077
FXCA62 TJSJ 301809
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
209 PM AST Tue Sep 30 2025

.KEY MESSAGES...

* Isolated to scattered showers with isolated thunderstorms are
  expected across the northwest quadrant of Puerto Rico this
  afternoon.

* Across the U.S. Virgin Islands, a showery pattern will tonight
  into Wednesday morning.

* A Heat Advisory remains in effect for all urban and coastal areas
  of Puerto Rico and the U.S. Virgin Islands until 5 PM AST. Stay
  hydrated and avoid prolonged sun exposure. On Wednesday,
  conditions will support an even warmer day, with the potential for
  additional Extreme Heat Warnings or Heat Advisories.

* A large, long-period northerly swell arriving by late Thursday
  night will promote hazardous marine and beach conditions across
  the islands, persisting through at least Sunday or early next
  week.

&&

.SHORT TERM...Tonight through Thursday...

Current satellite derived precipitable water (PWAT) values show that
a band of moisture, with values ranging from 1.9 to 2 inches,
trailing distant Humberto already over the Mona Passage and moving
westward. Drier air continues to filter into our area with values
ranging to around 1.6 to 1.8 inches over the region, with the
highest values observed over NW PR, where later convective activity
continues to be forecast. This drier air mass and overall
unfavorable conditions have served to limit today`s afternoon
convection, however increasing 700 to 500 mb lapse rates and
decreasing 500 mb temperates are still forecast during the rest of
the period. This is due to a series of short-wave troughs along a
ridge north of the region. Elevated, to possibly significant
tomorrow, heat risks will continue across urban and coastal
elevations of the islands. Heat Advisories will likely be issued
each day with an Extreme Heat Warning possible tomorrow, in part due
to weak southerly flow tomorrow. A more northern component can bring
some light relief Thursday. 925 mb temperatures are forecast at 2
standard deviations above normal while reaching 1 SD above normal on
Thursday. Available moisture reaching the region should remain with
below to normal PWAT values, reaching normal values with isolated
patches of higher moisture reaching the islands. Afternoon showers
and t-storms are expected to develop along the interior each
afternoon, steered by the weak above mentioned flow. Diurnal
heating, lighter flow will also help this convective activity
develop, and possibly linger longer. Passing showers continue to
be forecast across windward sectors each day. Lines of showers can
also develop downwind of the local islands during each afternoon.

.LONG TERM... Friday through Tuesday...

/FROM PREV DISCUSSION...Issued at 507 AM AST Tue Sep 30 2025/

Variable conditions are expected through the long-term forecast.
Winds will remain light and variable due to a col region
developing over the area, slightly increasing by Sunday, then
becoming lighter by the start of next workweek. As mentioned in
the previous discussion, a drier air mass is expected to filter
into the region due to the influence of a mid- level ridge that is
expected to linger northeast of the region. However, the latest
model guidance suggests that a mid-to- upper trough may move
south of the CWA by Friday and Saturday. The presence of this
feature is reflected in the projected 500 mb temperatures (between
-6.5 and -7.5 degrees Celsius), near below climatological
normals, enhancing the deep convection activity. As the
deterministic guidance of the GFS and ECMWF continues to suggest
low to seasonal PWAT values (1.6 - 1.8 inches), expect seasonal
weather conditions during the upcoming weekend, with isolated
showers moving over the local waters into windward sectors and
afternoon convection. Taking into consideration the wind speed,
theres a high chance that any shower developing over the area
will stay stationary, meaning that the flood potential will be
more localized, particularly over the mountain ranges of Puerto
Rico. By early next week, a gradual increase in moisture content
and favorables conditions for deep convection are likely due to a
series of upper level troughs approaching the CWA. From the latest
model guidance, PWAT values should increase close to near above
normal values (1.8- 2.0 inches, low chance of reaching 2.2
inches). This could increase the frequency of showers and
thunderstorms moving across the local islands on Monday and
Tuesday, elevating flood potential. Given the expected conditions,
the flooding threat will remain from limited to elevated for the
rest of the forecast period.

The tendency of warmer conditions across the islands is expected
to continue due to above-normal temperatures, available moisture,
and light winds. Hence, the heat threat will remain limited to
elevated for the urban and coastal areas of Puerto Rico and the
U.S. Virgin Islands.

&&

.AVIATION...

(18Z TAFS)

Mainly VFR condition`s expected. Afternoon VCSH will affect
TIST/TISX/TJPS and possibly TJSJ/TJBQ (VCTS for TJBQ) through 22Z.
Winds will continue from the SE up to 15 kts with sea breezes
(mainly over TJBQ) and higher gusts. Winds will be light and
variable overnight, gradually gaining a southerly component and
picking up again up to 10-13 kts with sea breeze variations after
01/14Z.

&&

.MARINE...

Moderate southeasterly to easterly winds will continue tonight,
becoming gentle by Wednesday night and Thursday. A large, long-
period northerly swell generated by distant hurricanes Humberto and
Imelda will likely promote hazardous marine conditions for small
craft across the Atlantic waters and local Caribbean passages from
late Thursday night through at least Sunday or early next week.
Mariners are urged to monitor the forecast over the coming days, and
those operating smaller vessels should consider avoiding navigation
during this upcoming period.

&&

.BEACH FORECAST...

Tonight, the risk of rip currents is expected to remain moderate for
the north-facing beaches of Puerto Rico, including Culebra, as well
as St. Thomas and St. John in the U.S. Virgin Islands. Beachgoers
are urged to exercise caution at all times, as a moderate risk means
life-threatening rip currents are possible within the surf zone. A
low risk remains in place elsewhere.

Conditions are expected to improve by tomorrow, Wednesday, however,
a large, long-period northerly swell anticipated to arrive by late
Thursday night will promote life-threatening rip currents through at
least early next week. Additionally, there is potential for large
breaking waves to trigger High Surf conditions. Beachgoers are
encouraged to continue monitoring the forecast and heed the advice
of the flag warning system over the coming days.

Additional beach hazards this afternoon and in the days ahead
include afternoon thunderstorms, especially along northwestern
beaches and heat risk, typically from 10 AM to 5 PM. Stay well
hydrated, and remember that if you hear thunder, you should seek
shelter in a safe indoor location.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

MRR/YZR