132
FXCA62 TJSJ 031922
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
322 PM AST Sun Aug 3 2025

.KEY MESSAGES...

* Trailing moisture from yesterday`s tropical wave will continue
  to support afternoon showers and thunderstorms over north-
  central to northwestern PR this late afternoon.

* Hazy skies are expected starting tonight into Monday due to the
  arrival of Saharan dust, which may lead to reduced visibility
  and deteriorated air quality.

* High heat indices expected again tomorrow, northeasterly winds
  could help promote somewhat lower temperatures on Tuesday.

&&

.SHORT TERM...Tonight through Tuesday...

The 12Z sounding detected a precipitable water (PWAT) value of 2.13
inches. Current satellite derived observations indicate PWAT values
above 2 inches, with drier air slowly filtering in from the east.
This trailing moisture from yesterday`s tropical wave, as well as
southeasterly steering flow, promoted high heat index values above
108 degrees over several coastal and lower elevation areas of the
islands. During the past hour strong thunderstorms also developed
over north-central to northwestern Puerto Rico. As of 242 PM AST,
accumulations have reached 0.8 to 1.10 inches. Predominant SE
steering flow will continue to bring tropical moisture over the area
with PWAT values remaining above 2 inches through at least tonight.
Showers and strong t-storms will continue to affect north-central to
northwestern PR during the afternoon hours, before gradually
dissipating and/or moving offshore. Late tonight into the overnight
hours PWAT values will sharply decrease, with more humid air
confined to the lower levels to start the workweek. This drier air
mass comes with Saharan Dust embedded in it, limiting passing shower
activity over windward sectors. This plume of saharan dust will
result in hazy skies and decreased rain chances through at least
Monday, with lingering concentrations early Tuesday. An upper trough
is also forecast to approach the area on Monday, increasing
instability in the upper levels. This, along with sea breeze
convergence, southwesterly flow, and local effects will promote
shower and t-storm activity, mainly over northwestern Puerto Rico.
Winds will back to become more northeasterly on Tuesday, which can
somewhat refresh temperatures and concentrate afternoon convection
over southwestern Puerto Rico during the afternoon hours. Overall
moisture will also increase, prompting passing showers over the
Atlantic towards windward sectors of the islands. 925 mb
temperatures will continue at near to above normal values, with the
lowest values forecast for Tuesday.

.LONG TERM...Wednesday through Sunday...

/FROM PREV DISCUSSION...Issued at 332 AM AST Sun Aug 3 2025/

For midweek next week into the early part of the weekend, weather
conditions are forecast to be relatively variable. Any shower and
thunderstorm activity will follow a typical seasonal pattern.
Wednesday will begin with above-normal columnar moisture and
marginal instability aloft. Passing showers will affect windward
coastal areas during the morning, followed by deeper convective
activity in the afternoon as the above-normal moisture combines with
surface heating, sea breeze convergence, and local effects. This
activity could lead to urban and small stream flooding, primarily
across the interior and western portions of Puerto Rico. On Thursday
and Friday, the areal coverage of showers is expected to decrease
due to mid-level dry air intrusion and decreasing low-level moisture
to below-normal levels, which will increase atmospheric stability.
While shower and thunderstorm activity will follow a similar pattern
to Wednesday, areal coverage is expected to be more limited.

During this period, model guidance suggests that 925 mb temperatures
will fluctuate near or above climatological normals. This will
result in hot surface temperatures, which, when combined with
sufficient low-level moisture, will lead to heat index values
exceeding 100F. As a result, at least a limited to elevated heat
threat is expected to impact coastal and urban areas, particularly
on Thursday. A localized significant heat threat during peak daytime
hours cannot be ruled out.

For the weekend, discrepancies begin to emerge in global model
guidance regarding the evolution of a tropical wave. Both the GFS
and ECMWF suggest increasing instability aloft due to the approach
of a Tropical Upper Tropospheric Trough (TUTT) north of the forecast
area. However, by the end of the weekend, the GFS depicts a vigorous
tropical wave combining with this instability as it approaches the
northeastern Caribbean. This scenario would result in a gradual
deterioration of weather conditions and an increased flooding threat
across the islands. In contrast, the ECMWF suggests some tropical
development of the wave, steering it northward before reaching the
Caribbean. Under this scenario, wetter conditions would be delayed,
and a more seasonal weather pattern would persist across the area.
Given the current uncertainty in model solutions, it is still too
early to determine any specific impacts for the region. Residents
are encouraged to stay alert and continue monitoring the forecast as
we move closer to the weekend.

&&

.AVIATION...
(18Z TAF)

Mainly VFR conditions should prevail, with brief MVFR conditions
possible over TJBQ through around 21Z, due to current
thunderstorm activity over north-central to northwestern PR. Lines
of convection are also forming from El Yunque and affecting TJSJ
through around 21Z. Light to moderate ESE winds will continue
decreasing after 03/22. A plume of Saharan dust can decrease
visibilities and bring HZ to the terminals tonight into tomorrow.

&&

.MARINE...

Trailing moisture from the tropical wave will continue to move over
the islands during the evening hours, before drier air filters in.
A surface high- pressure over the central Atlantic will promote
moderate to locally fresh winds through early this week. Hazy
skies are anticipated tonight into Monday due to the arrival of
Saharan dust embedded in the above mentioned approaching drier
air.

&&

.BEACH FORECAST...

A moderate risk of rip currents will continue through at least
tomorrow for northern and eastern PR, as well as Culebra and St.
Croix, with a low risk elsewhere. At least north-central to
northwestern PR are forecast to remain under a moderate chance of
rip currents after late monday. Even if the risk for rip currents
is low, life- threatening rip currents often occur in the
vicinity of groins, jetties, reefs, and piers. Deteriorated air
quality is forecast for late tonight through Monday as a plume of
saharan dust affects the islands.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning until 4 PM AST this afternoon for
     PRZ001>005-007-008-010-011.

     Heat Advisory until 4 PM AST this afternoon for PRZ012-013.

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

AM...None.
&&

$$

MRR/RVT/ERG/MNG