Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 190132
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
932 PM AST Thu Jan 18 2018

.UPDATE...No change to previous forecast discussion and inherited
short term grids at this time. While most of the shower activity
diminished over the interior sections of Puerto Rico, a few showers
lingered along the northwest coastal areas and just offshore over
the Mona passage. Most of the showers however were of light to
moderate intensity. Expect occasional passing showers to continue
to affect parts of the north and east coastal sections of some of
the islands overnight as the moderate easterly trades will continue
to transport occasional patches of moisture across the region. Otherwise
no change to the forecast for the next few days based on recent
model guidance and the present weather pattern.

&&

.AVIATION UPDATE...No change to previous AVN discussion. Prevailing
VFR conds durg entire fcst prd. Passing low level clds...nr FL025...FL050
with few SHRA enroute btw islands and ovr coastal waters. No sig wx
impacts at local terminals attm.

&&

.MARINE UPDATE...No change to previous marine discussion. Small craft
should continue to exercise caution over the offshore Atlantic and
the Anegada passage. A long period northwest to north swell is still
forecast to arrive across the Atlantic waters and local passages during
the weekend. This will result in hazardous seas and life threatening
rip currents across the Atlantic facing beaches of the islands.

&&

.PREV DISCUSSION... /issued 514 PM AST Thu Jan 18 2018/

SYNOPSIS...
Once again, a mid to upper level ridge pattern induced a trade
wind inversion, which limited the vertical development of showers
across the islands. Local weather will be driven by low level
moisture advection from the surrounding waters. As a result,
expect passing showers across the U.S. Virgin Islands and the
eastern half portion of Puerto Rico during the overnight and
morning hours. Then, showers are forecast to move across the
interior and western sections of Puerto Rico during the afternoon
hours.

SHORT TERM...the rest of today through Saturday... Satellite
imagery and the terminal Doppler detected clouds and showers
across the interior sections of Puerto Rico and downwind from the
U.S. Virgin Islands. This activity will continue through the
evening hours. Under a easterly wind flow expect showers across
the surrounding waters of the USVI and some of them will move
inland from time to time. Also, passing showers are expected
across the eastern half portion of Puerto Rico, especially
downwind from El Yunque into the surrounding municipalities. The
maximum rainfall accumulations are forecast across the interior
and western sections of Puerto Rico where rainfall amounts will
range between a quarter and half an inch.

Although much of the islands will observe mostly clear skies with
no rainfall activity, an easterly wind flow will advect clouds and
showers across the Atlantic and Caribbean Waters as well as across
the U.S. Virgin Islands and the eastern half portion of Puerto
Rico overnight and tomorrow morning. This weather pattern is
forecast to continue through the weekend. However, breezy trade
winds are expected by the upcoming weekend as high pressure ridge
is forecast to build north of the region and increase the local
pressure gradient. Which will increase the frequency of the
passing showers across the local islands.

LONG TERM...Sunday through Friday...
Model guidance suggest a seasonal weather pattern for Puerto Rico
and the U.S. Virgin Islands, as a mid to upper level ridge holds
through at least Monday. Then, as a short wave trough moves north
of the islands the upper level ridge is forecast to erodes.
Although no significant upper level dynamic will be in place,
low level moisture will induce shower development across the
windward sections each night and morning. Also, the available
moisture will combine with daytime heating, orographic effects and
sea breeze variations to produce showers over some locations of
the interior and western PR each afternoon.

AVIATION...
Mainly VFR conditions are expected to prevail across all
terminals. However, shra over western PR could impact the flying
area of TJMZ/TJBQ through at least 22z. Elsewhere, light trade wind
showers could move at times across the vcty of the sites. Low level
winds will continue from the east at 5 to 15 kt. Light and variable
winds are expected overnight due to land breeze variations.

MARINE...Local seas are diminishing, mariners can expect seas
between 3 and 5 feet across the local waters, except across the
Atlantic and Anegada Passage where seas can reach up to 6 feet.

A long period northwest to north swell will reach the Atlantic
waters and local passages during the weekend. This will result in
hazardous seas and life threatening rip currents across the
Atlantic beaches of the islands, specially by Saturday afternoon
and on Sunday.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  75  84  74  85 /  30  30  30  40
STT  74  84  74  84 /  30  20  30  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....RAM


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