Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230836

Area Forecast Discussion
National Weather Service San Juan PR
436 AM AST Tue Jan 23 2018

.SYNOPSIS...Surface high pressure across the west and central Atlantic
will continue to spread eastward and strengthen over the next few
days. This will maintain moderate to fairly strong east to
northeast trade winds across the local area, with breezy
conditions expected by the end of the week and into the weekend.
Upper ridge will continue over the local islands for the next few
days with an amplifying polar trough still forecast to move
eastward across the Atlantic and slightly erode the upper ridge by
the weekend. A cold front and associated boundary is forecast to
stall and remain north of the local islands over the weekend and
into early next week.


.SHORT TERM...Terminal Doppler Radar indicated only few showers mostly
 across the Atlantic waters overnight. Not precipitation was observed
elsewhere across the region overnight and so far this morning. Easterly
trades will continue across the region as a weak high pressure system
remains across the Atlantic, just to the north of the region. This
weak high pressure is expected to be absorbed by another high pressure
across the northwest Atlantic, maintaining an east to east northeast
low level wind flow through Thursday. Small patches of low level moisture
embedded in the trades will move across the local islands from time
to time during the next couple of days producing brief passing showers
across the U.S. Virgin Islands and eastern Puerto Rico from time to
time, especially overnight and early in the mornings for the next couple
of days. During the afternoons, some cloudiness with showers are expected
to develop across the interior and west sections of Puerto Rico.

A cold front is expected to move into the western Atlantic tonight.
A strong surface high pressure behind the cold front will push the
frontal boundary southeastward across the Atlantic waters during
the next couple of days. By Thursday it is forecast to be about
400 miles north northwest of Puerto Rico. By Thursday the low
level wind flow will begin to turn northeast bringing cooler and
slightly drier airmass across the region.

.LONG TERM...Friday through Wednesday of next week. The upper level
high pressure ridge will hold across the region through Friday. However
expect the prevailing east to northeast winds to continue to transport
sufficient shallow moisture across the region to support passing
late evening and early morning showers, followed by the development
of isolated to scattered afternoon convection. Saturday through Sunday,
a polar trough is forecast to amplify and spread eastward across the
west and central Atlantic causing a slight erosion of the upper level
ridge. This will provide better support for afternoon convection across
portions of the islands. By Monday through Wednesday, the mid to upper
level ridge will reestablish over the region and again limit shower

In the low levels, the a cold front is forecast to move across the
west and central Atlantic and stall well north of the local area
by the weekend. However, model guidance suggests a gradual increase
in the trade winds by late Friday and into the weekend with conditions
more favorable for moisture convergence across the islands. As a result,
cooler temperatures but more frequent shower activity is expected due
to the increasing northeast winds and frontal boundary moisture which
will sinks southwards and be transported across the region from time
to time. Gradually improving weather conditions are forecast Monday
through Wednesday with a return of more typical passing trade wind
showers and isolated to scattered afternoon showers mainly over the
west and interior sections of Puerto Rico.


.AVIATION...Mainly VFR conditions are expected across the local
flying area through the forecast period. However, -SHRA on trades
moving across the Leeward islands could move briefly across the USVI
and eastern Puerto Rico terminals this morning.
Thereafter, SHRA will begin to affect mostly TJMZ between 23/18Z and
23/21Z. Latest 23/00z TJSJ upper air sounding indicated ENE winds at
11-24 kt from SFC to FL100.


.MARINE...The northerly swell has subsided across the regional
waters and seas no longer meet small craft advisory criteria. However,
small crafts should continue to exercise caution across both Atlantic
and Caribbean waters and passages due to seas up to 6 feet and/or winds
up to 20 kt. High rip current risk will continue on Tuesday mainly
for the northwestern coast of Puerto Rico. Moderate risk continues
across the rest of the coastal areas of the islands.


SJU  85  75  84  74 /  40  50  50  30
STT  83  75  84  74 /  40  50  50  20


PR...High Rip Current Risk through this afternoon for Northwest-
     Western Interior.



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