Small Craft Advisory 31, NWS Key West, FL

Area Forecast Discussion
National Weather Service Key West FL
356 PM EDT Sun May 24 2020

.SHORT TERM...(Tonight through Wednesday)
Heavy rainfall will be the main concern through Monday. WPC
guidance forecasts widespread heavy rainfall amounts of 3 to 4
inches tonight through Monday, in addition to what has already
occurred.

The air mass is extremely moist right now. The 12z MFL sounding
observed a daily record-moist Precipitable Water (PW) value of
2.21", and KEY's value of 2.12" exceeded the 95th percentile.

Besides being extremely moist, the air mass is characterized as
having weak convective instability but an increasing amount of
low-level wind shear for organizing convection. Global models
point to falling pressures over the southeast Gulf this evening,
with the rowdier model solutions closing of a weak low center.
Either way, it will increase our E-SE surface winds tonight
beneath southerly flow in the 850-700 mb layer. Within the last
2-3 hours, radar has detected an increasing number of rotating
convective cells over the waters, especially the Gulf waters. SPC
has the Keys under a Marginal Risk of severe weather tonight and
Monday morning, with low probabilities showing up for tornadoes
and severe wind gusts. It is also possible that this evening's
pressure falls over the southeast Gulf will organize some curved
bands of heavy rainfall. If this happens, then the repeated
training of cells over the same locations could lead to our
heavier rainfall amounts exceeding WPC guidance.

This soupy air mass exists to the north of a negatively tilted
500 mb trough extending from the upper Texas coast to the Yucatan
Channel to near Grand Cayman. This placement puts the Keys under
difluent flow aloft, and this will continue until the slow-moving
trough axis lifts north of the Keys on Monday PM. The mere
presence of large-scale lift in a record-moist air mass would
support heavy rain.

Once the trough axis lifts north of the Keys on Monday night,
large-scale lift will exit, and the atmosphere will dry out above
700 mb. Rain chances will taper off back closer to June normals of
25-30 percent in a 12-hour period by Wednesday. (Yes, I know it
is not yet June.)

&&

.LONG TERM...(Thursday through next Sunday)
A weak and diffuse upper ridge will build over Florida and the
eastern Gulf between cutoff upper lows over northeast Texas and
south of Bermuda. It will not be enough to completely suppress
convection, especially with PW values remaining at or above
normal. Nonetheless, the return of more common east-southeast
surface winds and pockets of convective inhibition will amount to
near- or slight above-normal rain chances. In order to focus any
convection, we will have to rely on diurnal influences from nearby
land masses such as Florida, Andros Island and Cuba.

&&

.MARINE...
Weak low pressure over the Yucatan Channel or over far western
Cuba will move northward through the eastern Gulf from this
afternoon through Monday night. As the weak low moves northward
near our far western Gulf waters tonight, fresh to occasionally
strong southeasterly breezes are expected over the Keys waters.
This will likely require the eventual issuance of a Small Craft
Advisory. In addition, numerous showers and a few thunderstorms
are expected through Monday. These will contain particularly heavy
rainfall, therefore reducing visibility accordingly. Gusty
convective winds are also a concern. Though any shower could
produce gusty winds, keep an ear out for thunder, since that will
come with the strongest gusts.

From Wednesday through Friday, a large Bermuda high will expand
westward toward the U.S. Southeast coast, bringing gentle to
occasionally moderate easterly breezes.

&&

.AVIATION...
Showers and a few thunderstorms containing very heavy rainfall
will be the main aviation concern. The terminals are currently in
a lull, but thunderstorms are currently rolling north through the
western Straits and will bring deteriorating conditions at the EYW
and MTH terminals over the next few hours. Showers and
thunderstorms will become more widespread late this evening and
overnight, warranting a prevailing shower group in the TAFs. With
extremely heavy rain possible, visibility could fall to 1 mile or
less under the heaviest rain cores. There is also surface gust
potential to 35 knots or greater with nearby convection tonight.

&&

.CLIMATE...
On this date in 2001, Marathon set the daily record rainfall 2.30
inches. On this date in 1937, Key West set its daily record
rainfall of 3.15".

&&

.PRELIMINARY POINT TEMPS/POPS...
Key West  75  86  79  88 / 90 80 70 40
Marathon  75  86  79  87 / 90 80 70 40

&&

.KEY WATCHES/WARNINGS/ADVISORIES...
FL...None.
GM...Small Craft Advisory for GMZ031>035-042>044-052>055-072>075.

&&

Public/Marine/Fire...Haner
Aviation/Nowcasts....LIW
Data Collection......KBL

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