520
FXCA62 TJSJ 151857
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
257 PM AST Tue Jul 15 2025

.KEY MESSAGES...

* Hot temperatures, high humidity content, and southeasterly winds
  will promote an elevated heat risk across the lower elevations
  and urban areas of Puerto Rico through the end of the week.

* Another, more defined, tropical wave will increase shower and
  thunderstorm activity tomorrow.

* A plume of moderate to high concentrations of Saharan Dust will
  reach the islands, leading to hazy skies and deteriorated air
  quality. This will be particularly noticeable on Thursday and
  Friday, with lingering concentrations on Saturday.

* Increasing winds will lead to choppy seas and up to a moderate
  risk of rip currents.

&&

.SHORT TERM...Tonight through Thursday...

The shower activity continued during the morning, mainly over the
local waters moving inland. Based on Doppler Radar rainfall
estimates, some areas, including the San Juan Metropolitan Area,
received around 0.5 - 0.75 inches. The 12z sounding showed an
increase of low to mid relative humidity up to 80 - 84%, with
Precipitable Water values of 1.89 inches above climatological
normal. Nevertheless, the sounding`s 500 mb temperature was warmer
than normal, due to the mid-level ridge north of the region.
Convection activity already started, with scattered showers moving
over the Cordillera Central into northwestern portions of Puerto
Rico. Periods of locally heavy rainfall will enhance ponding of
water in roadways, urban and poorly drained areas, with a low chance
of urban flooding. Weather conditions should gradually improve
tonight, with a few passing showers moving across the regional
waters and some across windward sections.

A surface high pressure strengthening over the Western Atlantic will
promote breezy conditions for the rest of the forecast period. The
mid to upper level ridge should linger north of the CWA for the next
few days.  PWAT values should slightly increase tomorrow as another
tropical wave more defined located along 53W south of 20N approaches
the local islands, increasing the frequency of showers. 500 mb
temperatures should chill to seasonal values (-6 to -7 degrees
Celsius), which aids the development of thunderstorm activity.
Additionally, the Galvez-Davison Index suggests there`s a potential
for isolated thunderstorms in the afternoon. Hence, expect showers
moving over eastern Puerto Rico and the U.S. Virgin Islands in the
morning, and convection activity in the afternoon over the interior
and western portions of Puerto Rico. Rainfall accumulations will
increase the flooding risk; therefore, urban and small streams
flooding is very likely over the aforementioned areas. A dense SAL
is anticipated to approach the local islands by Thursday morning,
with moderate to high concentrations based on NASA GMAO. These
concentrations should limit deep convective activity, bringing hazy
skies, reduced visibility, and deteriorated air quality.

Seasonal to above normal temperatures and high moisture content will
keep the elevated risk of heat for the rest of the short term,
and it`s very likely to meet Heat Advisory Criteria.

&&

.LONG TERM...Friday through Tuesday...

/PREV DISCUSSION... /issued 500 AM AST Tue Jul 15 2025/

Hazy skies, reduced visibility, and poor air quality conditions will
continue through Friday due to a lingering Saharan Air Layer.
Winds will acquire a northeasterly component as a broad TUTT-
induced trough approaches the eastern Caribbean. The combination
of the limited moisture content with daytime heating and the sea
breeze convergence will trigger afternoon showers with isolated
thunderstorms over the southwest quadrant of PR. An advective
weather pattern at night will then promote showers across the
local waters and over portions of the USVI and northeastern PR.

The TUTT-low and the induced low-level trough are expected to
move across the northeastern Caribbean through the weekend. This
will bring an increase in shower activity once again across the
islands. A tropical wave is expected to move across the region on
Monday, followed by another SAL event on Tuesday. In general,
these weather features will enhance the early morning convection
between the USVI and eastern sections of PR, followed by afternoon
thunderstorms over portions of the interior and western PR. Even
though the SAL is expected to bring hazy skies and drier mid-level
air, the precipitable water content is forecast to range between
normal to above normal levels through the end of the long-term
period.

Meanwhile, a deep layer ridge building from the central Atlantic
into the northeastern Caribbean will push further west and away of
the area the trough pattern and promote southeasterly winds by
Saturday afternoon. This moist southeast flow will prevail through
at least Tuesday, and hot temperatures are expected each day.
Likely triggering Heat Advisory conditions across most coastal
areas of PR.

&&

.AVIATION...

(18z TAFs)

Mostly VFR conds across all terminals. -RA/+RA in JQB due SCTD SHRA
over NW Puerto Rico, with chance of VCTS. Winds from the NE btwn 16 -
18 kt with gusts up to 26 kt will cont until 15/23z, grdly decreasing
btwn 8 - 12 kt. VCSH for all TAF sites is likely after 16/12z.

&&

.MARINE...

A surface high pressure will strengthen over the western Atlantic
early this week. While the current tropical wave will depart late
tonight, another is expected to affect local waters tomorrow,
Wednesday. Consequently, moderate to locally fresh trade winds and
choppy seas will persist across the regional waters through midweek.
Increased shower and thunderstorm activity is expected with the
passage of these tropical waves. Saharan dust will lead to hazy
conditions and reduced visibility on Thursday and Friday, with
lingering concentrations on Saturday.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents for the north and east
facing beaches of Puerto Rico as well as for most beaches of
Culebra, Vieques, and St. Croix. This moderate risk will spread to
the southeastern beaches of Puerto Rico. Up to a moderate risk of
rip currents is forecast through the workweek. Although other areas
have a low risk of rip currents, these often occur in the vicinity
of groins, jetties, reefs, and piers. The potential for isolated
thunderstorm development will continue  tomorrow across most coastal
areas due to another tropical wave.  Saharan dust will result in
hazy conditions and reduced visibilities on Thursday and Friday,
with lingering concentrations on Saturday.

If caught in a rip current, stay calm and yell for help. Conserve
your energy by floating until assistance arrives. If you must swim,
do so parallel to the shore until you are out of the current, then
swim back to the beach. Do not attempt to swim directly against the
current, as this will quickly exhaust you.

&&

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

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

$$

SHORT TERM/AVIATION...MNG
LONG TERM...DSR
MARINE/BEACH FORECAST...MRR