward, eventually emerging
off the Virgina and North Carolina coasts late Tuesday. As the
Keys coastal waters come under the influence of strong gradients
on its southern periphery, easterly winds will become strong late
tonight and Tuesday, with wind veering from northeast to east.
Strong easterlies will continue Tue night and Wed, as the surface
high moves across the Western Atlantic to just north of Bermuda.
Over the open waters in the middle of the Straits, 10-11 foot
seas are expected on Tue afternoon and evening.

Winds will start to ease by Wed afternoon, as high pressure exits,
and a cold front moves off the Texas coast and surges toward the
Middle Gulf. As this front progresses into the Eastern Gulf on Wed
night, our winds will ease further and become southerly.

So the next cold frontal passage is expected on Thursday
afternoon. In its wake, breezes will shift northerly and become
fresh --- possibly strong --- on Friday.


.AVIATION...The few strato-cumulus clouds lingering near the EYW
and MTH terminals right now will soon give way to a completely
clear afternoon, as the low-level wind trajectory brings cool and
dry air down the Florida Peninsula and across the Keys.

Low-level flow will become veer around through the northeast and
then the east late tonight, quickly returning low-level moisture
and clouds that remain just off the Southeast Florida coast and
over the Gulf Stream. FEW-SCT cloud coverage is expected this
evening, but then BKN cloud coverage should prevail late tonight
and Tuesday morning as moisture increases further and some weak
warm advection upglide gets underway over the Keys. Could even
have some light showers or sprinkles in the vicinity of the
terminals after about 09z-10z. Showers are more likely over the
nearby waters of the Straits, so have handled that with VCSH in
the 15z EYW amendment.


In 1971, the daily record low temperature of 46 degrees was recorded
in Key West. Temperature records date back to 1872.




Data Collection......BT

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