Area Forecast Discussion
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FXUS62 KKEY 210728

Area Forecast Discussion
National Weather Service Key West FL
328 AM EDT Fri Jul 21 2017

.DISCUSSION...In the middle and upper levels, (700-200 mb) latest
available IR GOES 16-R satelitte imagery overlaid with model
streamline analysis as of 300 am, depict an anticyclonic northern
branch of the jetstream with 75 to 100 knot westerlies at 250 mb
moving from the Pacific Northwest to across the Canadian Prairie
Provinces coming back southeast across New England. South of that,
a typically weak middle and upper level pressure pattern is in
place reflective of the dog days of summer, with the center of a
strong (595 dm plus at 500 mb) deep and warm middle and upper
anticyclone centered near Arkansas expanding from the Southeastern
United States west to the Central Rocky Mountains. Closer to the
Keys, a couple of weak upper lows are located near Northern
Florida and East of the Bahamas.

At the surface and in the lower to middle levels(Surface to 700
mb), latest marine and land surface observations and analysis
detail broad 1018-1020 mb surface ridging in place across all of
the Southeastern United States including the Florida Peninsula,
extending just north of the Keys. The western edge of stronger
surface, (1020 mb plus), ridge reaches to just east of the
Northern Bahamas from the Central Tropical Atlantic near 25 N & 50

Across the Tropics, the leading edge of a well defined Saharan Air
Layer (SAL) is approaching the Central Bahamas from the east,
extending out to the longitude of Puerto Rico. GOES 16-R PWAT
imagery detects values within this airmass down between 1.25 and
1.50 inches. So, last evenings 00Z sounding at Key West
illustrate a light and variable surface to 700 mb flow becoming
gentle southwest to west above that, with a still fairly moist
lower to mid troposphere and PWAT at 1.87 inches.

.CURRENTLY...As of 300 am, skies are mostly clear across the
islands and surrounding waters, with IR and radar imagery only
detecting a few cumulus and all boundaries devoid of any
convection attm. C-man stations along the Florida Reef are
Southeast to south at 5 to 10 knots with island sensors recording
5 to 10 mph.

.SHORT TERM...Today thru Sunday Night, Deep lower to middle level
ridging will continue near the Keys thru the period with 590 dm
plus heights at 500 mb expected. At the surface and in the low
levels, the leading edge of the Saharan Air Layer will move across
Andros island today, filtering then across the Keys tonight. This
airmass will continue to move across the islands for the rest of
the weekend until more typical low to middle level moisture
advecting in across the islands not until Sunday Night. Forecast
soundings illustrate columnar PWAT falls to between 1.25 and 1.50
inches beginning tonight. As such, rain chances will be well
below average for late July with this SAL event.

Although the column still somewhat moist today, bifurcated
surface ridging will keep winds more between southeast and south,
unfavorable for mesoscale cloud lines. Then tonight thru Sunday
Night, the surface ridge will remain nearby just north of the
Keys, maybe sliding a but north of the Keys Saturday Night and
Sunday, before sliding back close to the Keys Sunday Night. Skies
will become mostly sunny/clear with the sky taking a hazy/milky
appearance during the day. Will maintain only a dime pop in the
grids for today thru Saturday Night as the crux of the driest
airmass and associated particulates move across, then just
increase slightly only to 20% for Sunday and Sunday night, given
slightly more low level convergence given MSLP gradient. Daytime
highs will reach 90 or higher during the day, with overnight lows
in the lower 80s, with the possibility of a record mid 80s reading
at either or both official ASOS sites.


.LONG TERM...Monday thru Thursday, a series of tropical waves
now analyzed from east of the Windward islands all the way to
Africa will begin to slide across the Keys longitude from Tuesday
thru Thursday. Exact impact remains hazy out at this point in
time. So given typical moisture in place, and unclear indications
of any strong mesoscale forcing, will maintain climatology for
widely scattered showers with isolated thunderstorms, 30% percent
for each day and night period. No changes here on this cycle.


.MARINE...Boating conditions will remain good thru the entire
period with no SCEC or SCA conditions expected. Light to gentle
breezes expected today thru Sunday, with perhaps a brief period
of moderate breezes expected Sunday Night at best. Light to gentle
breezes are also expected next week as well.


Until 00z/22nd, although VFR conditions will prevail, a MVFR bout or
two is possible within a cloud line especially at the Marathon
terminal this afternoon. Light and variable winds will turn southwest
before becoming variable again late this afternoon and evening.


In 1946, the daily record low temperature of 72 degrees was last
recorded in Key West. Temperature records date back to 1872.


Key West  91  83  91  83 / 10 10 10 10
Marathon  93  83  94  82 / 10 10 10 10





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