Area Forecast Discussion
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FXUS62 KKEY 180921

Area Forecast Discussion
National Weather Service Key West FL
421 AM EST Sun Nov 18 2018

Temperatures early this morning are in the mid 70s along the
island chain with mostly cloudy skies across the region. The Key
West WSR-88d is detecting scattered to numerous showers across the
area, with the highest concentration near a surface pressure
pertubation located in the Florida Straits about 20 to 60 NM from
the west end of Seven Mile Bridge to the Dry Tortugas.

The 00z KEY sounding shows a saturated layer from 925mb to near
800mb. A veering wind profile through this layer indicates warm
air advection, and isentropic analysis in the 300-310K layer
indicates modest upglide occurring to the east of the surface
pressure pertubation.

.SHORT TERM...(Today through Wednesday)
MOS guidance, high resolution CAMs, and global models are all in
good agreement that the aforementioned feature will keep clouds
and shower activity around today. The GFS slows the movement of
the pressure pertubation and briefly forms a closed surface low.
Low-level flow to the east becomes better aligned with the theta
gradient to allow more efficient isentropic upglide later this
morning. The HRRR is actually more aggressive forming a surface
low, tracking it nearly due north directly to the west of the
Lower Keys. This scenario would maximize rainfall potential today.
Regardless, the end result is expected to be mainly light to
moderate showers with no substantial source of lift above 800
mb... not to mention the much drier air above. Therefore I see no
reason to add thunder despite the increase in PoPs today.

The aforementioned upglide looks to fall apart this evening as the
theta gradient weakens and the surface low departs to the west.
Rain chances fall off accordingly, with perhaps some scattered
showers lingering over the waters north and west of the Lower
Keys tonight.

A longwave trough will move into the eastern CONUS over the next
couple of days. Unfortunately this feature will not bring
significant changes to our sensible weather with no major front
expected to make it this far south. Easterly winds will gradually
weaken through Tuesday, then a new ridge of high pressure will
build back into the southeastern CONUS on Wednesday in association
with the departing trough.


.LONG TERM...(Wednesday night through Saturday)
Not much change to the long term forecast, as uncertainty
increases substantially after Thursday. Due to drastic differences
between the 00z ECMWF and 12z GFS, decided to favor a heavily
weighted GFS forecast during this period. This maintains northeast
to east winds through the period, gradually increasing through
late week before diminishing into a col region by the weekend.
Contrast this with the 00z ECMWF solution which brings a low
pressure system west to east across the Gulf, blasting our area
with fresh to strong breezes from the west. Will wait for better
agreement between models before factoring such an extreme solution
into the forecast. PoPs will be held at near climatology (20%).


Moderate easterly breezes are expected across the coastal waters
through tomorrow. High pressure over the southeastern US will
gradually weaken, leading to gentle to moderate northeast to east
breezes by Tuesday. A fresh injection of high pressure to the
southeastern US on Wednesday will lead to moderate breezes
becoming likely again.


VFR conditions are expected to prevail at both KEYW and KMTH as a
layer of stratus and stratocumulus with bases between 3000 and 4000
ft persists over the Keys. Cannot rule out brief periods of MVFR
ceilings during the morning hours, however the general trend should
be upward with cloud layers eventually scattering out during the
course of the day. Winds will generally remain out of the northeast
at around 10 kts at both terminals, veering slightly to a more
easterly direction later this morning.


On this day in 1886, the daily record high temperature of 90F was
recorded. Temperature records in Key West date back to 1872.


Key West 81 75 83 76 / 60 20 10 10
Marathon 82 75 84 75 / 60 20 10 10




Data Collection......DR

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