Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 131354

Area Forecast Discussion
National Weather Service San Juan PR
954 AM AST Fri Dec 13 2019

Only minor changes were made to the inherited weather grid to
reflect the latest forecast. Passing showers will continue to
affect portions of the eastern half of Puerto Rico and the U.S.
Virgin Islands through the rest of the day and early into the
weekend. The TJSJ 13/12Z showed strong low level winds across the
area. Winds should decrease this weekend, coming out of the east
at 10 to 15 mph. Over the waters, hazardous marine conditions will
continue with seas up to 8 feet.


Mainly VFR conditions will continue across the local terminals.
Passing showers will continue to affect the area through the
forecast period. The latest TJSJ 13/12Z sounding shows FL050 winds
out of the east at 13 to 34 knots.


Hazardous marine conditions continue across most of the local
waters, except the coastal waters of southern and southwestern
Puerto Rico. Seas up to 8 feet and winds up to 20 knots are
expected to continue. Small craft advisories are in effect. For
the beachgoers, there is a high rip current risk for the beaches
along the northern coast of Puerto Rico, Culebra, and St. Thomas
and St. Croix in the U.S. Virgin Islands.


.PREV DISCUSSION... /issued 441 AM AST Fri Dec 13 2019/


Passing showers will continue to affect the area for the next few
days, as well as afternoon shower activity due to local and
diurnal effects. Winds are expected to subside somewhat as the
high to our north pulls away, but with another ridge making its
way into the Atlantic to take its place, this will be short-lived,
and the enhanced winds - and therefore waves - will return early
next week. The start of the work week looks slightly drier, but
sufficient moisture is likely to maintain some shower activity.

SHORT TERM...Today through Sunday...

An induced surface trough is forecast to move across the region
today; this will increase shower activity across the U.S. Virgin
Islands and the eastern sections of Puerto Rico during the morning
hours. As the trough cross the region from east to west, it will
enhance the diurnally induced afternoon showers over western
Puerto Rico, and streamer type activity off the USVI and from el
Yunque area into the San Juan metro area. As the trough exits the
region, easterly trades will turn more from the east-southeast.
Surface winds are expected to slightly decrease during the
weekend, from 15- 20 kts today to 10-15 kt by Sunday.

For the weekend, wind flow turns more from the southeast as a
surface high moves from the northern Atlantic into the central
Atlantic and a surface low moves over the northeastern coast of
USA, while its associated front remains over the western
Atlantic. At upper levels, a ridge building from the west will
provide some drier air and stability aloft. The 500mb temps are
forecast to increase from -8 degrees C today to -5C by Sunday.
This should inhibit thunderstorm development, but not the
development of moderate to locally heavy rain showers, as
tropical moisture is forecast to increase across the Caribbean Sea
during the weekend. Therefore, under a southeast steering wind
flow expect showers to reach portions of southern and southeastern
PR during the night and early morning hours, followed by
diurnally induced showers over the NW quadrant of PR and over the
San Juan area each afternoon.

LONG TERM...Monday through Saturday...

A surface ridge stretched across the central Atlantic will continue
to be the dominant feature on Monday. As it weakens and pulls away
to the northeast, a ridge stretching from James Bay in Canada to the
southeastern United States will build into the Atlantic - a process
that is expected to repeat near the end of the week next week.
Because of this, a zonal ridge is expected to be the major feature
to our north through the forecast period. This will aid in the
persistence of moderate, generally easterly trade winds, which
are forecast to weaken somewhat later in the week as the ridge
weakens its hold over the Atlantic somewhat. At the mid-levels, a
ridge is expected to dominate over the area through the week next
week; a trough to our north will weaken this ridge slightly later
in the week. Aloft, the TUTT will be located to the east of the
local islands on Monday. These conditions will likely act to
inhibit upward motion over the area. Even so, passing showers are
still expected, as well as afternoon showers due to local effects
and diurnal heating, especially in western Puerto Rico.

A weak disturbance is forecast to push westward into the area on
Tuesday, associated with deep, enhanced moisture. An upper level
trough will make its way eastward at the same time, stretching over
Hispaniola by Tuesday afternoon. Because of this, increased shower
activity is likely. As the upper level trough passes over and past
us on Wednesday, a dry slot is expected to push over the area.
Though the typical pattern of passing showers during the overnight
and morning hours, and afternoon showers due to local effects and
diurnal heating will still persist over the area, activity will be
lessened. As we move into the latter half of the week, patches of
moisture will push into the area, carried in on the trade winds.
Associated with this moisture is the potential for increased shower
activity over the area.


Mainly VFR conditions are expected. However, trade wind
showers will move at times across the USVI/Eastern PR terminals
through the morning hours, causing brief periods of -SHRA/MVFR cigs.
SHRA this afternoon developing over west PR could cause tempo MVFR
conds at TJMZ/TJBQ. East winds will increase between 15-20 kts with
higher gusts along coastal areas after 14z.


Enhanced easterly trade winds associated with a strong Atlantic high
pressure system continue to blow across the region at around 15-20
knots. Associated with these winds are waves of roughly 5 to 9
feet, with occasional seas of up to 11 feet. Continuing hazardous
marine conditions will persist this weekend, and Small Craft
Advisories are in effect for all zones of the local waters, with
the exception of nearshore waters of southern/southwestern Puerto
Rico. At the beaches, there is a high risk of rip current along
northern beaches of Puerto Rico and Culebra, and a moderate to
high risk of rip current at beaches of the U.S. Virgin Islands.
Though winds and seas are forecast to subside briefly, as the
current ridge moves off to the east, another high will push in
over the central Atlantic early next week, heralding a return to
the increased winds and waves across the region.


SJU  83  73  87  76 /  50  40  10  40
STT  85  76  85  76 /  40  20  30  40


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

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

     High Rip Current Risk until 6 PM AST this evening for
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 6 AM AST Monday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N-Caribbean Waters of Puerto Rico from 10
     NM to 17N-Coastal Waters of Northern Puerto Rico out 10 NM-
     Coastal Waters of Northern USVI and Culebra out 10 NM-
     Coastal Waters of Southern USVI Vieques and Eastern Puerto
     Rico out 10 NM-Mona Passage Southward to 17N.

     Small Craft Advisory until 1 PM AST Sunday for Coastal Waters OF
     Northwestern Puerto Rico out 10 NM.



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