819
FXCA62 TJSJ 171905
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
305 PM AST Thu Jul 17 2025

.KEY MESSAGES...

* Showers and isolated thunderstorms will continue through this
  evening, increasing flooding potential over western/northwestern
  PR.

* A Saharan Air Layer (SAL) arriving into the region will bring
  hazy skies, reduce visibility, and deteriorate air quality,
  affecting mostly people sensitive to these particles through
  Friday evening. Lower concentrations will linger through the
  weekend.

* Warmer-than-normal temperatures will continue, with higher chances
  of heat indexes reaching Heat Advisory Criteria, over urban and
  coastal areas.

* A moderate risk of rip currents continues along north- and east-
  facing beaches of Puerto Rico and parts of the U.S. Virgin
  Islands Islands.

&&

.SHORT TERM...Tonight through Saturday

Moisture trailing the tropical wave that crossed the region
yesterday contributed to a variable to showery day across most of
the region. Diurnal heating and local effects, along with saharan
dust particles filtering in, promoted thunderstorm formation over
the region. Highest accumulations were observed over the western
metro area and over the western interior. Although showers and
cloudiness have affected the region, several areas with clearer
skies have reported heat indices in the 100s. A limited to
elevated heat risk was in effect. High Heat indices around 105
degrees persisted in areas without stronger shower activity.

Showers will continue to be steered towards windward sectors tonight
while showers/thunderstorms currently over the islands will
gradually dissipate and or move offshore. This will be due to, in
part, of the plume of Saharan Dust (with moderate to high
concentrations) filtering into the region. This will result in
hazy skies, reduced visibilities and deteriorated air quality on
Thursday and Friday evening, with lingering lower concentrations
during the weekend (long term period). As moisture erodes later
this evening, peak Saharan Dust concentrations are expected
tonight into early Friday morning, which will limit radiational
nighttime cooling. Consequently, Friday morning may begin with
warmer-than-normal minimum temperatures, potentially making it
easier to reach Advisory or Extreme Heat Warning criteria. In
general, 925 mb temperatures will remain at or above normal, with
maximum temperatures in the upper 80s to low 90s along the coastal
and urban areas with heat indices above 100 degrees each day.
Minimum temperatures will generally be in the 60s over the
interior to mountains to the upper 70s or low 80s over coastal and
urban areas. Lingering low concentrations of saharan dust are
forecast afterwards. Regardless, the typical rain pattern is still
possible, with passing showers in windward locations followed by
afternoon convection across western Puerto Rico and downwind from
the U.S. Virgin Islands.

The mid to upper level high pressure affecting the region, will be
gradually displaced throughout the period. A retrograding TUTT is
forecast to approach the region on Friday and move north of the
islands on Saturday, according to model guidance. This could lead
to the formation of a surface trough, potentially introducing more
tropical moisture over Puerto Rico and the US Virgin Islands.
Model guidance Precipitable water (PWAT) values will remain at 1.5
to 1.9 inches, with local patches of moisture reaching the area
and afternoon convection reaching around 2 inches. ESE steering
flow will back to become more easterly to possibly northeasterly
tomorrow, and then veer to become more southeasterly on Saturday.
Showers will be steered towards windward locations, followed by
afternoon convection (showers and possibly isolated thunderstorms)
over sectors of western Puerto Rico (depending on the steering
flow) and downwind from El Yunque and the US Virgin Islands. A
risk to observe ponding of water in roads and poorly drainage
areas, as well as localized flooding in urban areas, roads and
small streams, will mainly continue in the afternoons during the
rest of the period.

.LONG TERM...Sunday through Thursday...

At the beginning of the long term period, the TUTT low will be
positioned to the northwest of the region along with an induced low-
level trough expected to move across the northeastern Caribbean
through the weekend, bringing showery weather. The latest model
guidance suggests that precipitable water content will be up to 1.8-
2.0 inches on Sunday, which is considered average to above average.
In general, these weather features will enhance early morning
convection between the USVI and eastern sections of Puerto Rico,
followed by afternoon thunderstorms over portions of the interior
and western Puerto Rico. It that scenario materializes, there will
be an elevated flood risk across the northwestern quadrant of Puerto
Rico due to a southeasterly wind flow. On Monday, a mid to upper
ridge building from the central Atlantic into the northeastern
Caribbean will shift further west and displace the trough pattern.
Winds will shift more from the east. Mid-level moisture is expected
to decrease, and rainfall will likely follow a more typical diurnal
pattern driven by daytime heating and local effects. Flood risk
should remain from none to limited starting the week. Monday night
onward, another TUTT low will approach the region bringing a winds
surge and a series of disturbances with patches of moisture Tuesday
and Wednesday, likely embedded with Saharan dust. At the moment,
there is some discrepancies in the global models, but we will
continue to monitor conditions for any forecast update. At this
time, expect variable weather conditions those days with early
morning convection between the USVI and eastern sections of Puerto
Rico, followed by convection with isolated thunderstorms over
portions of the interior and western Puerto Rico each afternoon.
Model suggest precipitable water will fluctuate around 1.6 to 1.7
inches, which is considered seasonal values. By Thursday, moisture
content will decrease to below average values helping to reduce
shower frequency.

Additionally, warm to hot temperatures are anticipated each day,
likely triggering Heat Advisory conditions across most coastal and
urban areas of Puerto Rico. Residents and visitors are encouraged to
stay hydrated, avoid outdoor activity during peak heat hours, wear
light clothing, and check on vulnerable individuals and pets.

&&

.AVIATION...
(18Z TAFs)

Mostly VFR conds across all terminals. SHRA moving towards the
islands can promote brief MVFR conditions, especially over TJBQ
late in the afternoon (also due to TSRA). Winds from the ESE
between 15 to 20 kts with higher gusts through 17/23z, later
gradually decreasing to light and variable and backing to become
more E/ENE tomorrow, increasing again after 18/13Z. Hazy skies
will limit visibilities with Saharan Dust concentrations peaking
after 18/00z through tomorrow evening.

&&

.MARINE...

A surface high pressure system over the Western Atlantic
migrating eastward will promote moderate to locally fresh easterly
winds through tonight, becoming moderate tomorrow into the
weekend. Additionally, remnants of a tropical wave will result in
showers and isolated thunderstorms moving across the regional
waters and passages; small craft should exercise caution. A
Saharan Air Layer over the region will promote hazy skies and
reduced visibility through Friday evening, with lower
concentrations lingering over the weekend. A surface trough
approaching the region could potentially increase shower activity
across the waters. Winds will increase from Monday night onward.

&&

.BEACH FORECAST...

The beach forecast remains on track. The moderate risk of rip
currents continues across northern and eastern beaches of Puerto
Rico, Vieques, Culebra, and St. Croix in the U.S. Virgin Islands
and should reduce tomorrow and Saturday. Nevertheless, winds will
increase once again by Sunday, spreading to most beaches of Puerto
Rico and the U.S. Virgin Islands by the first part of the
workweek. If visiting beaches under moderate risk, be aware of
possible life-threatening rip currents along the surf zone.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

MRR/MNG