Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 020818

Area Forecast Discussion
National Weather Service San Juan PR
418 AM AST Sat Dec 2 2023


A surface high pressure over the Central Atlantic will continue
to promote moderate to fresh easterly winds today. An increased
moisture from an old frontal boundary will move in late today,
increasing the potential for shower activity across the region.
Hazardous marine conditions will persist for the rest of the
weekend, with seas up to 7 feet. Regardless of the stable weather
conditions and periods of no rain with clear skies, fragmented
moisture will result in occasional periods of clouds and showers.


.SHORT TERM...Today through Monday...

Mostly clear skies with little to no rain prevailed across the
U.S. Virgin Islands and Puerto Rico overnight. The thermometers
reported the coastal low temperatures in the mid-70s degrees
Fahrenheit and around the low 60s near mountains and valleys.
Surface winds were mainly from the east-northeast at 10 mph or

An advective pattern will dominate the short-term period associated
with the surface high pressure across the Atlantic Ocean. Although
we expect plenty of periods of sunshine, the trade winds will bring
occasional passing showers, mainly across the windward sections in
Puerto Rico and the Virgin Islands. Limited afternoon convection
could occur along the Cordillera Central and over the southwest
quadrant today. The surface high pressure will push the remnants of
an old frontal boundary across the islands, increasing the frequency
of the passing showers late this afternoon into the evening and
early Sunday morning.

A tropical upper tropospheric trough (TUTT) will amplify from the
Central Atlantic into the eastern Lesser Antilles, pushing the mid-
high pressure eastward into the Western Caribbean and weakening the
upper-level ridge. Meanwhile, at the surface, the local effects and
sea breeze variations will promote the formation of afternoon
convection across the interior and southwest PR on Sunday, giving
place to ponding in periods of moderate to heavy rain.

The TUTT mentioned above will induce surface perturbations that
winds will advect into the region by Monday, creating the typical
December advective weather pattern across the U.S. Virgin Islands
and Puerto Rico. The amount of rain will be tied to the arrival of
patches embedded in the winds that may move inland. We forecast that
temperatures will range around the December-typical values for our


.LONG TERM...Tuesday through Saturday...

A seasonal weather pattern will persist for most of the long
term. Model guidance continues to suggest the development of a
Tropospheric Upper-Level Trough just east of the Leeward Islands
by the beginning of the upcoming workweek. Although the islands
will remain on the subsidence side of the TUTT, a weak induced
surface trough from the TUTT above will result in trapped moisture
at 800 mb. The shallow moisture embedded in the east-
southeasterly winds and the local effects will be enough to
increase the frequency of the passing showers across windward
sections on Tuesday into Thursday. By late Wednesday into
Thursday, the high surface pressure over the central Atlantic will
be the main feature, resulting in east-southeast wind, bringing
pulses of moisture. Global model guidance suggests a surface wind
variation due to the arrival of a pre-frontal trough moving
southward into the Caribbean for the last part of the long-term


VFR conditions will prevail today. Expect calm to light and variable
winds til 02/13z. Then, winds will be mainly from the E-ENE at 10-15
kt with stronger gusts mainly due to sea breeze variations. The wind
flow sometimes brings passing showers and may produce localized
brief lower visibilities. Clouds and mountain obscuration could
develop along the interior and southwest PR by the afternoon. The
frequency of SHRA will increase near JSJ/IST/ISX after 02/21z.



A broad surface high pressure building over the western Atlantic
will promote moderate to fresh easterly winds across the regional
waters. The combination of a northerly swell and the increasing
winds and northerly swells will maintain choppy to hazardous seas
across the offshore waters and local passages through at least
Sunday. Therefore, a Small Craft Advisory is in effect for the
offshore Atlantic waters from 8 AM AST today though at least 8 AM
on Sunday. Conditions will remain slightly hazardous on Monday
morning. For beachgoers, there is a high risk of rip currents
across the northern sections of Puerto Rico and Culebra on Sunday


PR...High Rip Current Risk through Sunday afternoon for PRZ001-002-

AM...Small Craft Advisory from 8 AM this morning to 8 AM AST Sunday
     for AMZ711.



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