877
FXCA62 TJSJ 130906
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
506 AM AST Mon Oct 13 2025

.KEY MESSAGES...

* More stable to typical weather conditions are expected over the
  next few days. However, from Thursday through Saturday,
  conditions will become more favorable for widespread showers and
  stronger thunderstorms, increasing the risk of urban and small-
  stream flooding, rapid river rises, frequent lightning, and
  gusty winds.

* There is a limited heat risk today; however, southerly wind
  flow will bring above-normal temperatures combined with elevated
  moisture, which could result in an elevated to significant heat
  threat, particularly from Tuesday onward.

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

* In the U.S. Virgin Islands, the main threats this week are heat,
  along with the return of life-threatening rip currents and
  hazardous seas by the end of the week into next week.

&&

.SHORT TERM...Today through Wednesday...

Isolated to scattered showers developed over the surrounding waters
and, steered by moderate easterly winds, some moved into eastern
Puerto Rico, including Vieques and Culebra, as well as the U.S.
Virgin Islands. The highest rainfall totals so far are around half
an inch in southeastern Puerto Rico. Temperatures showed a strong
landsea contrast, with lows dropping into the low 60s in higher
elevations, while exposed coastal areas remained warm in the 8082F
range. Conditions were slightly warmer and drier than the previous
night, reflecting a patchy wet/dry pattern. Across land areas, winds
were generally light to calm and variable.

A mostly quiet morning will turn more active this afternoon as
daytime heating and sea breezes trigger scattered to locally
numerous showers and isolated to scattered thunderstorms, mainly
across the western interior and west. A strong ridge aloft will
continue to limit widespread storm development by keeping the
atmosphere capped and somewhat dry above the surface, but low-level
moisture will increase by mid-afternoon, allowing for brief heavy
rain and localized flooding. Winds will remain moderate to locally
fresh out of the east due to surface high pressure to the north, and
while temperatures will be slightly cooler than recent days, heat
may still affect vulnerable individuals in urban and coastal areas
of north-central Puerto Rico. Tonight, winds will weaken and a surge
in moisture (higher PWAT) will allow slow-moving showers to drift
inland from the waters into eastern Puerto Rico and the U.S. Virgin
Islands, bringing a continued chance of additional rainfall before
hazards gradually decrease with cooling temperatures and fading
instability.

A gradual but important pattern shift is expected over the next
4872 hours as the mid- to upper-level ridge finally shifts
eastward, allowing a deep-layer trough over the western Atlantic to
support a coastal low and push the seasons first frontal boundary
and its pre-frontal trough southeastward toward the region. Model
guidance clearly shows this transition through veering and weakening
winds, increasing PWAT values, and cooling mid-level temperatures,
all of which point to a wetter and more unstable environment
developing by Wednesday. As winds shift from east to southeast and
then south to southwest, hotter air and rising moisture will
increase heat stress. By Wednesday night, a surge of tropical
moisture will push PWAT values above 2 inches, while mid-level
temperatures cool back to normal, reducing inhibition and enhancing
instability. With light winds, high moisture, and increasing
instability in place, slow-moving showers and thunderstorms will
become more likely, raising the risk of localized flooding and
lightning impacts, especially Wednesday afternoon and night.


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

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...
(06Z TAFs)

Variable weather expected. Patchy SHRA early, then coverage
increases this afternoon. SHRA/TSRA likely across all PR terminals
13/16-22Z, with brief MVFR/IFR VIS/CIG in heavier cells. USVI
terminals will also see periods of SHRA, steered by E winds 814 kt.
Most activity will fade after sunset but may linger at TJSJ, TJBQ,
and USVI terminals overnight. Winds LGT/VRB (06 kt)
overnight/morning, increasing to 1014 kt from ESEE with sea
breeze, then LGT/VRB again after 13/22Z.

&&

.MARINE...

Gentle to moderate east to southeast winds will prevail through
Tuesday as a surface high-pressure system dominates across the
central Atlantic. From Wednesday onward, light to gentle southerly
winds are expected. Today, isolated to scattered showers embedded
in the trade winds are anticipated, with showers and thunderstorms
possibly developing across the northwestern waters of Puerto Rico
this afternoon. Shower activity is expected to increase around
Thursday due to an approaching frontal boundary. Looking ahead, a
northerly swell could lead to deteriorating marine conditions by
next weekend and into early next week.

&&

.BEACH FORECAST...

A moderate risk of rip currents is present today across beaches
along the northern and eastern coasts of Puerto Rico, Culebra and
St. Thomas. That means life-threatening rip currents are possible
in the surf zone and beachgoers need to exercise caution. Tomorrow,
Tuesday, similar conditions are expected to prevail, followed by
improving conditions with low risk for Wednesday and Thursday. At
the end of the week into the weekend, the risk will gradually
increase again. For more information, please refer to the Surf
Zone Forecast for Puerto Rico and the U.S. Virgin Islands (SRFSJU)
or visit weather.gov/beach.

Additional beach hazards over the coming days will include:
elevated heat risk and afternoon thunderstorms. Beachgoers are
advice to remain hydrated and seek shelter immediately if you hear
thunder.

&&

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

$$

SHORT TERM/AVIATION...ICP
LONG TERM/MARINE/BEACH...YZR