223
FXCA62 TJSJ 170906
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
506 AM AST Thu Jul 17 2025

.KEY MESSAGES...

* Lingering moisture from a tropical wave will result in showers and
  isolated thunderstorms today, especially across the interior and
  western Puerto Rico during the afternoon hours.

* Moderate to high concentrations of Saharan dust particles are
  expected to gradually return today and prevail through at least
  Friday night, resulting in hazy skies, reduced visibility, and
  poor air quality.

* Limited heat risk today across urban and coastal areas of the
  islands. An elevated heat risk is expected over the next few days,
  with heat indices likely to meet Heat Advisory Criteria.

* Moderate to locally fresh winds will lead to choppy seas and a
  moderate risk of rip currents through Friday, an improvement is
  expected during the weekend through early next week, then
  increasing winds and a moderate risk returning by Tuesday.

&&

.SHORT TERM...Today through Saturday...

The skies were mostly clear across PR`s west, south and interior
sections, while north and east windward locations in PR and the US
Virgin Islands observed partly cloudy skies. Once again, overnight
minimum temperatures ranged from the low 60s in the mountains to the
upper-70s or low 80s in coastal urban areas. The lingering moisture
of yesterday`s tropical wave caused the Doppler Radar to detect
scattered to numerous showers across the surrounding waters and
moving inland over the USVI and the windward locations of PR. Winds
were mainly from the east at 10 mph or less, with land breeze and
locally higher gusts near showers.

The trailing moisture of yesterda`s tropical wave will bring
additional rounds of showers during the morning hours, before the
arrival of a Saharan Air Layer (SAL) with moderate to high
concentrations of suspended dust particles later this morning into
the Friday. Regardless of the expected hazy skies associated with
the SAL, the lingering moisture will result in shallow rain showers
affecting potions of the windward locations in the USVI and PR
during the day. Then, local effects, diurnal heating and sea breeze
variations will promote the formation of afternoon showers downwind
from the USVI and the Luquillo Mountain Range, as well as across the
interior and western PR. This activity will result in ponding of
water in roads and poorly drained areas, without ruling out the
potential for urban and small stream flooding. One or two intense
thunderstorms could develop this afternoon, enhanced by the presence
of the suspended dust particles. Tempereature-wise, we expect muggy,
warm to hot heat indices during the maximum heating today, posing a
particular danger to the most vulnerable communities.

Moisture will erode later this evening, as we forecast the SAL
concentrations to peak tonight into early Friday morning, limiting
radiational nighttime cooling. Thus, Friday morning may begin with
warmer-than-normal minimum temperatures, making it easier to reach
Advisory or Extreme Heat Warning Criteria. Regardless, we cannot
rule out the typical rain pattern, consisting of passing showers
across the windward locations, followed by afternoon convection
across the western portions of PR and downwind from the US Virgin
Islands.

Model guidance suggest a retrograding TUTT will move into the region
around Saturday. This will induce a surface trough, potentially
bringing additional tropical moisture over Puerto Rico and the US
Virgin Islands. Under these conditions, there is a moderate chance
of wet and unstable weather, which could lead to a heightened risk
of flooding rains in the afternoon and evening on Saturday.
Residents and visitors in PR and the USVI should stay tuned for
further updates regarding this weather pattern.

.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...
(06Z TAFs)

Expect occasional SHRA/+SHRA across the local terminals of E-PR/USVI
thru 17/16z. Local effects and sea breeze variations will result in
afternoon SHRA/TSRA across the interior and western PR, which could
impact JPS/JBQ or even JSJ. This activity could result in brief MVFR
or even IFR conditions, especially between 17/16-22z. Hazy skies
will return later this morning, limiting visibilities, from 17/12z,
peaking after 18/00z. Expect calm to light and VRB winds thru
17/13z, then E/ESE between 15-20 kt through 17/13z with higher gusts
near 25-30kt and sea breeze variations.

&&

.MARINE...

A surface high pressure system over the Atlantic will promote
moderate to locally fresh easterly winds through tonight across the
regional waters and local Caribbean passagesexpect choppy seas as
well. Remnant moisture from a tropical wave will result in showers
and isolated thunderstorms today. Additionally, a Saharan Air Layer
will gradually arrive today, bringing hazy skies through Friday
night. Another disturbance is anticipated by the weekend,
potentially increasing shower activity across the waters. Winds will
remain moderate through the weekend, increasing again from Monday
night onward.

&&

.BEACH FORECAST...

Today, there is a moderate risk of life-threatening rip currents
across the northern and eastern beaches of Puerto Rico, Vieques,
Culebra, and St. Croix. A low risk is expected elsewhere; however,
life-threatening rip currents may still occur, especially near
groins, jetties, reefs, and piers. Always swim near a lifeguard and
follow the advice of local beach patrols and flag warning systems.

Similar conditions will likely persist tomorrow, Friday. After that,
a low risk is mainly expected through early next week, with some
exceptions across northern exposed beaches. Increasing winds
starting around Tuesday may once again raise the risk to moderate.

Hazy conditions are expected to persist through at least Friday,
with heat index values likely exceeding 100F. Stay well hydrated,
seek shade frequently, and avoid outdoor activity during peak sun
hours.

&&

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

$$

SHORT TERM/AVIATION...CAM
LONG TERM/MARINE/BEACH FORECAST...YZR