Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 270827

Area Forecast Discussion
National Weather Service San Juan PR
427 AM AST Tue Oct 27 2020

An easterly disturbance will continue to cross the northern
Leeward islands and move across the region today. This will result
in an increase in low level moisture across the forecast area and
consequently more frequent passing early morning showers which
will be enhanced by the upper trough/Tutt located just east of the
area. By mid-week, conditions are expected to become even more
favorable for showers and thunderstorm development due to the
repositioning of the upper level trough and associated area of low
pressure as well as an approaching tropical wave. This fairly
moist and unstable weather pattern will result in unsettled
weather conditions which is so far forecast to linger through the
end of the workweek.


.SHORT TERM...Today through Thursday...

The upper level atmospheric pattern will play an important role in
our weather for the next few days. During the past 48 hours, an
upper level low has developed to our east, just east of the Leeward
Islands. This low helped to induce and enhance a low level
disturbance with high moisture content that is currently impacting
the region. With precipitable water values around 2 inches, this
disturbance is producing scattered showers and isolated
thunderstorms across the local waters, the eastern Puerto Rico
coastal areas, and the USVI. By this afternoon, this moisture will
help to kick of showers and isolated thunderstorms across western
Puerto Rico and possibly the San Juan Metro area as well. With the
upper level low still to our east by this afternoon, it is not in a
favorable location to enhance thunderstorm activity, but some
lightning is possible this afternoon with near average lapse rates
and 500mb temperatures around -6C. With strong high surface pressure
over the central Atlantic, vigorous low-level winds will help
showers to move quickly and reduce their impacts. Still, with ample
moisture in place, there will still be a threat for isolated areas
of flooding, most likely in western Puerto Rico this afternoon for
the municipalities around Anasco. Finally, for the more vigorous
showers that form, gusty winds at around 30 mph are possible.

Over the past several days, a tropical wave has been moving across
the Atlantic towards our region, almost catching up to the
aforementioned low-level disturbance. By late tonight and tomorrow
morning, there will only be a small break between the disturbance
and the tropical wave. Thus the overnight hours tonight will see
showers exiting to our west as the disturbance moves out of our
region, followed by brief tranquil conditions with only isolated
showers before the tropical wave arrives late in the day tomorrow.
At this point, it appears the moisture from the wave will move to
our north Wednesday morning and not move over the USVI and
especially PR until later Wednesday night. Still, there will be
sufficient moisture to produce showers over southwestern Puerto Rico
Wednesday afternoon. Furthermore, the aforementioned upper-level low
will be just to our north by Wednesday afternoon, and in a better
position to enhance thunderstorm activity across the region. As the
upper-level low moves to our west overnight, and better moisture
moves in from the tropical wave, scattered showers and isolated
thunderstorms are expected overnight and through Thursday. Gusty
winds around 30 mph can again be expected with these showers. Decent
rainfall amounts can be expected for eastern Puerto Rico overnight
Wednesday and Thursday during the day. Though northwestern Puerto
Rico can also expect showers Thursday afternoon, cloud cover and
vigorous low-level winds may limit the impacts of these showers.

.LONG TERM...Friday through Wednesday...

Friday is still forecast to remain fairly active and unstable due to
the proximity of an upper trough just west of the area and moisture
trailing a tropical wave. These conditions will favor good potential
for enhanced showers and thunderstorm development over eastern
Puerto Rico and the USVI during the overnight and early morning
hours, then over portions of the central interior and northwest
sections of Puerto Rico during the afternoon. Meanwhile activity
over the U.S. Virgin Islands should gradually taper off during the
day with lesser chance afternoon convection expected. Winds will
become more from the ESE, which will steer the shower activity
over the northwestern sections of the islands during the afternoon
hours. Overnight and early morning passing showers will remain
possible over the coastal waters through Saturday with a gradual
drying trend by Saturday afternoon into Sunday. Expect continued
improvement in the overall weather pattern for the rest of the
weekend as a high pressure ridge will build across the region and
low level moisture transport erodes as a drier airmass filters in
and lingers across the region at least until Tuesday. By
Wednesday, another surge in trade wind moisture is so far forecast
as the tradewinds increase and a weak easterly disturbance
approaches the forecast area. By then expect a better chance for
overnight and early morning passing showers, followed by afternoon
convection across the islands and coastal waters.


Mostly VFR conds expected at all terminals. However, fast moving
scattered showers could produce SHRA and VCTS for
TISX/TIST/TJSJ/TNCM/TKPK during the period, plus TJMZ/TJBQ in
afternoon. SHRA will have gusty winds around 25 to 30 kts.
Otherwise, winds easterly around 10 kts through 14z, then 10 to 20
kts from the E to ESE through 22z with sea breeze variations, then
around 10 kts after 22z.


.MARINE...Confused seas will continue to produce hazardous marine and
surf zone conditions across portions of the local island through
much of the forecast period. Mariners can expect seas between 5 to
8 and winds around 20 knots with higher gusts. Therefore Small
craft advisories are in effect for much of the local waters. The
best marine conditions or the safest areas are along the southern
beaches and coasts of the islands. However, mariners should
continue to exercise caution due to winds around 20 kts and
occasionally higher gusts.


SJU  90  79  86  77 /  60  30  50  60
STT  88  78  87  79 /  60  40  50  60


PR...High Rip Current Risk through Wednesday afternoon for Culebra-
     North Central-Northeast-Northwest-San Juan and Vicinity-

     High Rip Current Risk through this afternoon for Mayaguez and

VI...High Rip Current Risk through Wednesday afternoon for St Croix.

     High Rip Current Risk through this afternoon for St.Thomas...St.
     John...and Adjacent Islands.

AM...Small Craft Advisory until 8 AM AST Wednesday for Caribbean
     Waters of Puerto Rico from 10 NM to 17N.

     Small Craft Advisory until 8 PM AST this evening for Coastal
     Waters of Northern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 PM AST Wednesday for Anegada
     Passage Southward to 17N-Atlantic Waters of Puerto Rico and
     USVI from 10 NM to 19.5N.

     Small Craft Advisory until 2 AM AST Wednesday for Coastal Waters
     of Northern USVI and Culebra out 10 NM-Coastal Waters of
     Southern USVI Vieques and Eastern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 AM AST this morning for Coastal
     Waters of Northwestern Puerto Rico out 10 NM-Mona Passage
     Southward to 17N.



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