Area Forecast Discussion
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FXUS62 KKEY 201906

Area Forecast Discussion
National Weather Service Key West FL
306 PM EDT Fri Oct 20 2017

.DISCUSSION...In the middle and upper levels(700-200 mb), latest
available satelitte imagery overlaid with model streamline
analysis as of 200 am depict a NE to SW arcing middle and upper
trough axis from south of Newfoundland to well east of North Florida.
Upstream of that, the axis of a longwave ridge is located from
North Central Ontario down to the Eastern Gulf of Mexico. Hence,
water vapor imagery shows lots of middle level dry air across
Florida and the Keys.

At the surface and in the lower to middle levels, (Surface to 700
mb), latest available satelitte imagery overlaid with marine and
land surface observations and analysis as of 200 am detail the
1026 mb center of surface high pressure system from the Upper Ohio
Valley to North Carolina. The Florida Keys remain along the
southern periphery of this anticyclone. Enough moisture in the
boundary layer allowed for widely scattered showers to move across
portions of the Keys through the morning and late afternoon, and
several hundreths of an inch fell between Lower Matecumbe and
Marathon, with trace amounts everywhere.

.CURRENTLY...As of 200 pm, Key West and Miami radar and visibile
satellite imagery detects only isolated showers impacting the Keys
and surrounding waters. Skies remain partly cloudy. Temperatures
over the islands have reached the middle 80s with with dewpoints
in the mid 70s. C-man stations along the Florida Reef are 15 to
20 knots with gusts over 20 knots, with island sensors recording
near 15 mph over land.

.SHORT TERM...Tonight through Sunday night, 1025-1027 mb autumnal
ridging positioned from the Northeast to the Carolinas will build
to near 1030 mb during Saturday and Saturday night, then move
slowly eastwards during Sunday and Sunday night. This will allow
for fresh breezes to slowly veer from east tonight to between east
and southeast tomorrow and tomorrow night, then become southeast
and only slowly diminish Sunday and Sunday Night. This setup when
combined with ample moisture will allow for development of fast
moving scattered showers across the islands and surrounding waters
through the period. On balance though, skies will be partly cloudy
with breezy conditions across the islands. Any showers will be
quick movers, so not expecting a great deal of rainfall with

The latest available model GFS and ECMWF model forecast soundings
allow for some lower to middle level moistening tonight through
the weekend. As such, will be maintaining middle of the road
chances, 40%, for showers for tonight and Saturday, with a high
chance for showers during Saturday Night when both model forecast
soundings are maintaining columnar PWAT near 50%, but trim to 30%
during Sunday and Sunday Night when MSLP gradient begins to loosen
just slightly. Have reinserted lightning back into the grids
beginning tomorrow, when slightly more instability is present
with deeper moisture in the column.


.LONG TERM...Monday and Tuesday, expect a slackening of the
gradient, although forecast soundings till maintain decent
moisture in the lower to mid levels, so will maintain a low chance
of showers with a slight chance for thunder, since upper level
heights ill begin to fall slightly in advance of a deep chilly
trough progged by both models to be developing over the Central
and Eastern United States. Highs and lows will still be above
normal with higher dewpoints.

Then Tuesday Night through Wednesday Night, as mid level energy
carves out a cold 545-555 dm middle level low pressure system
passing across the TN Valley, the system will likely swing a
decent cold front southwards down the Peninsula Tuesday Night and
early Wednesday. The GFS is still faster with this scenario, so
taking it more into account have the actual surface front moving
through before 12z Wed. For now, will maintain middle of the road
pops for Monday Night thru Tuesday Night, but some dynamics could
support a more organized frontal passage given this type of
synoptic setup.

For Wednesday afternoon through Friday, expect shorter period of
NW winds immediately behind the front Wednesday afternoon and
night and this could bring over some decent stratocu cigs across
the Gulf, given typical 1000-850 mb thickness advection pattern.
Do expect typically windy conditions developing for Wednesday and
Wednesday night, with winds still coming off the Peninsula through
Thursday Night. Took pops out for Wednesday afternoon through
Thursday night, as winds come right down the Florida Peninsula
with PWAT falling down to 1 inch. Temperatures will reach 80 and
lows near 70 for Wednesday and Wednesday Night, only lower 80s
Thursday, and lower 70s Thursday Night before recovering to normal
Friday, with comfortable dewpoints likely near 60.


.MARINE...Small craft advisory (SCA) conditions expected across
all waters tonight thru Saturday Night except SCEC conditions
across Florida Bay, with SCA conditions remaining in place at
least across the Florida Straits, the Offshore Gulf waters and the
extreme SE GULF of Mexico waters on Sunday with SCEC conditions
across the rest of the waters. SCEC conditions will gradually
disappear across the rest of the waters by Sunday Night. No
headlines or advisories are expected Monday thru Tuesday Night,
but SCA conditions will probably impact all waters by late


.AVIATION...Breezy northeast to east winds expected tonight at the island
terminals. VFR conditions will prevail. With that said, isolated to
scattered fast moving showers may deliver brief spells of sub VFR
conditions at the island terminals.


Key West  77  85  78  86 / 40 40 50 30
Marathon  77  86  78  86 / 40 40 50 30


GM...Small Craft Advisory for GMZ032>035-042>044,GMZ052>055-



Upper Air/Data Collection......DR

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