Area Forecast Discussion
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FXUS62 KKEY 221900

Area Forecast Discussion
National Weather Service Key West FL
300 PM EDT Mon Apr 22 2019

The next change in the weather is expected on Friday and
Saturday, when the Florida Keys will experience an uptick in the
chance of showers and thunderstorms.

The air mass is very dry right now, leading to mostly sunny skies
and shower-free weather. The 12z KEY sounding revealed a dry PW
of just 0.54". Surface dewpoints also reflect the dry air mass,
running near 60F right now. Gentle to moderate easterly breezes
prevail across the marine waters this afternoon.

.SHORT TERM...(Tonight through Thursday night)
Fair and rain-free weather will continue through Thursday under
the influence of a very dry air mass. NW mid-upper level flow will
remain through Wed night, then a flat upper ridge axis will
progress across the Keys on Thu. A surface high pressure ridge
will be laid out across North Florida today, then Central Florida
on Tue and Wed. Through Tue, Precipitable Water (PW) values over
the Keys will remain well under one inch, then creep up to near 1
inch on Wed. Surface dewpoints will take until Wed night to even
climb above the mid 60s. Shallow surface-based moisture will
remain confined to the lowest 4,000 feet, and the air mass above
4,000 feet will be bone dry.

The first early signs of change start on Thu night. By that time,
surface high pressure will have moved well off to the east,
allowing low-level flow to pick up from a moister southeast
direction. The upper ridge axis will have passed east by then,
with southerly mid-level flow developing. In this regime, leftover
diurnal convection and convective boundaries over Cuba on Thu
afternoon could drift north across the Straits on Thu evening.
Precipitable Water values over the Keys will quickly climb on Thu
night to near 1.5" by sunrise Friday. So the first chance of
showers will arrive over the islands after midnight on Thu night.


.LONG TERM...(Friday through next Monday)
Shower and thunderstorm chances will be picking up on Friday and
Saturday. The upstream culprit is a southern stream upper trough
that is currently settling into the Desert Southwest. By
Thursday, it will be moving east across East Texas, with a
trailing trough moving into the far Western Gulf. As the upper
trough moves across the Gulf on Friday, difluent W-SW flow aloft
will develop over the Keys. Upper height falls and cooling aloft
will occur on Friday and Friday night, as the surface-based air
mass reaches its peak for the week in terms of temperature and
dewpoints (dewpoints in low-mid 70s). The convective instability
of the atmosphere will quickly increase, with CAPE values likely
approaching 2000 J/kg on Fri night. Concurrently, 500 mb winds of
35-40 knots from the W-SW will provide a shear profile that could
loosely organize convection.

The upper trough axis will drag across South Florida late
Saturday. Precipitable Waters values in excess of 1.5" on Sat
still indicate a juicy air mass. Upper dynamics and the shear
profile will favor less organization on Saturday, but 500 mb
heights near 580 decameters and cyclonic flow aloft will bring
peak rain chances on Saturday. A broad surface low over the
eastern Gulf will also help keep the sensible weather stirred up.

Rain chances will be slow to taper off next Sunday, emphasis on
slow. Even though the upper trough axis will be off to our east
by then, 500 mb flow still shows some cyclonic curvature. Ensemble
mean sea-level pressure maps show a washed out inverted surface
trough lingering somewhere between the Bahamas and the eastern

PoPs trend down in the direction of climatology on Monday. The
lingering surface trough from Sunday should dissipate on Monday.
Surface pressures will rise, as the periphery of an east- west
ridge axis over the Eastern U.S. spreads in from the north. PW
values will fall back closer to 1.25" on Mon. Given those factors,
we will mainly rely on moderate to fresh E-NE surface flow to
help focus weak showers at the leading edge of mesoscale wind


A surface ridge axis is aligned from the Florida Panhandle to the
Georgia coast this afternoon. Gentle to moderate easterly breezes
over the waters will undergo some diurnal freshening this evening,
necessitating a SCEC headline for the Straits and Hawk Channel.

High pressure will gradually settle south toward Central Florida
through Wed, then weaken and lose definition as it reaches South
Florida on Thu. A low pressure trough will enter the western Gulf
on Thu and reach the central Gulf on Fri. This feature will
gradually weaken over the eastern Gulf on Saturday and Sunday,
with light and gentle breezes over the waters by Sunday.


VFR conditions are expected to continue through Tuesday afternoon
and beyond. A dry air mass is in place right now, limiting
coverage of fair weather cumulus to mainly FEW coverage, with a
few patches of SCT coverage. Surface dewpoint depressions of
15-20F are propping cloud bases up in the 035-050 range.


On this date in 1940, the daily record low temperature of 62
degrees was recorded in Key West. Temperature records date back
to 1872.


Key West  72  81  73  82 / 0 0 0 0
Marathon  73  86  74  87 / 0 0 0 0




Data Collection......BT

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