National Weather Service Today's Weather Impact Levels

872
FXCA62 TJSJ 131845
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
245 PM AST Mon Oct 13 2025

.KEY MESSAGES...

* Tomorrow, winds will shift from the east-southeast, bringing
  above-normal temperatures, which could lead to an elevated
  heat threat with heat indices reaching 108 degrees.

* A seasonable weather pattern is forecast from Tuesday into
  Wednesday as an upper level ridge lingers over the region.

* Deteriorating weather conditions are expected during the second
  part of the workweek as a frontal boundary moves north of the
  area, increasing the moisture content across the islands.

* For the U.S. Virgin Islands, from Tuesday into Wednesday, there
  will be fair weather condtions and an elevated heat threat.

* A northerly swell could lead to deteriorating marine and
  coastal conditions by the end of the week into next week.


&&

.SHORT TERM...Rest of this afternoon through Wednesday...

Sunny to partly cloudy skies were observed across Puerto Rico and
the U.S. Virgin Islands during the morning hours. Passing showers
were moving from the Anegada Passage into the eastern sections of
PR. Additional cloudiness and showers were developing downwind of
the islands and along portions of the Cordillera Central and
western PR. Thunderstorms are expected to develop across these
areas through the rest of the afternoon hours. Maximum temperatures
were from the upper-80s to low-90s across the lower elevations of
the islands, to the mid-70s and upper-80s across the higher elevations
of Puerto Rico.

For tonight, a surge in moisture embedded in the trades will bring
showers with brief periods of heavy rainfall across portions of
the USVI and the eastern half of PR. A mid-to upper-level ridge
will continue over the region through at least late Tuesday night.
This will continue to support a similar weather pattern for the
next 2 days. On Wednesday, a pre-frontal trough will increase
moisture content over the eastern Caribbean and winds will become
lighter and turn more southerly. This will cause an increase in
afternoon shower and thunderstorm activity over the north-central
and eastern portions of PR, where the flood threat will be
elevated. Thereafter, a front over the Atlantic waters is
expected to gradually push further southward into the local area,
bringing an increase in shower coverage across the islands from
late Wednesday night into Thursday.


.LONG TERM...Thursday through Monday...

/From prev discussion issued at 506 AM AST Mon Oct 13 2025/

The long-term period remains generally unchanged, but a transition
into more favorable conditions for increased shower coverage and
stronger thunderstorms is expected from Thursday onward. According
to the latest guidance, the influence of a deep-layer trough and
its associated frontal boundary will linger to the north of the region,
while southerly flow continues to transport tropical moisture
over the area. Model guidance also indicates that precipitable
water (PWAT) values will stay near average, ranging between 1.9
and 2.0 inches. In addition, mid- to upper-level relative humidity
is forecast to increase to above-normal levels. Cooling
temperatures at 500 mb, from around -4C to -6 or -7C, will
enhance stronger thunderstorm development during the afternoon
hours. Winds will shift to a more southerly direction and remain
light. Also, the southerly flow will also contribute to elevated
heat risk, with 925 mb temperatures exceeding two standard
deviations above normal, likely prompting the issuance of Heat
Advisories for urban and coastal areas.

Overall, the combination of these factors will support increased
shower activity and stronger thunderstorms each afternoon, at
least through the end of the week. The flood risk will remain
elevated on a daily basis, especially in areas with saturated
soils or poor drainage. Potential impacts include ponding of water
on roads and in poorly drained areas, urban flooding (especially
in low-lying zones), and rising water levels in small streams,
rivers, and washes.

By Sunday onward, conditions are expected to become more typical,
still with some afternoon convection due to local effects, as
surface high pressure building over the central Atlantic helps
lift the frontal boundary. A mid-level ridge becomes dominant
aloft, ushering drier air into the mid-levels. However, the
tropics should continue to be monitored, as some forecast models
are indicating the possible approach of a tropical wave after
Monday.


&&

.AVIATION...

(18Z TAFS)

SHRA/TSRA should remain mainly in the VCTY of TJSJ/TJBQ thru
13/22z. Meanwhile, SHRA en route from the Leeward terminals is
expected to move over TIST/TISX from late this afternoon through
at least 14/06z, causing brief periods of MVFR conds. Similar
conditions are expected to reach TJSJ/TJPS by this evening. Brief
isolated thunderstorms could develop between the Anegada Passage
and the eastern PR waters. The 13/12z TJSJ sounding indicated east
winds up to 22 kt blo 2500ft.


&&

.MARINE...

A surface high pressure over the Central Atlantic will continue to
result in moderate easterly winds across the local waters. As a
result, seas will remain up to 5 feet along the offshore exposed
waters and slightly higher across the northwestern coastal
waters of PR. A change in winds is forecast from Wednesday
onward, as a frontal boundary moves southward into the islands.
This will increase shower and thunderstorm activity on Thursday
into the weekend. For the latter part of the week, a northerly
swell could lead to deteriorating marine conditions.


&&

.BEACH FORECAST...

No changes to the inherited forecast. There is a moderate risk of
rip current along the north and eastern coast of Puerto Rico.
Similar conditions will remain for the upcoming days. By the
upcoming weekend, model guidance suggests the arrival of a northerly
swell that could increase the breaking waves along the north coast
of Puerto Rico, Culebra, and St. Thomas and St. John.


&&

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

$$

AVIATION/SHORT TERM...DSR
KEY/BEACH/MARINE...LIS

NWS SJU Office Area Forecast Discussion