Area Forecast Discussion
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FXUS62 KKEY 192013

Area Forecast Discussion
National Weather Service Key West FL
313 PM EST Sat Jan 19 2019

Main focus of the forecast is with weather that will result from
the passage of a strong cold front on Sunday morning. The front
is now crossing the Middle Gulf, trailing from a 995 mb parent low
that is moving E-NE across Middle Tennessee. The front will pass the
Eastern Gulf tonight. Cold frontal passage is expected near Dry
Tortugas and the deep Southeast Gulf waters around 12z Sunday (7
am), the Lower Keys around 15z (10 am), exiting off the Upper Keys
around 16z- 17z (11 am-noon). Deep layer southwesterlies
preceding the front will quickly increase atmospheric moisture,
with Precipitable Water values forecast to peak near 1.75" in an
axis immediately preceding the front. The air mass will also
become marginally to moderately unstable, with CAPE values peaking
near 800-1000 J/kg over the Keys, 1500+ J/kg over the Straits,
and less than 800 J/kg over Gulfside waters. This will support a
broken line of thunderstorms along or just in advance of the
front. Shear profiles in the pre-frontal convective environment
will be nearly unidirectional with modest speed shear, so gusty
winds, brief downpours, and a few lightning strikes will be the
main impact of convection. The line of showers and thunderstorms
will be moving east at a pretty good clip, so storms will be quick
to move through.

Following the front, a sharp wind shift is expected, with
northwesterlies developing. Will become windy, with land-based
wind gust around 30 mph expected as wind speeds peak on Sunday
afternoon. Otherwise, rain and showers should end pretty quickly,
except perhaps for some sprinkles that fall out of cold air
strato-cumulus clouds streaming from the Gulf across the Lower

The coldest night will occur on Sunday night, with quite a dry air
mass and clear skies spreading down the Peninsula and across the
Keys. Surface dewpoints will fall into the 40s, and low
temperatures on Monday morning will be in the 50s throughout the

Surface winds will be quick to flip northeasterly on Monday
morning, veering further to easterly or southeasterly on Tuesday.
This is in response to a meridional surface ridge axis up and down
the Mississippi Valley late Sunday, which will consolidate into a
strong 1035 mb surface high center near the Outer Banks on Tuesday
afternoon. Once winds become easterly on Tuesday, the strong
pressure gradient on the periphery of this high center will lead
to windy conditions over the Keys and the gradual return of a
milder maritime air mass. Wind gusts of 25 to 30 mph will be
common on Tue and Wed, especially as easterlies peak on Tue PM.

Isolated to scattered showers will return to the forecast on
Monday night and Tuesday, as surface-based moisture deepens to
about 850 mb and the strong easterlies help a few showers develop
in confluent cloud lines that orient parallel to the low-level


.LONG TERM...The next front is expected to cross the Keys around
next Thursday PM. Ensembles members still vary on timing, but
there is growing consensus with the Thu PM timing. A tropical
maritime air mass will precede the front, with PW values
approaching 2" in advance of the front. Continue to have isolated
thunder in the forecast next Thu and Thu night. Once the front
passes, models are somewhat slow pushing it past the Straits of
Florida, so have had to hang on to low PoPs into next weekend.


Marine hazards will emerge tonight and Sunday in response to the
passage of a strong cold front across the Florida Keys on Sunday
morning. Preceding the front tonight, southerly winds will pick up
and gradually become southwesterly. Winds speeds will pick up as
the front approaches, with Small Craft Advisories likely for a few
zones as early as tonight. The front itself will first reach the
Dry Tortugas and deep waters of the Southeast Gulf around 7 am,
cross the Lower Keys waters around 10 am, the exit into the
Straits off the Upper Keys by noon. The front itself will be
accompanied by a broken line of showers and thunderstorms,
containing gusty winds brief visibility reductions in transient
downpours. The front will be followed by a sharp shift to
northwest wind, with sustained speeds near 25 knots over nearly
all the waters on Sunday afternoon. Wave heights will peak in the
10-12 foot range early Sunday evening over the deep Gulf waters
and the lower Straits of Florida due to strong northwest winds.

Winds will ease on Sunday night and bottom out as a moderate
northerly breeze on Monday morning.

A strong surface high center will consolidate near the North
Carolina coast on Tuesday. Strengthening gradients on the
periphery of this feature will drive an increase to strong
easterly winds over the waters for Tue and Wed. Marine MOS shows
speeds rising above 20 knots early Tue morning for all but Long

The next cold front is expected around next Thursday PM, though
there are still differences with timing the next cold front.


VFR and dry conditions will prevail through this evening at the EYW
and MTH terminals. Most few to scattered cloud bases should be near
FL030-035 and spreading near FL060 through sunset. Southeast winds
will continue 7-12 knots through at least 20/03Z. Winds will turn
south after that time, gradually increasing to near 15 knots with
gusts at or above 20 knots by 20/08Z. Showers and isolated
thunderstorms will begin to increase in coverage from 20/08Z-10Z,
with some bringing MVFR ceilings, brief IFR visibilities and west
wind gusts possibly near 30 knots. After the line of showers and
possible thunderstorms passes, sustained winds will increase to 15
to 20 knots out of the west. After 20/15Z, MVFR ceilings may
linger as showers decrease, with winds becoming northwest 15 to 20
knots with frequent gusts near 25 knots.


On this date in 1997, the daily record low temperature of 48F was
last recorded at Key West. Temperature records date back to 1872.




Data Collection......DR

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