017
FXCA62 TJSJ 151857
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
257 PM AST Sun Jun 15 2025

.KEY MESSAGES...

* An upper-level low will meander north of the region through
  early in the workweek. This feature will support the development
  of showers and thunderstorms across local waters and portions of
  the islands.

* Hazy skies are expected through most of the workweek due to a
  weak Saharan Air Layer. This may lead to slightly reduced
  visibility and a decline in air quality across the region,
  particularly on Monday and Tuesday.

* Winds are expected to increase by midweek, resulting in breezy
  to locally windy conditions across coastal areas. Choppy seas
  are likely, especially in exposed waters.


&&

.SHORT TERM...Today through Tuesday...

Mid-to upper-level cloudiness persisted once again over the region
under the influence of a developing upper-level low to our north
and a broad upper-level ridge over the southeastern Caribbean Sea.
Shower and thunderstorm activity persisted mainly across the
offshore Caribbean waters and the Mona Passage. Although it was
mostly cloudy over land areas, little to no rainfall activity was
detected during the morning and early afternoon hours. However,
the onset of afternoon convection is delayed due to the
cloudiness, and showers are still expected to develop late this
afternoon over portions of western PR. Maximum temperatures were
from the mid- to upper-80s in general across the lower elevations
to the low 90s across the southern coast of Puerto Rico. The wind
was from the east to southeast at 14 to 18 mph with sea breeze
variations and higher gusts.

Latest global model guidance suggest a faster solution regarding
the position of the upper-level low, with the low now shifting
further westward by Tuesday, and drier air moving earlier than
expected across the eastern Caribbean. This pattern suggest that
the best window for shower and thunderstorm development is from
tonight through Monday afternoon. Meanwhile, the broad surface
high pressure over the central Atlantic will continue to promote
moderate to locally fresh trades through the short-term period.
Hazy skies will prevail across the islands through most of the
workweek, due to a weak Saharan Air Layer bringing minor to
moderate concentrations of suspended Saharan dust.

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

/From Prev Discussion issued at 507 AM AST Sun Jun 15 2025/

Model guidance indicates changes in the forecast, as the TUTT low is
now expected to move away from the region much faster than suggested
in previous model cycles. By Wednesday, the TUTT low is forecast to
be positioned northwest of Puerto Rico, with upper-level dynamics
showing reduced favorability for deep convective activity compared
to earlier runs. 500 mb temperatures are now projected to range
between -5 and -6 Celsius, which is slightly warmer than the
climatological normal. This will result in marginal instability,
which is expected to persist through the remainder of the long-
term forecast period. As a result, POPs have been slightly reduced
to reflect the expected decrease in convective potential.
However, further adjustments may be necessary as models come into
better agreement.

The determining factor for shower activity will be moisture
availability, with significant fluctuations in precipitable water
values. These values are expected to range from as low as 1.45
inches on Wednesday to nearly 2.0 inches by Thursday, with a similar
pattern expected to repeat from Friday through Sunday. Moisture will
remain mostly concentrated south of the local islands, with
intermittent patches of drier and more humid air streaming across
the region.

Surface high pressure over the central Atlantic will maintain an
east-southeast wind flow across the northeastern Caribbean. The
local pressure gradient is also expected to tighten, resulting in
the return of breezy to locally windy conditions across coastal
areas through the long-term forecast period. As a result, the
weather pattern will favor shower and thunderstorm activity that
follows a typical diurnal cycle, with overnight and morning showers
over windward coastal areas, followed by afternoon convection
developing over interior and western Puerto Rico. The areal coverage
of this activity will largely depend on available moisture.

The prevailing east-southeast wind flow will also contribute to
warmer temperatures across the local islands. The heat threat will
largely depend on moisture content, with the greatest risk occurring
on days with high humidity, as this will significantly increase heat
index values and the potential for heat-related impacts. Residents
and visitors are advised to take precautionary measures during peak
heating hours, including staying well-hydrated, wearing light and
breathable clothing, limiting direct sun exposure, and taking
regular breaks when engaging in outdoor activities.


&&

.AVIATION...

(18Z TAFS)

Mainly VFR conditions expected across most terminals. However,
SHRA/TSRA expected near TJBQ thru 15/23z which may cause brief
MVFR conds. Additional SHRA/TSRA expected to develop overnight
into early Monday morning across portions of the local waters,
which could cause tempo MVFR conds at TIST/TISX/TJSJ. ESE winds up
to 21 kt blo FL050 with higher gusts and sea breeze variations
thru 15/21z, bcmg light and variable overnight.


&&

.MARINE...

The surface high pressure north of the region will continue to
gradually shift east over the coming days. While some fluctuations
in the wind pattern are expected, winds will generally remain from
the east to east-southeast through the period, gentle to moderate
through Tuesday, then increasing to moderate to locally fresh by
Tuesday night across the regional waters. As winds increase, seas
will become choppy, especially in exposed areas. A plume of suspended
Saharan dust is expected to linger through midweek, potentially
causing a slight reduction in visibility. Meanwhile, increasing
moisture and instability associated with an upper-level trough
will likely lead to a rise in thunderstorm activity from tonight
through Monday.


&&

.BEACH FORECAST...

There is a moderate risk of rip currents across most beaches
exposed to the prevailing winds. While some western beaches tend
to be more sheltered, caution is still advised near jetties,
piers, and channels where stronger currents may still develop.


&&

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

$$

DSR/GRS