464
FXCA62 TJSJ 120808
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
408 AM AST Wed Mar 12 2025

.SYNOPSIS...

A broad surface low over the western Atlantic will move into the
central Atlantic by Thursday, while an associated surface front
extending north of Hispaniola will continue to sink further into
the northeastern Caribbean during the next few days. At upper
levels, a series of troughs will provide some instability to
enhance showers with possible isolated thunderstorms over the
local area. The wettest period is expected from Thursday through
Saturday. A drying trend is forecast for early next week.

A long-period northwesterly swell will spread across the Atlantic
waters and local passages on Thursday, resulting mainly in
life-threatening rip currents. Another larger northerly swell is
expected to fill across the local waters during the weekend,
promoting hazardous seas and rough surf conditions.


&&

.SHORT TERM...Today through Friday...

Clear skies prevailed across most of Puerto Rico overnight, with fog
developing along the Cordillera Central. Overall, calm and stable
conditions dominated across Puerto Rico and the U.S. Virgin Islands.
Temperatures remained in the low to mid-70s in coastal and urban
areas, while mountainous and rural regions remained in the 60s.
Winds were light and southerly.

Today, the ridge aloft will begin to weaken as an upper-level trough
approaches the Northeast Caribbean from the west, gradually
increasing atmospheric instability. Afternoon convection driven by
sea breezes and local effects may develop over interior and western
Puerto Rico today.

On Thursday, winds will become variable, shifting from the south-
southwest before transitioning to a north-northeasterly flow.
Instability will further increase as mid-level temperatures drop to
around -7C while moisture levels extend up to 500 mb. These
conditions suggest that any developing showers will have a higher
potential for vertical growth, increasing the likelihood of stronger
development. As a result, the risk of thunderstorms and heavy
rainfall will rise, with further intensification expected by the end
of the workweek.

By Friday, an approaching frontal boundary will enhance shower
activity throughout the day, with stronger afternoon convection
anticipated over interior and southwestern Puerto Rico. Isolated
thunderstorms are expected, particularly with the heaviest rainfall.
At this time, there is an elevated risk of flooding across the
islands, meaning that flooding in urban areas, roads, small streams,
and washes is expected, along with a low chance of isolated flash
floods.

.LONG TERM...Saturday through Wednesday...

A mid-to upper-level trough will move over the Anegada Passage on
Saturday, and the 500 mb temperature will remain around -6/5
degrees C. This will promote marginal instability aloft for
isolated thunderstorm development. A stalled surface front over
the islands will continue to promote above normal moisture content
across the local area with precipitable water (PWAT) content
fluctuating between 1.50 to 1.70 inches. Therefore, a wetter
pattern is expected to continue across Puerto Rico and the U.S.
Virgin Islands, with a limited to elevated risk of excessive
rainfall, leading to urban and small stream flooding and potential
minor landslides in steep terrain. Lingering moisture across the
area will continue on Sunday as the front moves further south
under moderate to locally fresh northeast winds.

For the first half of the week, an upper-level ridge building
from the western Caribbean and a developing TUTT-low northeast of
the Leeward islands will promote a drying trend across the local
area, with PWAT values dropping between 1.00-1.20 inches.
Meanwhile, the surface high pressure over the western Atlantic
will strengthen, and the low-level winds will increase up to 20
knots across the Atlantic waters and the Anegada Passage,
particularly on Monday. Therefore, breezy conditions and an
advective weather pattern are expected, with shallow passing
showers moving at times across the windward areas of the islands
and limited afternoon shower development over the SW quadrant of
PR.


&&

.AVIATION...

(06Z TAFs)

All terminals should experience VFR conditions. VCSH will affect
TJPS and TJBQ after 12/17Z. and TJSJ, TIST, and TISX after 12/23Z.
Winds will prevail from the SE around 4 to 10 kt with sea breeze
variations. A gradual increase in rainfall is anticipated across all
TAF sites from this evening onwards.


&&

.MARINE...

A surface high pressure over the far eastern Atlantic and a
surface low and associated front moving over the western Atlantic
will maintain a southerly wind component through this morning.
Winds will become light and variable, but with a northerly
component later in the afternoon. By the end of the week, the
potential for showers over the local waters will increase as the
front moves closer to the area, and winds turn from the east to
northeast. A long-period northwesterly swell will spread across
the Atlantic waters on Thursday and linger through Friday. Late in
the weekend into early next week, a larger northeasterly swell
will bring hazardous seas once again across the Atlantic waters
and passages.


&&

.BEACH FORECAST...

The surface low over the western Atlantic will produce a long
period northwesterly swell that will arrive across the local
waters on Thursday. Life-threatening rip currents are likely
across the north-facing beaches of the islands. Seas are forecast
to build up to 6 feet across the Atlantic waters, but model
guidance could be underestimating the swell height and high surf
advisory conditions cannot be ruled out across some of the
northern beaches of Puerto Rico. This swell will increase the risk
of life-threatening rip currents across the northern exposed
beaches of the islands through at least Friday.

A larger northeasterly swell with seas building up to 8 feet is
expected late in the weekend into early next week, causing large
breaking wave action along the north-facing beaches of the
islands. High surf advisory conditions and life-threatening rip
currents are likely, particularly on Sunday and Monday.


&&

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

$$

AVIATION/SHORT TERM...MMC
BEACH/MARINE/LONG TERM...DSR