203
FXCA62 TJSJ 061408
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1008 AM AST Sat Sep 6 2025

.UPDATE...

Although some showers are present in the region, they are more
isolated in nature and mostly confined to the eastern third of the
area. Cloud coverage is also less than expected across the region,
with mostly clear skies over western Puerto Rico. This will increase
heating, and in combination with well above normal moisture,
conditions will become much hotter during the peak hours of the day,
particularly across urban and coastal areas. Therefore, a Heat
Advisory was issued for Urban and coastal areas of Puerto Rico, as
well as the valleys of the eastern interior, and the U.S. Virgin
Islands. Most weather stations have already reported heat index
values above 100F, with some even exceeding 108F. However, shower
and thunderstorm development in the afternoon should bring some
relief to the heat threat in those areas.


&&

.AVIATION...
(06z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hour. However, locally induced TSRA is expected
to develop in and around the terminals throughout the period,
causing tempo MVFR to brief IFR conditions. The 06/12Z TJSJ
sounding showed up to 15 kt ESE winds blo FL050.


&&

.PREV DISCUSSION... /issued 516 AM AST Sat Sep 6 2025/

KEY MESSAGES...

* Unsettled conditions will continue for the rest of the weekend,
  with the most active periods expected during the afternoons and
  evenings across Puerto Rico and the U.S. Virgin Islands.

* For the U.S. Virgin Islands, warm to hot conditions will
  continue through the next few days.

* The National Hurricane Center continues to monitor Invest 91 over
  the Central Tropical Atlantic. Residents and visitors should
  continue to closely monitor official updates in the coming days.

SHORT TERM...Today through Monday...

The departing tropical wave interacting with the mid to upper-level
trough promoted showers and thunderstorms across the regional
waters, with some of them moving inland across the US Virgin Islands
and the windward locations of PR. Winds were mainly from the east-
southeast at 5 to 10 mph, but locally higher in and near rain
activity. Minimum temperatures were warmer than normal overnight,
especially across the urban and coastal areas, from the upper-70s to
low 80s.

We expect another day with muggy heat indices between 100 and 111
degrees Fahrenheit across many urban and coastal sites in the US
Virgin Islands and Puerto Rico, due to the warmer-than-normal low
temperatures and the above-normal moisture content associated with
the departing tropical wave. However, the tropical wave trailing
moisture interacting with the lingering mid to upper-level trough
will promote another active afternoon, especially downwind from the
US Virgin Islands, Vieques, and Culebra. The activity will then form
downwind from El Yunque into the San Juan Metropolitan Area and
along the Cordillera Central, as well as to the north and west of
it. Thunderstorm activity will also promote gusty winds and frequent
lightning. The downpours will promote a slight to elevated flooding
risk across these locations, especially during the afternoon.

Residents and visitors are encourage to monitor the evolution of the
weather conditions, as a Heat Advisory might be required if the
expected rain activity and subsequent cloud cover do not form.

A trade wind perturbation will result in a showery Sunday across the
region. Showers will move across the local waters, moving inland
over the windward locations of the US Virgin Islands and Puerto Rico
overnight and early Sunday morning. Then, heat indices will reach
values around 110 degrees Fahrenheit again in the mid-morning and
afternoon. The excessive heating combined with sea breezes and local
effects will then promote strong afternoon convection along the
Cordillera Central and western PR, as well as across the southeast
coast and San Juan Metropolitan area. The activity is also expected
to be felt downwind from the US Virgin Islands, where ponding of
water could impact the west end. Moisture content will slowly erode
on Monday. However, the typical hydrological pattern involves AM
passing showers across the windward locations, followed by strong
afternoon convection across the western locations, due to various
factors, such as excessive heating.

LONG TERM...Tuesday through Friday...

Changes were introduced to the long-term forecast, with uncertainty
remaining high for the second part of the next workweek. Based on
satellite-derived products, ahead of Invest91L, theres a dense
Saharan Air Layer (SAL) likely to approach the local islands by
Tuesday. From the latest model guidance, Precipitable Water (PWAT)
values should drop to well below climatological normal (around 1.5
inches). Additionally, relative humidity in the low and mid levels
should plummet to below normal as well, with 850 - 700 mb lapse rate
below the 10th percentile (< 5.0 C/km). Although the shower activity
may be triggered due to daytime heating and local effects, deep
convection should be limited. As for the Saharan Dust, low to
moderate concentrations may bring hazy skies, reduced visibility,
and deteriorated air quality.

The National Hurricane Center continues to monitor Invest 91L, but
from the latest Tropical Weather Outlooks (TWOAT), has lowered the
formation chance in the past 24 hours. In the 05/06z Tropical
Weather Outlook (TWOAT), Invest AL91 had a 60% formation chance
through 48 hours (medium) and 90% through 7 days (high).  From the
latest TWOAT, the system has a 30% (low) chance in the next 48 hours
and 60% (medium) chance for the next 7 days due to a drier air mass
that is limiting shower and thunderstorm activity, unfavorable for
cyclonic development. Nevertheless, AL91 it`s still likely to
upgrade to a tropical depression early to the middle part of next
week. According to the latest deterministic guidance, global
models continue to disagree and fail to reach a consensus on the
potential trajectory and intensity of the system. While the ECMWF
projects the system moving northeast of the region, GFS is
currently positioning the tropical wave south of the CWA.
Nevertheless, the grand ensemble maintains PWAT values above
climatological normal (2.0 - 2.2 inches), as moisture content
should gradually increase by late Tuesday. With plenty of tropical
moisture, the frequency of showers and thunderstorms will likely
increase across the islands, affecting mainly windward sections
during the morning. Taking into account SE winds, daytime heating,
and local effects, convection activity should develop over the
mountain ranges and northwestern portions of Puerto Rico.

At the moment, the flooding risk will remain limited to elevated
over the aforementioned areas, nevertheless, this may  change over
the next few days as the weather pattern will depend mostly on the
development of Invest AL91. Interest in Puerto Rico and the U.S.
Virgin Islands should continue to monitor the progress of this
system.

AVIATION...

(06Z TAFs)

Mainly VFR conds are expected across al terminals during the fcst
period. However, tropical moisture will promote SHRA/TSRA across the
local area today. VCTS are likely across TJSJ/TJBQ/TIST/TISX thru
06/23Z (especially btwn 06/16-23z). Tempo MVFR to brief IFR conds
are possible across TJSJ/TJBQ aft 06/16Z. Expect calm to light and
VRB winds thru 06/13z, then from the E-ESE at 10-15 kt with sea
breeze variations thru 06/23Z. Then, becoming light and variable
thru 07/13Z.

MARINE...

The trailing moisture of a tropical wave moving into the central
Caribbean, interacting with a mid to upper level trough will promote
showers and thunderstorms across the regional waters today. The
surface high pressure across the Central Atlantic, promoting moderate
to locally fresh easterly winds will result in moderate chops to
choppy conditions this weekend into early next week. The National
Hurricane Center is monitoring Invest 91L over the central tropical
Atlantic. While a drier air mass is limiting the potential for
development, this system is likely to be near the Lesser Antilles by
the middle to latter part of next week. Mariners should remain
attentive to forecast updates in the coming days.

BEACH FORECAST...

No major changes were introduced to the beach forecast. Beaches
across the CWA continue under a low risk of rip currents, expected
to continue through early next week. Although the risk remains low,
isolated stronger rip currents may occur in the vicinity of groins,
jetties, reefs, and piers. Besides rip currents, beachgoers must
also stay aware of the weather conditions across the area,
particularly in the afternoons and evenings, due to shower and
thunderstorm activity that may approach the surf zone. Beachgoers
must leave the water and seek shelter if thunderstorms are close by.
The National Hurricane Center continues to monitor Invest AL91 over
the central tropical Atlantic. Although uncertainty remains high
regarding the trajectory and intensity of the system, interest
should continue to monitor the progress of this system.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

UPDATE/AVIATION...CVB
IDSS AND PUBLIC DESK...GRS