Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 202111
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
511 PM AST Sat Jan 20 2018

.SYNOPSIS...Frontal boundary will remain north of the region. Mid
to upper level ridge will continue to build and hold for the next
several days. Surface ridge to the northwest of the region will
merge with another surface high by early next week and continue
through the end of the week. Moderate to fresh trades will
continue to carry passing showers across the region through next
week.

&&

.SHORT TERM...Tonight through Tuesday...
The cold front continues about 240 miles north of San Juan, but is
now denoted by a rope cloud more than a solid deck of clouds. As of
20/1730Z showers were just beginning to form under partly cloudy
skies over the interior portion of Puerto Rico. Early indications
suggest less rain coverage than yesterday. The front is expected to
undergo frontolysis as it lays more east/west over the waters to the
north of us. High pressure at the surface is moving east out of the
western Atlantic and the southeastern U.S. and is reinforcing the
trade wind flow now through Sunday. This high pressure will merge
with a high in the northwest Atlantic Tuesday before moving east
into the northeast Atlantic, but trade wind flow will persist just
the same. Patches of moisture will continue to pass in that flow,
but GFS is showing less pronounced peaks to the precipitable water
than in previous forecasts and more general fluctuations. The most
pronounced is from 1.04 inches at 21/12Z to 1.61 inches 23/12Z.  All
fluctuations between Tuesday and the following Monday remain within
these bounds. Scattered trade wind showers will continue--minimally
tonight and Sunday, but increasing somewhat Monday and Tuesday.

.LONG TERM...Tuesday through Sunday...From previous
discussion...Low level moisture will periodically pass over
Puerto Rico and the U.S. Virgin Islands during the long term
period. Several short wave troughs will move north of the region
with little to no effect on the area due to the troughs energy
remaining north of Puerto Rico. Therefore, the long term weather
will be influenced by the mid to upper level ridge and low level
moisture moving in from the east. The ridge of high pressure will
feel the effects of the the troughs and become elongated from east
to west. The weakening high will allow low level moisture to
sneak in from the south which will induce showers across the
windward islands and eastern Puerto Rico during the night and
morning hours. Also, low level moisture combined with daytime
heating along with orographic effects and sea breeze variations
will produce showers over portions of the interior and western PR
during the afternoon.

&&

.AVIATION...Limited mtn obscurations over the highest trrn,
otherwise wrn PR btwn 20/18-22Z. Sfc winds easterly 10 to 15 kt with
sea breeze influences til 20/21Z. Maximum winds 30-35 kt from FL410-
480 diminishing durg the next 24 hrs.

&&

.MARINE...Long period northwest swell will continue to affect the
regional waters through the weekend with seas ranging between 6-8
feet. Moderate to locally fresh easterly trades will continue
through the next several days. Hazardous seas will continue
through early next week. Small craft advisories are in effect for
the Atlantic waters and passages as well across portions of the
offshore Caribbean waters. High surf advisory is in effect for the
Atlantic beaches of Puerto Rico. High rip current risk continues
in effect for the northern beaches of the islands.


&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  74  85  75  85 /  30  40  50  50
STT  76  85  76  79 /  20  20  40  30

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Sunday night for Culebra-
     Mayaguez and Vicinity-North Central-Northeast-Northwest-San
     Juan and Vicinity-Southeast-Western Interior.

     High Surf Advisory until 6 PM AST Sunday for Mayaguez and
     Vicinity-North Central-Northeast-Northwest-San Juan and
     Vicinity-Southeast-Western Interior.

     High Rip Current Risk through late Sunday night for Vieques.


VI...High Rip Current Risk through late Sunday night for
     St.Thomas...St. John...and Adjacent Islands.

     High Rip Current Risk through late Sunday night for St Croix.


AM...Small Craft Advisory from 2 AM to 2 PM AST Sunday for
Caribbean Waters of Puerto Rico from 10 NM to 17N.

     Small Craft Advisory until 2 PM AST Sunday for Coastal Waters of
     Northern USVI and Culebra out 10 NM.

     Small Craft Advisory until 2 AM AST Monday for Coastal Waters of
     Northern Puerto Rico out 10 NM.

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

     Small Craft Advisory from 2 AM Sunday to 2 PM AST Monday for
     Anegada Passage Southward to 17N.

     Small Craft Advisory until 2 PM AST Monday for Mona Passage
     Southward to 17N.

     Small Craft Advisory until 2 PM AST Sunday for Coastal Waters OF
     Northwestern Puerto Rico out 10 NM.

&&

$$

SHORT TERM...JA
LONG TERM....DS



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