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858
FXCA62 TJSJ 160924
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
524 AM AST Tue Dec 16 2025

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

.KEY MESSAGES...
Issued at 426 AM AST Tue Dec 16 2025

* Showers will move across the US Virgin Islands at times today,
  producing moderate to locally heavy rainfall.

* Showers and isolated thunderstorms this afternoon may lead to a
  slight chance of flooding and thunderstorm impacts, extending
  from El Yunque into the San Juan metropolitan area and across
  the northwest quadrant.

* A long-period northerly swell will arrive late tonight, creating
  a high risk of rip currents along the exposed north-facing
  beaches in PR, spreading to the rest of the north-facing beaches
  from Aguadilla to Fajardo, Culebra, and the northern US Virgin
  Islands on Wednesday.

&&

.Short Term(Today through Thursday)...
Issued at 426 AM AST Tue Dec 16 2025

We had calm weather with clear skies, which allowed for nighttime
cooling, resulting in pleasant low temperatures across Puerto
Rico and the US Virgin Islands overnight. Winds were calm to light
and variable, allowing the land breeze to form overnight.

Early this morning, a high likelihood of generally calm
conditions prevailed across the U.S. Virgin Islands; however, a
low-level perturbation embedded in the easterlies is expected to
introduce periods of cloudiness and passing showers,
intermittently affecting the local waters and islands. As this
feature progresses westward, the probability of showers is
expected to increase across the St Croix and the eastern third of
Puerto Rico by mid-morning, with precipitation chances rising
from low (10-20%) to moderate (30-40%) from late morning through
the afternoon across St. Croix and eastern Puerto Rico.

By early to mid-afternoon, model guidance suggests a moderate to
high likelihood (40-70%) of showers developing across the San
Juan metropolitan area and the northwest quadrant of Puerto Rico.
As a perturbation aloft approaches from the west, a slight chance
of isolated thunderstorms is anticipated, particularly during the
afternoon and early evening when instability peaks. Shower
activity is expected to gradually diminish after approximately 8
PM AST, as convection shifts offshore, allowing for a higher
probability of decreasing cloud cover and calmer conditions.
Nonetheless, another surge of moisture embedded in the easterlies
late tonight into early Wednesday morning may increase the
likelihood of showers, with locally moderate to heavy rainfall
possible.

On Wednesday, the weakening of the mid- to upper-level ridge
associated with an approaching upper-level trough, combined with a
jet streak maximum north of the region, will likely enhance
vertical development. At the surface, another low-level
perturbation is expected to increase moisture availability, and
when combined with diurnal sea-breeze convergence, will maintain a
moderate to high probability of showers, particularly across
western Puerto Rico during the afternoon. Consequently, the risk
of flooding rainfall ranges from slight across eastern Puerto Rico
to elevated across the interior and western sections. While a
flooding risk remains unlikely across the U.S. Virgin Islands,
brief periods of moderate to heavy rainfall cannot be ruled out.

By Thursday, model consensus indicates a strengthening ridge over
the Northeast Caribbean, promoting subsidence and drier air aloft.
Despite this, persistent easterly flow will likely sustain a
moderate chance (3050%) of passing showers, especially across
windward areas of Puerto Rico and the U.S. Virgin Islands during
the afternoon and overnight periods.

&&

.Long Term(Friday through Tuesday)...
Issued at 426 AM AST Tue Dec 16 2025

Mid- to upper-level ridging will dominate the pattern at the
beginning of the long-term period, maintaining generally stable
conditions across the northeastern Caribbean. Subsidence associated
with the ridge will limit vertical development for the most part,
while mid-level relative humidity remains near climatological
values, occasionally dropping to very dry levels near or below 10
percent. This will tend to cap convection and keep shower activity
limited early on.

At the lower levels, precipitable water values will remain somewhat
near normal for most of the period. Patches of moisture will
continue to stream across the area from time to time, resulting in
passing showers, mainly across windward sections and during the
overnight and early morning hours. Rainfall amounts should remain
light and localized with minimal impacts.

Toward the latter portion of the long-term forecast, the mid-level
ridge begins to erode as a polar trough exiting the eastern U.S.
seaboard approaches from the northwest. This will introduce cooler
air aloft, with 500 mb temperatures falling into the -7 to -8 C
range. As a result, lapse rates will steepen, and the environment
will become increasingly unstable. At the surface, an associated
frontal boundary is expected to approach from the northwest late in
the forecast window. Columnar moisture will begin to increase, with
PWAT values trending upward with this setup. The combination of
cooling aloft, increasing moisture, and improved forcing suggests a
more unsettled pattern developing heading into the Christmas period.

This evolving setup could support a higher potential for
thunderstorms and periods of locally heavy rainfall, increasing
the risk for flooding in prone areas. Confidence remains lower at
this time, given that these features occur late in the forecast
window, where uncertainty in timing and strength remains higher.
Subsequent forecasts will continue to refine the potential
impacts, if any.

&&

.AVIATION...
(06Z TAFS)
Issued at 426 AM AST Tue Dec 16 2025

Mainly VFR conditions across all TAF sites. Winds will continue
calm to light and VRB, becoming E-SE winds at 10 to 15 kts, with
higher gust and sea breeze variations aft 16/13Z. -SHRA/SHRA will
approach from the east, first near IST/ISX this morning, spreading
to PR (JSJ) by mid-morning and near JBQ around noon into the
evening. There is a slight risk of TSRA near JBQ/JSJ this
afternoon and evening.

&&

.MARINE...
Issued at 426 AM AST Tue Dec 16 2025

A cold front moving eastward across the Western Atlantic with a
prefrontal trough interacting with an Azores surface high will
promote a moderate to locally fresh easterly wind flow across the
islands today. The cold front will approach the northeast Caribbean
on Wednesday and Thursday and remain off to the northwest of the
islands, bringing trade wind perturbations across the islands,
resulting in showers and afternoon or early evening thunderstorms. A
long period northerly swell will arrive across the local waters late
tonight into Wednesday, deteriorating marine and coastal conditions.
Thus, Small Craft Advisories will be in effect for the middle of the
week.

&&

.BEACH FORECAST...
Issued at 426 AM AST Tue Dec 16 2025

The north and east-facing beaches of PR and the US Virgin Islands
can expect a moderate risk of rip currents through this evening.
The risk of rip currents is low along the west and south-facing
beaches. Then, the arrival of a long-period northerly swell will
increase the risk to high levels for the PR Atlantic Coastline. By
Wednesday, the risk will spread to high from Rincon to Fajardo,
Culebra, St Thomas, and St John. We encourage beachgoers to
monitor the beach forecast, as there is a moderate chance of High
Surf Conditions due to dangerous breaking waves, which could
promote life-threatening coastal conditions along these beaches,
especially on Wednesday.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from this evening through Wednesday
     morning for PRZ001-002-005-008.

VI...None.
AM...Small Craft Advisory from midnight tonight to 6 AM AST Thursday
     for AMZ711.

     Small Craft Advisory from 6 AM Wednesday to midnight AST
     Wednesday night for AMZ712.

&&

$$

KEY MESSAGES/SHORT TERM/AVIATION/MARINE/BEACH FORECAST...CAM
LONG TERM....CVB

NWS SJU Office Area Forecast Discussion