Urb Linda Vista, Puerto Rico Weather Forecast Discussion

710
FXCA62 TJSJ 061828
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
228 PM AST Sat Sep 6 2025
.KEY MESSAGES...
* Heavy showers and isolated thunderstorms will continue across
  portions of Puerto Rico fueled by lingering moisture from a
  tropical wave located south of Hispaniola. This activity will
  result in frequent lightning and minor flooding across the area.
* For the U.S. Virgin Islands, showers and occasional
  thunderstorms may lead to ponding of water on roadways this
  afternoon.
* A trade wind perturbation will move into the area by Sunday,
  promoting another round of shower activity across the local
  waters and portions of the islands.
* The heat threat will persist through the next several days, with
  heat indices over 100F expected across the urban and coastal
  areas of Puerto Rico and the U.S. Virgin Islands.
&&
.SHORT TERM...Rest of Afternoon through Monday...
Heavy showers and thunderstorms have developed across the region
this afternoon, with additional streamer activity observed downwind
of the smaller islands. This activity is being fueled by the
trailing moisture associated with the departing tropical wave, in
combination with instability aloft provided by the proximity of the
upper-level trough, intense diurnal heating, and orographic lifting.
These factors will continue to promote widespread convection through
the evening hours. As a result, urban and small stream flooding will
remain likely, along with gusty winds and frequent lightning in the
stronger thunderstorms. In addition, the saturated soils across
interior Puerto Rico will increase the risk for landslides and
rockfalls, particularly across steep terrain. Reports of such
incidents were received yesterday from Utuado and Arecibo, and
similar hazards may occur again in localized areas today.
Overnight into Sunday, a trade wind perturbation will move into the
area, promoting another round of shower activity across the local
waters, moving inland across the windward portions of the U.S.
Virgin Islands and Puerto Rico. By late Sunday morning into the
afternoon, heat indices are expected to climb once again to near or
above 110F, particularly across urban and coastal areas, prompting a
continued heat threat. Combined with sea breezes and local effects,
this heating will help fuel additional strong afternoon convection,
especially across the interior and western portions of Puerto Rico,
as well as the San Juan Metropolitan Area. Expect similar hazards of
flooding, gusty winds, and frequent lightning.
By Monday, moisture levels are expected to decrease somewhat due to
the arrival of a narrow dry slot. However, moisture values will
remain near normal for early September, sufficient to sustain the
typical diurnal pattern of morning coastal showers across windward
sections of the islands and afternoon convection over the interior
and western Puerto Rico. The heat threat will persist, with heat
indices above 100F expected across the urban and coastal areas of
Puerto Rico, including the  the valleys of the eastern interior, as
well as the U.S. Virgin Islands.
.LONG TERM...Tuesday through Friday...
(from previous discussion)
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...
(18Z TAFS)
Reduced VIS/CIGs due to SHRA/TSRA across most terminals will prevail
thru the aftn resulting in brief MVFR/IFR. Sfc winds ESE 10-15 kt
with sea breeze variations and ocnl higher gusts thru 06/23z. Aft
07/03Z, SHRA/ISOLD TSRA expected across USVI and ern PR terminals
(TIST/TISX/TJSJ) as a wind perturbation approaches the area. deeper
moisture from a tropical wave arrives. By Sun aftn, SHRA/TSRA will
favor once again interior and wrn PR, affecting mainly TJBQ,  with
mtn obsc likely. Streamer downwind of El Yunque could also affect
TJSJ.
&&
.MARINE...
Lingering moisture from tropical wave located south of the
Hispaniola will continue to interact with a mid to upper level
trough, promoting showers and thunderstorms across the regional
waters for the rest of the afternoon into the evening hours. 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...
Beaches across the CWA will continue under a low risk of rip
currents through at least mid-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.
&&
.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.
&&
$$
CVB/GRS

NWS SJU Office Area Forecast Discussion