Area Forecast Discussion
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FXUS62 KKEY 211935

Area Forecast Discussion
National Weather Service Key West FL
335 PM EDT Sat Oct 21 2017

.DISCUSSION...In the middle and upper levels(700-200 mb), latest
available satelitte imagery overlaid with model streamline
analysis as of 300 pm depicts a developing significant long wave
trough over Western Canada and the CONUS, with its axis from
Alberta down to New Mexico. Downstream of that and conversely, the
axis of a full latitudinal ridge is oriented from near James Bay
southwards to Cuba, connecting to a upper high over the Central
Caribbean Sea. Middle and upper level dry air is gradually eroding
given advection on southwesterlies on given convergence between
the aforementioned deepening longwave trough and ridge.

At the surface and in the lower to middle levels, (Surface to 700
mb), latest available satelitte imagery overlaid with marine and
land surface observations and analysis as of 200 pm details the
center of the strongest surface ridging over the Northeast located
from Eastern PA to northern VA. To the south of the Keys, there is
a weak tropical wave/trough axis analyzed in the Western
Caribbean Sea. Hence, there is a decent MSLP gradient between
these two features across the Keys and surrounding waters.

.CURRENTLY...As of 300 pm, Scattered showers and isolated
thunderstorms continue to move rapidly across the Florida
Straits. Across the Keys, there was just one band that worked thru
the Upper Keys right after the noon hours. Other than that, only
another band is situated across the Offshore Gulf waters.
Temperatures have reached the middle 80s with dewpoints over the
middle 70s over the islands. C-man stations are recording
sustained northeast to east winds around 20 knots with gusts over
25 knots. Over the islands, east winds are 15 to 20 mph with gusts
between 20 and 26 mph.

.SHORT TERM...Tonight thru Tuesday, Middle fall and upcoming
Fantasy Fest week will be marked by a noticeable change in the
weather by midweek. For tonight through Monday, strong 1030 mb
plus surface ridging located from New England to the Mid Atlantic
will slide eastward into the NW Atlantic to just east of North
Carolina during Sunday and Sunday night, with the strongest center
of the ridging moving east of the Canadian Maritimes during
Monday, then sliding more east late Monday Night. Both latest
available GFS and ECMWF Forecast soundings are indicating the
deepest moisture moves in from the surface up to 400 mb tonight
thru Sunday, but given the radar coverage pattern, still believe
that showers and isolated thunderstorm coverage will be greatest
across the Florida Straits. Hence, have maintained high chance
pops tonight and tomorrow for the Islands and immediate waters,
except likely pops, 60%, across the Florida Straits for tonight.

On Sunday Night thru Monday night forecast soundings are showing
some drying in the 700-500 mb layer, with weaker lower level
convergence (as the MSLP gradient collapses). For now will
maintain a low chance for showers across the islands. Again, a
much weaker steering flow from the south, with deeper moisture is
indicated Monday and Monday Night, so the forcing may be lessened,
if Cuban Boundaries do not make it up here, so 30% pops could be
generous, and may need to be lowered for those two days.

Tuesday thru Thursday...On Tuesday, the aforementioned deep
trough will migrate eastward, swinging a rapidly moving cold
front southeast across the Florida Peninsula during the day, and
reaching the Florida Keys very late at night, between 06Z and 12Z
Wed. The ECMWF trough is sharper on this 12Z run and farther
south, but unlike last runs, there doesn`t appear to be as well a
defined 500 mb low moving across the TN Valley like the last
several runs in both models. So given weak dynamics, will keep in
the threat for thunderstorms as forecast soundings still also
show decent moisture available in advance of the surface frontal
boundary, the first of the season! Northwest winds daybreak
Tuesday may go more to the north during Wednesday early enough
that 1000-850 weak cold thickness pattern may not be favorable for
stratocumulus. Any showers post frontal will depart south of the
Keys Wed morning with no chance of rain thereafter. High
temperatures will only reach near 80 with comfortable dewpoints.
Cooler air Wednesday Night and Thursday morning may allow
temperatures to fall back near 70 and highs remaining only a few
degrees from 80 Thursday with mostly clear less humid near breezy
to breezy conditions and 0 chance of rain.


.LONG TERM...Thursday Night thru Saturday, appears very quick
modification back to near normal temperature and humidity and
this is common this early in the season, so temperatures in the
mid 80s and lows in the mid 70s with dewpoints back to near 70
degrees. Have reinserted isolated showers back in the grids, 20%,
which is normal in late October.


.MARINE...SCA conditions expected tonight all waters except
Florida Bay, where an SCEC is posted. SCA conditions likely again
in Hawk Channel and the Florida Straits in Hawk Channel and the
Florida Straits, not quite as sure in the Offshore Gulf and
extreme SE of Gulf of Mexico waters where a SCEC is advertised
attm. SCEC conditions still possible, but only in the Straits for
the first part of Monday. Winds and seas let down pre frontal
Monday night thru Tuesday Evening. Winds and seas pick up late
Tuesday Night with advisory conditions expected for portions of
the waters Wednesday and Wednesday Night, with cautionary
conditions still likely in place Thursday, before winds clock back
into a more easterly direction late Thursday Night and the
gradient relaxes again.


.AVIATION...Gusty easterly breezes are expected throughout the
forecast period  at the island terminals, with sustained winds of
12 to 15 knots and gusts of 20 to 25 knots, however this poses no
crosswind issues. Winds will lull a bit through the night hours,
but remain breezy just above the surface. Their is a good
potential for a round or two of brief sub VFR conditions tonight
due to fast moving showers racing westward across the area.


Key West  78  85  78  86 / 50 50 30 30
Marathon  78  85  78  86 / 50 50 30 30


GM...Small Craft Advisory overnight for GMZ032>035-



Upper Air/Data Collection......DR

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