Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 200132
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
932 PM AST Fri Jan 19 2018

.UPDATE...Shower activity over Puerto Rico quickly diminished over
land areas by early evening, leaving partly cloudy to variably cloudy
skies and a few lingering showers, mainly over the coastal waters
and the northeast coastal sections of Puerto Rico. Expect passing
cloud and occasional showers to affect parts of the islands overnight,
most of which should be of light to moderate intensity and of short
duration. The overall weather pattern is unchanged for most of the
weekend except for the increasing east to northeast trade wind flow
at high pressure ridge builds north of the region and spread into
the central Atlantic. In the meantime shallow low level moisture
transport and occasional showers can be expected through the weekend
but no significant rainfall accumulations is forecast at this time.
No change to previous forecast package and reasoning at this time.

&&

.AVIATION UPDATE...Prevailing VFR conds at all terminals durg entire
prd. SCT- BKN cld lyrs nr FL025...FL050..FL090 w/passing SHRA across
local waters and en route btw islands. Wnd fm E-NE 10-15 kts blo FL200.

&&

.MARINE UPDATE...No change during the overnight periods, However,
moderate to locally fresh trade winds are expected during the weekend
as the local pressure gradient tightens across the region and increases
the trade wind flow. A long period northwest to north swell will arrive
and spread across the Atlantic waters and local passages by Saturday.
Therefore Hazardous seas can be expected through most of the weekend.

&&

.PREV DISCUSSION... /issued 437 PM AST Fri Jan 19 2018/

SYNOPSIS...
A surface high pressure will drift into the western Atlantic
through the weekend, tightening the local pressure gradient. As a
result breezy trade winds will continue to bring clouds and
showers across the islands from time to time.

SHORT TERM...Tonight through Monday...
Showers continue over eastern Puerto Rico from Gurabo to Bayamon
and within 5 miles of the airport TJSJ. Radar shows some amounts
have exceeded one half inch over San Juan and Guaynabo. Flash
flooding along roads and urban areas were reported across these
municipalities. Other cells developed from Orocovis to Rincon and
the area of convection is expected to diminish around sunset.

Trade winds dominate the synoptic picture at this time. High
pressure from the Alabama coast eastward into the western Atlantic
will continue north of the area through the period and trade
winds will increase somewhat during the period. At 700 mb a high
pressure just north of the area will drag some of the moisture
associated with the front about 200 miles north into the area, but
it will move west and out of the area by Saturday morning. This
will lead to decreasing showers late Saturday and Sunday.
Moisture begins to return on Monday and eastern slopes of Puerto
Rico will be most affected most with scattered showers in the U.S.
Virgin Islands. Upper levels will continue to be dominated by the
ridge that continues to be reinforced in the westerly flow. This
will keep it nearby first to the east then to the west and back
again.

LONG TERM...Monday through Saturday...
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...
Areas of mtn obscurations may be seen across western PR,
but otherwise VFR conds are expected to prevail. Any MVFR conds in
the Leeward islands should disappear before 19/18Z. Mtn obscurations
in the Luquillo range should end by 20/04Z.  Sct SHRA have developed
across wrn PR but end by 20/00Z. Sfc winds east 10-15 kt. Maximum
winds genly 30-40kt btwn FL380-500, but 19/12Z sounding showed 50 kt
at FL450.

MARINE...
Mariner can expect seas between 4 and 6 feet across the Atlantic
and Mona Passage overnight. Elsewhere, seas at 5 feet or less are
expected. The winds will be from the east at 10 to 15 knots,
except across the Caribbean and Mona Passage where are expected
between 15 and 20 knots.

A long dominant period northwest to north swell will reach the
Atlantic waters and local passages by Saturday. As a result, small
craft advisories will be in effect across most of the local
waters.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  73  85  74  85 /  30  30  20  20
STT  74  84  74  85 /  30  40  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from 6 AM AST Saturday through late
     Saturday night for Culebra-Mayaguez and Vicinity-North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast-
     Western Interior.

VI...High Rip Current Risk from 6 AM AST Saturday through late
     Saturday night for St.Thomas...St. John...and Adjacent
     Islands.

     High Rip Current Risk from Saturday evening through late
     Saturday night for St Croix.

     Small Craft Advisory from 6 PM Saturday to 2 PM AST Sunday for
     St.Thomas...St. John...and Adjacent Islands.

AM...High Rip Current Risk from 6 AM AST Saturday through late
     Saturday night 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 from 6 PM Saturday to 2 PM AST Sunday for
     Anegada Passage Southward to 17N-Coastal Waters OF
     Northwestern Puerto Rico out 10 NM-Coastal Waters of
     Northern USVI and Culebra out 10 NM-Mona Passage Southward
     to 17N.

     Small Craft Advisory from 8 AM Saturday to 2 PM AST Sunday for
     Coastal Waters of Northern Puerto Rico out 10 NM.

     Small Craft Advisory from 8 AM Saturday to 8 AM AST Monday for
     Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N.

&&

$$

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



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