Area Forecast Discussion
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FXUS62 KKEY 170748

Area Forecast Discussion
National Weather Service Key West FL
348 AM EDT Thu Aug 17 2017

Starting with satellite interrogation, we find an upper level low
centered over the Windward passage between Cuba and Dominica.
Embedded in the core of this upper low is a pronounced plume of
Saharan dust, and is the likely culprit for the lack of moisture
and any convection near the core of the upper low. This will be
important for the following forecast. Lifting on the leading edge
of the low is active with developing showers and thunderstorms
moving westward. On the lee side of the low is an advancing
tropical wave. High pressure in the low to mid levels over the
Atlantic remains in place, with the ridge axis crossing the
Florida Peninsula. The radar is showing a lull in returns over the
Keys area at this hour, with echoes advancing westward from
Andros. winds are out of the east to southeast at the marine
stations surrounding the Keys Island Chain, and a recent ASCAT
satellite pass offers up 10 to 15 knots over the Florida Straits.
The island terminals are east to southeast near 10 knots.

Guidance has been persistent with raising rain chances with the
arrival of the upper low, but the timing has been a bit
questionable. Using the analog method, timing should bring the
center of the upper low across the Keys by Friday around sunset.
Rain chances will be enhanced along the leading edge of the low,
then as the center of this low moves over the area Friday night
and Saturday, expect that the SAL embedded in the low will cross
the Keys area, and drying out the mid and upper levels. Guidance
is much more bullish than this forecaster, and for some reason
fills in the low, removing the dry air. Timing of the arrival and
departure of this low, and keeping the dry air embedded, have
adjusted rain chances downward Friday night and Saturday to a
slight chance of showers and thunderstorms. Will increase pops to
medium to high end chance of showers and thunderstorms as the low
passes west of the Florida Keys, and the tropical wave slides
south of Cuba Saturday night and Sunday. Will keep a medium chance
of showers and a slight chance of thunderstorms in for the rest
of the week. Along with the higher rain chances, do expect a wind
surge on the back end of the low with the tropical wave moving
through. The Atlantic ridge will rebuild and provide peaks and
lulls in moderate to fresh winds across the area late Sunday
through Wednesday.


The high pressure over the Atlantic will be nudged northward as an
upper level low moves across the Windward Passage and Cuba today
and tonight. The upper low will move across the Keys area on
Friday, with the core of the low centered on the Keys Friday night
and Saturday. A tropical wave will pass south of Cuba as the low
exits westward Saturday night through Sunday. Expecting scattered
showers and thunderstorms Sunday, with thunderstorms becoming
isolated Monday through Wednesday. The Atlantic high pressure will
rebuild on Sunday, with peaks and lulls in moderate to fresh
winds through Wednesday. A small craft advisory is likely late
Sunday through Tuesday night.


Prevailing VFR conditions are expected through 17/06z. Atmospheric
moisture will be on the rise across the area on Thursday, resulting
in an increase in shower and thunderstorm coverage after 17/15z.
Episodes of MVFR/IFR VIS/CIGs will be plausible with any encounter
near the terminals, along with convective wind gusts to around 25
knots. Outside of any shower or thunderstorm interaction, winds will
remain gentle out of the east southeast at less than 10 knots.


On this date, back in the year 1881, the daily record cold high
temperature of 81 degrees was observed in Key West. This ties as the
coldest high temperature to ever be recorded during the month of
August. Temperature records in Key West date back to 1872.


Key West  90  83  90  83 / 40 50 40 20
Marathon  92  82  92  82 / 40 50 40 20




Data Collection......DR

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