184
FXCA62 TJSJ 161916
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
316 PM AST Tue Dec 16 2025

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

.KEY MESSAGES...
Issued at 314 AST Tue Dec 16 2025

* A prefrontal trough north of the area and a low level
  perturbation moving over the islands will continue to promote
  showers and isolated t-storms, steered by ESE flow, during the
  afternoon hours over Puerto Rico.

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

* A long-period northerly swell will arrive late tonight, creating
  a high risk of rip currents along the north-facing beaches in PR
  (with High Surf Conditions likely tomorrow) and then along the
  rest of the north-facing beaches of PR/Northern USVI tomorrow
  through at least Thursday.

* Marine conditions will also deteriorate tonight through at least
  Thursday, prompting Small Craft Advisories over the Atlantic
  waters.

&&

.Short Term(This evening through Thursday)...
Issued at 314 PM AST Tue Dec 16 2025

Current satellite derived precipitable water (PWAT) values
indicate around 1.75 inches, above seasonal values, as moisture
from a low level perturbation continues to be steered over the
area under ESE steering flow. This ESE steering flow helped
promote seasonal 925 mb temperatures, reported highs until now
have reached the mid to upper 80s across urban and coastal
sectors, with some stations reporting in the low 90s. A front,
well north of the area is also inducing a prefrontal trough to our
north. The low level perturbation brought showers over the USVI
to start the day and then promoted afternoon showers and isolated
t-storms over Puerto Rico, particularly over interior to
north/northwestern PR, being steered by the moderate ESE flow.

Afternoon showers are also being detected downwind of the local
islands and El Yunque towards eastern PR and over the southern
plains. This afternoon activity will gradually dissipate and or move
offshore this evening. However, showers will continue to be steered
towards the eastern region tonight. Tomorrow, patches of more humid
air due to low level perturbations will continue to filter into the
islands with PWAT values around 1.6 to 1.75 inches. An upper level
trough will also continue to move north of the area, aiding in
instability. ESE flow will continue tomorrow as the result of two
surface highs northwest and northeast of the islands, and their
interaction with the prefrontal trough. This steering flow, sea
breeze convergence and local effects, will promote afternoon shower
activity mainly across western PR and downwind of El Yunque and the
local islands, prompting a limited flooding risk. Passing showers
over the eastern region are expected throughout the day and early
tonight. Current model guidance has a drier air mass then moving
over  the islands late tomorrow night into Thursday. Another patch
of more humid air is forecast to arrive on Thursday under more
easterly steering flow (as the western Atlantic high establishes
itself north of the region), once again increasing PWAT values to
high end normal to above normal values to end the period. By
Thursday, the upper level trough, however, will be further northeast
and will continue moving away from the local area. Available
moisture will continue to steer showers towards the eastern region,
with afternoon showers also over interior to western PR. 925
temperatures will continue at seasonal to high end seasonal values.
&&

.Long Term(Friday through next Monday)...
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...
(18Z TAFS)
Issued at 314 PM AST Tue Dec 16 2025

The cold front moving eastward across the Western Atlantic with a
prefrontal trough interacting with an Azores surface high will
continue to promote a moderate to locally fresh easterly winds
across the islands through tonight. The cold front will approach the
northeast Caribbean by midweek and remain off to the northwest of
the islands, bringing trade wind perturbations across the islands,
resulting in showers and afternoon to 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.

&&

.MARINE...
Issued at 314 PM AST Tue Dec 16 2025

The cold front moving eastward across the Western Atlantic with a
prefrontal trough interacting with an Azores surface high will
continue to promote a moderate to locally fresh easterly winds
across the islands through tonight. The cold front will approach the
northeast Caribbean by midweek and remain off to the northwest of
the islands, bringing trade wind perturbations across the islands,
resulting in showers and afternoon to 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 314 PM AST Tue Dec 16 2025

A long period northwesterly swell will deteriorate marine and
coastal conditions over the Atlantic waters and coastline by
tonight through at least Thursday. The northern PR coastline will
have a high risk of rip currents tonight from Aguadilla to
Fajardo. By tomorrow, Rincon to Fajardo, Culebra and the northern
USVI will have a high risk of rip currents. We encourage
beachgoers to monitor the beach forecast, as High Surf Conditions
are also possible and a High Surf Advisory can be issued later
this afternoon for tomorrow, pending observations from offshore
buoys. Dangerous breaking waves, which could promote life-
threatening coastal conditions are forecast for tomorrow,
Wednesday.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from 6 PM AST this evening through late
     Wednesday night for PRZ001-002-005-008.

     High Rip Current Risk from Wednesday morning through Wednesday
     evening for PRZ010.

     High Rip Current Risk from Wednesday morning through late
     Wednesday night for PRZ012.

VI...High Rip Current Risk from Wednesday morning through late
     Wednesday night for VIZ001.

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.

&&

$$

MRR/MMC/CVB/MNG

NWS SJU Office Area Forecast Discussion