Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 271248

Area Forecast Discussion
National Weather Service San Juan PR
848 AM AST Sat Nov 27 2021

.UPDATE...Mostly sunny to partly cloudy skies prevailed across the
islands during the early morning hours, and are expected to
continue through the rest of the day. Shallow moisture content and
cloudiness from an old front, will gradually move over portions
of the U.S. Virgin Islands and eastern sections of PR today. Based
on the latest 27/12z TJSJ upper air sounding, there is a slight
bump in PWAT content from 1.11 inches last night to 1.43 inches
this morning. However, the 500 mb temp continues to warm (-3.6C)
and plenty of dry air remains above 750 mb in response to the
building ridge aloft. Having said that, the islands should see a
small increase in low-level cloudiness and showers, but mostly
minor rainfall amounts are anticipated. The main threat today and
through the rest of the weekend will be the hazardous marine and
coastal conditions, refer to marine section below.


.AVIATION...Mainly VFR conditions are expected to prevail across
all terminals during the next 24 hours. SCT/BKN cigs btw FL025-060
possible across the USVI terminals thru 18z. The 12z TJSJ sounding
indicated N-NE winds up to 17 kt blo 5000 ft, sea breeze
variations are expected across south-southwest PR.


.MARINE...The offshore Atlantic buoy 41043, about 170 NM north-
northeast of San Juan indicated swell heights between 7-8 feet at
11-13 seconds through the overnight/early morning hours. Taking
into consideration the northeasterly direction and the swell decay,
seas around 7 feet are likely across the local Atlantic waters
and portions of the local passages through at least this evening
or early tonight. Therefore, the small craft advisories were
extended accordingly. Please refer to the latest Coastal Waters
Forecast (CWFSJU) and Marine Weather Message for more information.

In addition, the lingering long-period swell continues to impact
the Atlantic coastline of the islands, as well as some of the
east facing beaches of the islands. Life-threatening rip currents
will continue across most of the north and east facing beaches of
the islands through the rest of the weekend and into next week.
There is a High Surf Advisory in effect from the northwest to
northeast coast of PR, including Culebra, and a High Rip Current
Risk for the same areas, and across the western beaches of PR,
Vieques and all the USVI. Please refer to the latest Coastal
Hazard Message (CFWSJU) and Surf Zone Forecast (SRFSJU) for
detailed information.


.PREV DISCUSSION... /issued 501 AM AST Sat Nov 27 2021/

Recent satelite imagery and the doppler weather radar suggest that
a much drier and stable weather pattern continued to spread across
the region during the overnight and early morning hours. Little
or no shower activity was noted over land areas with mostly light
showers over the coastal waters. This upper level ridge will maintain
its hold and remain the dominant feature through the weekend with
overall limited showers and pleasant weather expected. A slight
increase in trade wind moisture forecast by Monday and Tuesday as
winds become more east southeast and remnants of the old frontal
boundary returns to the area. However so far upper ridge is to
remain sufficiently strong to limit and inhibit significant convective

SHORT TERM...Today through Monday...

GOES-16 water vapor imagery shows strong subsidence above with the
frontal boundary southeast of the forecast area. A band of moisture
is producing cloudy conditions and some showers but it has stayed
mainly east of the U.S. Virgin Islands throughout the night.
Precipitable Water imagery at 0550Z continues to indicate values
below 1.0 inches over eastern half of Puerto Rico, which is more
than two standard deviations below normal for this time of year,
with up to 1.20 inches elsewhere putting values still below normal.
This has produced fair weather conditions across Puerto Rico and
U.S. Virgin Islands and is forecast to persist at least through
early afternoon as a mid-level ridge builds over the area. This
ridge will strengthen and remain throughout the short-term period.

Model guidance suggests a gradual increase of low-level moisture by
this evening as fragments from the frontal boundary get advected
across the islands. Precipitable water is anticipated to rise
slightly. However, shower activity will still be hindered with drier
air and stable weather conditions aloft. A few isolated showers
across windward coastal areas could be possible tonight and early
morning hours.

Winds will become light and shift to more east on Sunday and later
southeast by Monday. This will push the remnants of the frontal
boundary back across the forecast area. Low-level moisture is
expected to further increase with Monday peaking at near-normal to
normal values. The mid-level ridge will continue to inhibit convective
development. However, a slight increase in cloud coverage with isolated
to scattered showers at times could be expected as the remnants of
the front interact with the islands. No significant rainfall
accumulations is expected with any shower activity in the next
few days.

LONG TERM...Tuesday through Sunday
The mid to upper level ridge slightly erodes as a short wave
rough crosses the west Atlantic. This  will aid in weakening the
tradewind cap inversion and allow for an increase in moisture
transport as a weak easterly perturbation will move across the
area with winds remaining east southeast through Tuesday. Any
afternoon shower development should therefore be in the west and
northwest sections of Puerto Rico and mainly on the downwind side
of the U.S. Virgin islands or just offshore over the coastal
waters. By Wednesday and through the end of the work week, winds
will again become east to northeast and slightly increase as as
surface high builds north of the region.  This will favor better
change for quick low level cloud fragments and passing early morning
showers along portions of the north and east coastal areas. However
no significant rainfall accumulations area anticipated. By late Friday
and into the weekend an upper level short wave trough will move north
of the region to further erode the upper ridge while inducing a low
level surface trough, in advance of a cold front which will enter
the western Atlantic and should stall well north of the region. As
a result light winds and the slight increase in low level moisture
and instability will provide a better chance for afternoon shower
development over part of the interior and west sections of Puerto
Rico ,and mainly on the west-end of the U.S. Virgin Islands. For
the most part however, no significant rainfall or weather feature
feature is anticipated at this time as a more seasonal pattern
should prevail.

AVIATION....Mainly VFR conditions are expected to prevail across all
the terminals during the next 24 hours. Surface winds from the ENE
at 10-15 kts with higher gusts at times, with southerly sea breeze
at TJPS aft 14z.

Based on recent data from the surrounding Atlantic buoys, hazardous
marine and surf zone conditions are expected to continue at least
through later this afternoon due to a northerly swell and the gentle
to moderate northeast to east trade winds. Small craft advisories,
High surf advisories and high rip current risk continue for portions
of the regional waters and local islands respectively. Please refer
to the latest Coastal waters forecast (CWFSJU), Marine weather message
(MWWSJU) and the Coastal Hazard message (CFWSJU) issued by the NWS WFO
San Juan for more details and updates about these ongoing marine
and coastal conditions.


SJU  86  75  84  74 /  20  40  40  40
STT  86  75  85  76 /  20  30  30  50


PR...High Rip Current Risk through Tuesday afternoon for Culebra-
     Mayaguez and Vicinity-North Central-Northeast-Northwest-San
     Juan and Vicinity-Southeast-Vieques.

     High Surf Advisory until 6 AM AST Sunday for Culebra-Mayaguez
     and Vicinity-North Central-Northeast-Northwest-San Juan and

     High Rip Current Risk through late tonight for Southwest.

VI...High Rip Current Risk through Tuesday afternoon for St Croix-
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 8 PM AST this evening for Coastal
     Waters of Northern Puerto Rico out 10 NM-Coastal Waters of
     Northern USVI and Culebra out 10 NM-Coastal Waters of
     Northwestern Puerto Rico out 10 NM-Mona Passage Southward
     to 17N.

     Small Craft Advisory until 2 AM AST Sunday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

     Small Craft Advisory until 8 AM AST this morning for Anegada
     Passage Southward to 17N.



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