Area Forecast Discussion
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FXUS62 KKEY 240129

Area Forecast Discussion
National Weather Service Key West FL
929 PM EDT Tue May 23 2017

.DISCUSSION...In the middle and upper levels(700-200 mb), latest
available IR imagery overlaid with model streamline analysis as of
900 pm, depict an unseasonably deep longwave trough building
slowly eastward with an associated well aliened cold core cyclone
moving southeast away from Iowa. Much of the Gulf of Mexico is
beneath the southern periphery of this trough, or underneath deepening
middle and upper cyclonic flow that is pushing towards Florida.
South Florida and the Florida Keys are not that far east of the
right entrance region of the jet, somewhat unusual for late May.

At the surface and in the lower to middle levels,(Surface to 700
mb), latest IR imagery overlaid with land and marine surface
observations and analysis as of 900 pm, detail a strong late
season cold front trailing south southwest to East Texas from a
1005 mb low over Southern Minnesota. Well southeast of that, the
western periphery of an atlantic surface ridge is poking across
South Florida from the West Central Atlantic. The latest 00Z
evening sounding is illustrating veering and moistening, as well
as a destabilizing lower to middle level troposphere with a gentle
to fresh south to southwest flow right off the surface up to 700
mb, then becoming strong above that, with columnar PWAT up to
1.66 inches.

.CURRENTLY...As of 900 pm, Skies are partly cloudy owing to mainly
some altocu with cirrostratus and cirrus aoa 20 kft. This is well
 by the latest available IR imagery showing these clouds
continuing to roll in from the west. Temperatures across the
islands remain in the lower 80s with typically humid dewpoints in
the middle 70s. C-man stations along the Florida Reef are
southeast to south near 10 knots attm. Radar detects mostly non
precip echoes with the exception of a few cells which have
developed in situ out over the Cay Sal Bank well SE of the Keys.

.SHORT TERM...Overnight, a surface low pressure system will
develop over the Middle Ohio Valley after midnight tonight along
the aforementioned cold front and deepen rather quickly to 999 mb
by 12Z, deepening to 996 mb over Western Pennsylvania by days end.
Available forecast soundings will illustrate the deepening low to
middle level cyclonic flow with freshening southeast to southerly
winds overnight and tomorrow morning. Southeast to south winds
near 10 mph will increase to near 15 mph after midnight and become
gusty by daybreak, with breezy conditions likely during
Wednesday. Given the thermodynamics and approaching dynamics,
(with low level confluent flow ensuing already), will keep
isolated showers and thunderstorms in the grids overnight, but
this would be very late (well after midnight). Hence, Only changes
will be to adjust the winds up a bit and add slight chance of
thunder as well.

During Wednesday, the aforementioned jet entrance region will be
approaching South Florida and the Keys at 500 mb (18 to 24 hours
from now). As such, deep cyclonic flow and 500 mb PVA (positive
vorticity advection) combined with deep moisture will contribute
to rounds of strong showers and thunderstorms, with GFS forecast
soundings illustrating 0-6 km storm motion around 30 knots by 18Z,
allowing any Cuban convection that develops during Wednesday
afternoon to move rapidly northwards towards the Keys. This first
round is expected to be late in the afternoon and evening, but if
convection does develops earlier over the Cuban Archipelago in
the afternoon, then the first round could race quickly northwards,
reaching the Keys by mid afternoon. SPC has placed the Florida
Keys in a marginal risk for severe thunderstorms Wednesday night
thru Thursday morning.


.MARINE...Overnight, will be issuing SCEC headlines for Hawk
Channel, all of the Florida Straits, and the offshore Gulf waters
20 to 60 nm West of Mainland Monroe and the extreme Southeastern
Gulf of Mexico waters around the Dry Tortugas and Rebecca Shoal
Channel. SCEC conditions appear likely outside of showers and
thunderstorms across all of the waters during Wednesday, however,
mariners encountering showers and thunderstorms will encounter
some wind gusts between 35 and 45 knots in squalls later in the
day and evening. Special Marine warnings will be likely by late


.AVIATION...VFR conditions expected overnight at both KEYW and
KMTH island terminals.




Data Collection......DR

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