Area Forecast Discussion
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FXUS62 KKEY 191315

Area Forecast Discussion
National Weather Service Key West FL
Issued by National Weather Service MIAMI FL
915 AM EDT Tue Sep 19 2017

Surface high pressure over the northern Gulf of Mexico continues
to influence the weather over the Keys this morning, but a small
surface feature off the southeast coast of Florida will allow for
a wind shift during the afternoon. Surface winds will have more
of a westerly component after the shift. Otherwise, no updates
are necessary this morning.

.PREV DISCUSSION... /412 AM EDT Tue Sep 19 2017/

Dry and warm weather will persist through the
middle of the week with slightly above normal temperatures and
below normal shower chances across the Keys. A return to more
typical and seasonable weather will begin Thursday with shower and
thunderstorm chances increasing into the weekend. Winds will
generally be light from the north to northeast through Wednesday,
becoming more east to northeast the latter half of the week.

Diffuse pressure pattern and exceedingly dry air for mid-September
will create continued quiet, warm, and dry conditions across the
Keys. PWATs today and Wednesday around 1.3 inches will prevent much
more than shallow CU, and despite a tongue of more moist air across
central and south Florida favorable for at least some showers, the
dry air locally will squelch any of this activity as it tries to
advect southward on N/NE winds. By Thursday, moisture return will
begin as easterlies re-develop and heights begin to fall ahead of
still distant Hurricane Maria. PWATs climb above 1.5 inches and
lapse rates subtly steepen causing better instability, so will
reintroduce a slight chance for showers and tstms, with higher
probability across the Upper Keys. However, the area will be in
the unfavorable RFQ of a 300mb jet streak pushing across the
northern Gulf Coast, so anticipate only isolated coverage of
showers/tstms Thursday despite the increasingly favorable
environment. Temps will remain slightly above seasonable values
through Wednesday, 89-92, with lows 79-81, before falling back to
near normal values with a slightly lower diurnal range.


All eyes will be on Hurricane Maria through the upcoming wknd, which
at this time appears that it should stay well east of the Keys.
Please refer to latest National Hurricane Center products for more
information. Despite Maria forecast to remain well away from
Florida, increasing easterlies will advect tropical moisture
associated with the Hurricane, and the chance for showers and
thunderstorms will ramp up through Saturday. It is interesting that
although Maria may get closest Sunday, guidance showing
negative omega and a drying/strengthening inversion, suggests the
Keys will be within the subsident region surrounding Maria, and
have dropped POP back to below climo Sun/Mon after a more active
period. Temps will be near seasonable both for highs and lows
through the Keys.


Continued generally benign flow amid a weak pressure gradient will
keep winds light and variable into Thursday. Beginning on Thursday,
the gradient tightens allowing winds to take on a predominantly NE
direction and increase in speed, to 10-15 kts Friday, and possibly
as high as 20 kts Saturday, highest in the far eastern Straits of
Florida. Hurricane Maria is currently progged to pass east of the
Bahamas over the weekend, and as she lifts NE away from the area
winds will turn to the NW and ease into early next week.

Wave heights of generally 1-2 ft are expected through Friday, except
across the Straits of Florida where a continued long-period swell
sneaking through the Florida Channel will drive 2-4 ft seas, highest
east. As winds increase into the weekend significant seas will
likely rise an additional 1 ft all waters except within Florida Bay
and waters north of the Keys due to wave shadowing on the NE winds
off Southern Florida.


Today, VFR conditions will prevail at both the KEYW & KMTH Int`l
island terminals. Surface winds will average from 340-360 degrees
aoa 7-10 knots thru about 16Z, then backing to between 290-310
degrees. Expect a few clouds with bases aoa FL025-030.




Public/Marine/Fire...MD (MFL)
Aviation/Nowcasts....Futterman (KEY)

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