Area Forecast Discussion
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FXUS62 KKEY 192008

Area Forecast Discussion
National Weather Service Key West FL
308 PM EST Sun Feb 19 2017

.DISCUSSION...In the middle and upper levels, (700-200 mb), latest
available satelitte imagery overlaid with available model streamline
analysis as of 200 pm depicts a split flow over the Western CONUS
with a narrow deep and sharp southern trough oriented from Salt
Lake City, south into the Eastern Pacific just offshore of the
Central Mexican coast. Downstream of that, the main axis of a
unseasonably warm and deep anticyclone extends from the Yucatan
Peninsula up to Northern Minnesota.

At the surface and in the lower to middle levels, (surface to 700
mb), latest available marine and land surface observations and
analysis as of 200 pm details broad weak near 1020 mb surface
ridging centered over the Middle Ohio Valley. Well east of that,
there is a weak albeit defined area of low pressure well east of
Myrtle Beach SC. That system is trailing a cold front analyzed
through Central Florida attm.

.CURRENTLY...As of 200 am, temperatures have reached into the
lower 80s. Small cloud lines have developed near the Middle Keys,
where KAMX total 3 hr precip has indicated that a trace has
fallen near the 7 mile bridge from this short cloudline. This was
given the fact that depth of the low level moisture was
underestimated at morning analysis time. Given the aforementioned
increased moisture, its not surprising that a narrow band of
showers developed over South Dade and moved across the extreme
northernmost Upper Keys near Ocean Reef, resulting in several
tenths of an inch. As such a quick early afternoon update was
performed to split off Upper Keys. Now attm, winds along the
Florida Bay and the Reef are now all mostly northwest near 10

.SHORT TERM...Tonight thru Thursday, The combination of the
weaker ridging (now just over the Ohio Valley) intensifying
slightly as it migrates to the Northeast, with a developing low
well east of the Carolinas, will swing a weak front, shear line
thru the Keys this evening, with moderate veering breezes
following late tonight through the first half of Washingtons
Birthday. A weak ridge will follow keeping east gentle to moderate
east winds across the Keys thru Tuesday Night. It should remain
quite dry in the lower to mid troposphere until Tuesday Night,
when it begins to moisten up given increasing low level confluent
flow from the southeast. As such have inserted slight chance pops
back in the forecast.

On Tuesday night, the next well defined are of middle level
vorticity, defined in both ECMWF and the GFS, migrate a near
562-565 at 500 decameters disturbance across much of the Gulf
upstream of the Keys and South Florida at our latitude. Like 24
hours prior, these two models diverge in the vertical continuity
of the evolving surface to 500 mb low pressure which moves across
our area, with the 12Z GFS continuing to let go of a well defined
surface reflection when the middle level system reaches the
Florida Straits. But given the ECMWF insistence, will continue to
hold with high chances for showers with isolated thunderstorms.
The synoptic scenario remains rather anomalous for the Keys. So
will hold with 50 percent now for Wednesday and Wednesday Night
across the Keys. The low is still in close proximity on Thursday,
but given uncertainty, will be holding with middle of the road
pops for now for Thursday. Although winds will be weaker given the
GFS scenario, uncertainty remains with the magnitude and
direction given the same split in these solutions


.LONG TERM...After the systems dampens out Thursday night, a much
weaker pressure pattern will remains locally across the islands
through the weekend. The forecast soundings maintain a light to
gentle flow with slightly elevated near surface moisture. As such
will keep a dime pop in the grids for Friday thru Sunday.


.MARINE...Expect gentle west winds to become moderate to fresh
overnight and at least the first half of Monday. Do not have a
caution for these first two periods given 15 knot and gusty winds,
however its possible that SCEC conditions could develop across the
Florida Straits. Expect 10 to 15 knot easterly second half of
Monday through Tuesday, except possibly 15 knots in the Straits.
Then uncertainty prevail regarding wind direction and magnitude
for the Tuesday Night thru Thursday period so winds are capped at
15 knots and gusty. If the ECMWF solution becomes more reliable,
then SCEC or SCA conditions will be likely during this period.
Timing of the wind direction and magnitude will need to be fine
tuned before advertising these conditions. Any elevated winds and
seas will let down thereafter for the end of the week.


.AVIATION...Tonight, VFR conditions expected at the island
terminals this evening. Surface winds near 10 knots west to
northwest, clocking around to north


Key West  68  81  70  80 / -  -  -  -
Marathon  68  82  72  80 / -  -  -  -




Data Collection......Chesser

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