National Weather Service Today's Weather Impact Levels

436
FXCA62 TJSJ 311911 CCA
AFDSJU

Area Forecast Discussion...CORRECTED
National Weather Service San Juan PR
311 PM AST Sat May 31 2025

.KEY MESSAGES...

* An extensive and dense plume of Saharan Air Layer (SAL) will
  affect the local islands through midweek next week.

* Warmer than normal temperatures are likely to continue during
  the next few days across most coastal and lower elevations of
  the islands.

* An increase in surface winds will induce some moderate to
  locally fresh winds, resulting in wind-driven seas. Therefore,
  small craft operators should exercise caution.


&&

.SHORT TERM...Tonight through Monday...

The mid to upper level cloud deck that affected the islands during
the past couple of days has mostly moved away towards our east,
however some mid to upper level cloudiness has remained. These cloud
decks are mainly due a disturbance over South America and an upper
trough, that is now well north of the local islands. However,
another trough is moving in from our west and should linger through
the weekend. A mid-level ridge is also present and will remain
nearly stationary over the region during the period and will
continue to support a trade wind inversion (detected by the 12Z
sounding), effectively suppressing deep convective development
across Puerto Rico and the U.S. Virgin Islands. This pattern is
forecast to continue with drier air in the mid levels, as a Saharan
Air Layer (SAL) also moves in, cutting off moist air in the upper
levels and maintaining low level moisture below 800mb. Precipitable
water (PWAT) will be at seasonal values during the period, gradually
decreasing today and tomorrow. A wind surge will also cross the
region from today into Sunday and will increase moisture
advection towards windward sectors while another increase in winds
is forecast late in the period and into the long term forecast
period. In general up to breezy east to east-southeast winds will
continue due to a surface high over the Atlantic. A dense Saharan
Air Layer (SAL) is soon to arrive, its bulk is currently reaching
the Windward and Leeward Islands, and it`s leading edge is
filtering into the local islands. Current model guidance continues
to indicate that the bulk of this SAL will move into the area
this evening from the east/southeast, further deteriorating air
quality and resulting in hazy skies, drier air aloft, lower
visibilities and adverse conditions for immunocompromised,
vulnerable and sensitive groups. This is a moderate to high SAL
event, please follow your doctor`s and your Health Department`s
recommendations.

Despite the Saharan Dust and accompanying dry air, above mentioned
instability and diurnal heating can continue to promote shower
and isolated thunderstorm development, with afternoon activity
concentrating over NW PR and downwind of El Yunque and the local
islands. Overnight and morning trade wind showers will also
continue over windward sectors of the islands. Breezy east- to-
east-southeast winds will continue to promote a limited to
elevated heat threat for urban and coastal areas, the presence of
Saharan Dust and the absence of the mid to upper level cloud deck
that was over the islands during the next few days will also
contribute to this heat threat. Drink plenty of fluids, stay in an
air-conditioned room, stay out of the sun, and check up on
relatives and neighbors. Take extra precautions when outside. Wear
lightweight and loose fitting clothing. Try to limit strenuous
activities to early morning or evening. Take action when you see
symptoms of heat exhaustion and heat stroke. Please refer to the
latest Hazardous Weather Outlook (HWOSJU) and it`s graphical
format (weather.gov/erh/ghwo?wfo=sju) for more information.

.LONG TERM...Tuesday trough Saturday...From Prev Discussion...

The long-term forecast remains on track. According to the NASA
Goddard Earth Observing System Model V5, a dense plume of Saharan
Air Layer (SAL) will continue to affect the northeastern Caribbean
through midweek. The model indicates that the highest
concentrations are expected early in the period, with aerosol
optical thickness values nearing 0.5. This pattern will result in
hazy skies, reduced visibilities, and deteriorated air quality
through at least Wednesday. Although the most intense dust
presence is forecast for the first part of the week, low to
moderate concentrations are expected to linger into the upcoming
weekend. Residents and visitors, particularly those with
respiratory conditions, are advised to take appropriate health
precautions during this period. At the surface, a high-pressure
system will promote southeasterly winds through Tuesday, gradually
shifting from the east as the high moves into the central
Atlantic.

A mid- to upper-level ridge is forecast to strengthen over the
region during the early part of the week, favoring a drier and
more stable environment aloft. Precipitable water values will
remain below climatological normals, near or below 1.5 inches,
limiting overall shower activity across the area. However, locally
induced showers and isolated thunderstorms remain possible each
afternoon across western Puerto Rico, due to diurnal heating and
local effects. Temperatures will remain above normal, with highs
ranging from the upper 80s to low 90s Fahrenheit in coastal and
urban areas, and in the low 80s across higher elevations. Heat
indices may exceed 108F during the peak of the event,
particularly across north-central Puerto Rico. These hot and dry
conditions, combined with poor air quality, could pose additional
health risks, especially for the more vulnerable populations.


&&

.AVIATION...

(18Z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, brief SHRA/TSRA this afternoon
may cause tempo MVFR cigs at TJBQ/TJSJ thru 31/22z. The 31/12z
TJSJ sounding indicated ESE winds up to 19 kt blo FL100. HZ due to
Saharan dust will gradually move across the local area tonight,
and linger through at least early Wednesday. VSBY could drop to
around 6SM during the next few days.


&&

.MARINE...

A surface high pressure building over the Atlantic Basin will
continue to promote moderate to locally fresh trade winds over the
next few days. Southeast winds will prevail through the weekend,
becoming more easterly early next week. Small craft should
exercise caution across portions of the offshore waters and local
Caribbean passages. Hazy skies and reduced visibilities are
expected through mid-week as another dense plume of Saharan dust
moves toward the area.


&&

.BEACH FORECAST...

A moderate risk of rip currents will continue across the south to
southeast beaches of the islands through the rest of the weekend.
Elsewhere, the risk remains low. Nonetheless, swimmers are urged
to use caution, especially near piers, reefs, and other structures
where dangerous rip currents are more likely to form unexpectedly.


&&

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

VI...None.
AM...None.
&&

$$

SHORT TERM...MRR
LONG TERM...LIS
AVIATION/MARINE/BEACH...DSR

NWS SJU Office Area Forecast Discussion