Area Forecast Discussion
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FXUS62 KKEY 201411

Area Forecast Discussion
National Weather Service Key West FL
1011 AM EDT Fri Oct 20 2017

.DISCUSSION...In the middle and upper levels(700-200 mb), latest
available satelitte imagery overlaid with model streamline
analysis as of 900 am depict a NE to SW oriented positively
tilted middle and upper trough well east of Nova Scotia to
Florida. Upstream of that, the axis of a longwave ridge is located
from Upper Michigan down through the Central Gulf of Mexico. Water
vapor imagery delineated lots of dry air across Florida and the
Keys at the bottom of the departing trough.

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 900 am detail the
1028 mb center of a building surface high pressure system located
over the Middle Appalachians. The Florida Keys are along the
southern periphery of this anticyclone. As such, the 12Z sounding
at Key West illustrated a moderate northeast to east wind from
the surface to 700 mb with dry air above that and total PWAT at
1.82 inches.

.CURRENTLY...As of 900 am, Key West and Miami radar and visibile
satellite imagery detects widely scattered showers across the
Florida Straits, with fast moving isolated showers impacting
portions of the Upper and Middle Keys, but they are delivering
light amounts of rainfall, with several hundreths of an inch of
rainfall at most in those locations. Skies on balance over the
Keys are mostly sunny, except mostly cloudy over the Florida
Straits. Temperatures over the islands are in the lower 80s with
dewpoints in the mid 70s.

.SHORT TERM...Rest of the day and the evening. Slow eastward
movement of the aforementioned surface high pressure system, will
result in a tightening pressure gradient, locally, with winds
mixing down through the rest of the morning, and fresh breezes
continuing through the afternoon and evening. Latest available
GFS and ECMWF forecast soundings are still not that dry and with
enough moisture below 700 mb, and columnar PWAT between 1.50 and
1.75 inches. Hence, low level convergence will result in continued
development of fast moving ENE to WSW moving isolated to widely
scattered low topped showers. The best chance of a lightning
strike would be over the Florida Straits. Rainfall amounts will be
very light with most locations receiving a trace. A few locations
could receive several hundreths of an inch of rainfall. Given the
scarcity and overall coverage of the showers, a minor update was
performed earlier just to adjust shower coverage to isolated and
take out the chance for lightning over the islands.


.MARINE...A small craft advisory is now in effect for the Florida
Straits through at least Sunday Morning. SCEC headlines are in
place across the rest of the waters, but expect a SCA to be
issued this afternoon thru Sunday Morning across the Offshore
Gulf waters 20 to 60 NM west of Mainland Monroe County, The
extreme SE Gulf of Mexico waters around Rebecca Shoal Light and
the Dry Tortugas, All of Hawk Channel, and the Florida Straits.
Its still possible that an SCA will be needed across the
shallower landshadowed Florida Bay and nearshore Gulf waters
inside the Five Fathom Line. SCEC conditions still likely Sunday
and Sunday Night across most of the marine district before winds
and seas let down early next week.


.AVIATION...Northeast to east breezes will continue to trend
upwards this afternoon. Expect breezy conditions with winds of 10
to 15 knots, with gusts of 20 to 25 knots. VFR conditions will
prevail. With that said, isolated to scattered fast moving showers
may deliver brief spells of sub VFR conditions.


GM...Small Craft Advisory this afternoon for GMZ052>055- 072>075.



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

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