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788
FXCA62 TJSJ 040745
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
345 AM AST Sun Jan 4 2026

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST...

.KEY MESSAGES...
Issued at 247 AM AST Sun Jan 4 2026

* Light southeasterly winds will promote warmer than normal
  temperatures across the lower elevations and coastal areas of
  the islands.

* The flood risk increases on Monday and Tuesday across Puerto
  Rico, as well the potential for lightning due to the influence
  of a front and associated upper-level trough.

* Across the USVI, showers will increase on Monday through early
  Tuesday morning. Followed by a drying trend for the rest of the
  workweek.

* A small northerly swell will maintain a moderate risk of life-
  threatening rip currents along northern and east-facing beaches
  of the islands today. A high risk of rip currents is expected by
  midweek with the arrival of a longer-period northeasterly
  swell.

&&

.Short Term(Today through Tuesday)...
Issued at 247 AM AST Sun Jan 4 2026

Mostly fair conditions prevailed during the night hours, with
isolated to scattered showers moving across local waters and
southeastern and eastern portions of Puerto Rico. Based on WSR-88D
rainfall estimates, accumulations ranged between 0.3 and 0.5 inches,
with higher accumulations over Arroyo. Nevertheless, there were no
flooding reports. Based on CWOP and RAWS stations across the CWA,
temperatures dropped between the mid-to-upper 70s along coastal
areas and lower elevations, while stations at higher elevations
reported temperatures in the low to mid-60s. Current satellite-
derived products show lingering moisture across the CWA, with PWAT
values between 1.4 and 1.5 inches, and winds from the southeast.
Based on HIRES models, showers will continue to move across the
regional waters into windward sections of the local islands during
the morning hours, with late afternoon convection, particularly over
portions of interior and northwestern Puerto Rico, including the
U.S. Virgin Islands streamers. Isolated showers will likely result
with brief periods of heavy rainfall that may lead to ponding of
water over roads and poorly drained areas, although accumulations
should not pose a flooding threat.

The unsettled weather conditions are still expected to return early
next week, as another polar trough and associated frontal boundary
are expected to approach the CWA by Monday. The proximity of the
trough will veer winds from the north, with a col region that will
weaken surface winds, becoming light and variable. From the
deterministic guidance of the GFS and ECMWF, theres a high chance
of an increase in low to mid-level moisture, with PWAT values
reaching above climatological normal (between 1.6 and 1.8 inches,
medium chance of exceeding 1.8 inches). In terms of instability, the
presence of the upper-level trough should introduce instability
across the area, as the latest guidance suggests cooler-than-normal
500 mb temperatures (between -9 and -11 degrees Celsius) and strong
250 mb winds (between 60 and 80 kt), allowing ventilation, which are
key factors for cloud growth. Additionally, guidance keeps
suggesting strong signals of upper-level divergence across the CWA
during that period, which is favorable for deeper convection
activity. At the moment, the most likely scenario relies on an
advective pattern throughout the day, affecting mainly portions of
the U.S. Virgin Islands and northeastern/eastern Puerto Rico. The
highest potential of flooding and lightning is on Monday afternoon,
with slow-moving showers and thunderstorms that could result in
gusty winds and periods of heavy rainfall, leading to urban and
small streams flooding, with a low chance of flash flooding.

A drier air mass is still forecast to filter into the region at some
point during Three Kings Day, which may be an inhibiting factor for
the afternoon convection. Nevertheless, model solutions do not rule
out the chance of showers and thunderstorms over the mountain ranges
of Puerto Rico in the afternoon. Hence, the flooding and lightning
will remain limited. Weather conditions should gradually improve by
Tuesday night. As winds will likely turn from the NE on Monday, 925
mb temperatures will likely decrease and become seasonal on Tuesday,
with minimum temperatures expected between low to mid 70s at lower
elevations and low to mid 60s at higher elevations across the
islands, and even upper 50s in localized areas.

&&

.Long Term(Wednesday through Sunday)...
Issued at 247 AM AST Sun Jan 4 2026

Compared to the earlier part of the week, drier and more stable
conditions are expected to return during the long term period.
After the frontal passage, a downward trend in the precipitable
water content is expected, as PWAT values drop to near 1 inch by
Wednesday, well below seasonal values, and remain in the 50th
percentile for the rest of the period. This is due to a ridge
pattern building from the western Atlantic early in the period,
and a TUTT-low developing northeast of the region during the
weekend. The driest period is expected between Wednesday and
Thursday, with the latest GFS guidance suggesting low-to mid-level
RH values of 10-40%, which are within the 25th-50th percentile.
Therefore, a very dry layer is expected to suppress cloud/shower
development in general. Thereafter, moisture content slowly
recovers between 1.25-1.40 inches, mostly due to shallow patches
of moisture arriving with the trade winds, and favoring isolated
to briefly scattered showers across the windward areas of the
islands during the nighttime.

Minimum temperatures could reach the mid 50s across the higher
elevations of PR during the early morning hours of Wednesday and
once again on Thursday. Mostly due to the expected clear nights
and a northeasterly component in the winds. Across coastal areas,
lows are expected to range from the upper 60s to low 70s. Maximum
temperatures should range from the mid-70s across the higher
elevations to the mid-80s across coastal areas.

&&

.AVIATION...
(06Z TAFS)
Issued at 247 AM AST Sun Jan 4 2026

Mostly VFR conds are expcd across all TAF sites. -SHRA/+SHRA across
the islands will lead to VCSH and reduced CIGs over JSJ, JPS, IST,
and ISX, with light and VRB winds through 04/12-14z. By then, SELY
winds will increase, btwn 7-11 kt, while JPS may have gusts up to 20
kt. Aftn convctn will likely bring SHRA over interior and western
Puerto Rico, bringing VCSH over JPS and JBQ by 04/18z. VRB and light
winds are once again expcd after 04/22z.

&&

.MARINE...
Issued at 247 AM AST Sun Jan 4 2026

A surface high pressure building across the central Atlantic into
the northeast Caribbean will maintain light to moderate east to
east-southeast winds through at least early Monday. A frontal
boundary and associated polar trough are expected to approach the
regional waters late Monday into Tuesday, resulting in a shift to
light northerly to northeasterly winds and an increased potential
for showers and isolated thunderstorms early next week. A small
northerly swell will spread across the Atlantic waters and
passages today, followed by another pulse of a longer period
northeasterly swell by the middle of the week, which may lead to
deteriorating marine conditions, particularly for small craft and
across exposed Atlantic waters and Caribbean Passages.


&&

.BEACH FORECAST...
Issued at 247 AM AST Sun Jan 4 2026

Life-threatening rip currents are possible along the northern
beaches of the islands today due to the arrival of a small northerly
swell(~3ft at 10-11 seconds). A longer period northeasterly
swell(~5ft at 12-14 seconds) is expected to arrive across the local
waters by midweek, increasing the risk of life- threatening rip
currents across most east and north-facing beaches of the islands.

&&

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

$$

SHORT TERM...MNG
LONG TERM....DS
AVIATION...MNG

NWS SJU Office Area Forecast Discussion