Area Forecast Discussion
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FXUS62 KKEY 261916

Area Forecast Discussion
National Weather Service Key West FL
316 PM EDT Tue May 26 2020

Fairly normal and routine early summer weather is expected as we
head toward the start of June.

.CURRENTLY...(Rest of this afternoon)
Southerly low-mid level flow will continue through this evening.
For the next few hours, this will mean suppressed convective
activity under the familiar afternoon Cuban shadow. BTW, Cuban
shadow is just a subsident downwind response to rising thermals of
hot air over Cuba...what goes up (over Cuba) must come down (over
the Straits). In southerly flow, it suppresses cloud development
over the Straits during the early and mid-afternoon hours.

.SHORT TERM...(Tonight through Friday)
Once the Cuban shadow eases in the hour or two before sunset, a
few showers or perhaps a thunderstorm should develop and move
north through the Straits. Any showers and storms will curve
northeast as they approach the Keys, which will cause them to
merely brush by the Keys...most notably the Middle and Upper Keys
this evening. Overnight, the southerly flow will end as a surface
ridge axis from the Atlantic starts to nose westward across the
Keys. This will support a decreasing trend in overnight convective

Tamer and more routine late May weather is expected late tonight
through Friday. First, a broad surface ridge axis will build
westward across the Keys and South Florida tonight and Wednesday.
As the ridge axis lifts northward to Central Florida on Thursday
and Friday, today`s southerly flow will become easterly. Model
time-height sections show drying of the atmosphere starting late
tonight above about 850 mb, lowering Precipitable Water (PW)
values back to near-normal levels for this time of year. In the
absence of any large-scale weather features, we will mainly rely
on second-generation showers and thunderstorms that initially
form over nearby land masses such as the FL Peninsula, Cuba, and
Andros Island. We could even locally generate a shower or
thunderstorm if east winds take on a slight northerly component
and align favorably parallel to the Keys. This would enable mid-
day island cloud line development and a few short-lived showers.


.LONG TERM...(Saturday through next Tuesday)
Little overall change is expected through Monday, June 1. The
surface ridge axis over Central Florida will settle southward to
South Florida and dissipate next Sunday. If anything, weakening of
the E-NE low-level winds will be more favorable for mid-day island
cloud line shower development on Sunday and Monday, since lighter
winds are less disruptive to rising thermals of air over the
narrow island chain.

Easterly breezes will freshen next Tuesday, thanks to continental
high pressure moving off the North Carolina coast. In addition,
models and ensemble means start to lower surface pressures
somewhere between the western Caribbean and the Bay of Campeche
next Tuesday, which would also contribute to tightening pressure
gradients over the Keys. Stronger low-level easterly flow
typically deepens the surface-based moist layer and comes with an
increase in fast-moving showers. Have therefore indicated an
upward trend to 40 percent rain chances on Day 7.


No advisory or cautionary headlines are expected through Sunday,
other than an evening enhancement of easterlies over the distant
Straits on Thursday evening.

From synopsis...A broad Atlantic ridge axis will start to nose
westward across the Keys and South Florida late tonight and
Wednesday. It will lift northward to Central Florida on Thursday
and remain in place through Saturday. The ridge will dissipate as
it retreats back down across South Florida next Sunday, leading to
only light and gentle easterly breezes.


Showers and thunderstorms may develop over Cuba late this
afternoon. They would then move north at first and then curve
northeast as they approach the Keys. Therefore, any convection
that survives the transit across the Straits is more likely to
clip MTH than EYW. However, confidence in this evolution is not
high enough to mention in the TAFs at this time. S-SW surface
winds will become southeasterly overnight.


Key West  78  88  79  88 / 30 10 10 10
Marathon  78  91  80  90 / 30 10 20 20




Data Collection......SDR

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