186
FXCA62 TJSJ 300907
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
507 AM AST Tue Sep 30 2025

.KEY MESSAGES...

* Warmer conditions will persist for the next few days. A Heat
  Advisory is in effect for all urban and coastal areas from 10
  AM AST through 5 PM AST. Stay hydrated and avoid prolonged sun
  exposure.

* For the U.S. Virgin Islands, shower activity will increase later
  tonight, with limited flooding threat.

* A large, long-period swell arriving by late Thursday night will
  promote hazardous marine and beach conditions across the
  islands, persisting through the weekend.

&&

.SHORT TERM... Today through Thursday...

Passing showers embedded in the trade winds moved over portions of
the U.S. Virgin Islands, and over the eastern third and south coast
of Puerto Rico overnight. The Doppler radar estimated up to half an
inch of rain between Las Piedras and Humacao. Minimum temperatures
ranged once again from the upper-60s across the higher elevations to
the upper 70s and low 80s across coastal areas. The wind was from
the southeast at 10 mph or less. Showers today will be enhanced by a
band of precipitable water content of around 2.00 inches. However, a
drier air mass of around 1.50 inches will gradually filter over the
region from the southeast through Wednesday. Meanwhile, a low-level
trough is expected to develop over the eastern Caribbean late in the
period, enhancing moisture transport into the local area by
Thursday. Moderate southeast winds will prevail today, and become
lighter and more southerly on Wednesday, by Thursday winds will
abate furthermore and acquire a north-northeasterly component due to
the influence of the trough and a col area developing just north of
the region.

An elevated heat threat will persist across the lower elevations of
the islands through the shor-term period, with heat advisories
likely issued each day, with a possible extreme heat warning on
Wednesday due to the expected deep layered southerly flow. Across
the lower elevations of the islands, maximum temperatures are
expected to reach the low to mid-90s through at least Wednesday,
while the northerly component on Thursday may bring temperatures
down by a degree or two. Regardless, heat concerns will continue.

Although a drier air mass will filter over the region early in the
period, the 850-700mb and 700-500mb RH layers are expected to
increase from the 25th percentile today to the 50th percentile on
Wed/Thu. Also, in terms of stability, conditions are forecast to
become gradually more unstable at the mid-levels, as warmer lapse
rates and colder 500mb temperatures are anticipated on both days.
This is supported by a series of short-wave troughs moving along the
base of weak ridge north of the region. Therefore, afternoon
thunderstorms and showers are expected to develop each day, mainly
along portions of the Cordillera Central and in areas favored by the
weak steering winds, with the combination of intense heating,
lighter winds, and increasing instability favoring stronger
thunderstorms and higher rainfall accumulations on Wednesday and
Thursday. Across the USVI, the frequency of passing showers will
increase early in the period, but as winds decrease, the locally
induced late morning/early afternoon showers over land areas could
linger longer than usual.

&&

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

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...

(06Z TAFS)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, tempo MVFR conds are possible at
TJBQ btw 30/16-22 due to TSRA/SHRA developing over NW PR. Elsewhere,
passing SHRA should cause mostly VCSH at times. Winds will continue
from the SE up to 16 kt blo 1000 ft. Sea breeze variations expected
mainly at TJBQ aft 30/13z.

&&

.MARINE...

Moderate southeasterly to easterly winds will continue for the next
few days, becoming gentle by Thursday. A large northerly swell will
promote hazardous marine conditions for small craft by late Thursday
night through the upcoming weekend.

&&

.BEACH FORECAST...

Today, the risk of rip current is expected to remain moderate for
the north-facing beaches of Puerto Rico, including Culebra, and
St. Thomas and St. John in the U.S. Virgin Islands. Although the
risk is moderate, life-threatning rip currents are still possible
along the surf zone, beachgoers should exercise caution.
The risk is expected to decrease on Wednesday, but a large, long-
period northerly swell arriving by late Thursday night will
promote life- threatening rip currents through at least Sunday.
Hence, beachgoers are urged to exercise caution and heed the
advice of the flag warning system.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION...DS
LONG TERM...MNG
MARINE/BEACH FORECAST...DS/MNG