848
FXCA62 TJSJ 290921
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
521 AM AST Sun Jun 29 2025

.KEY MESSAGES...

* A tropical wave will continue to cross the islands today,
  promoting increased cloudiness, showers and thunderstorm
  activity through this afternoon.

* Up to moderate concentrations of Saharan Dust, will move in
  after the wave, leading to hazy skies and deteriorated air
  quality through Monday, with lingering dust after.

* Breezy winds will sustain choppy seas and maintain a moderate
  risk of rip currents along exposed beaches over the next few
  days.

* Another tropical wave is forecast to affect the islands on
  Friday, again increasing shower and thunderstorm activity.

&&

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

Showers and isolated thunderstorms associated with a tropical wave
crossing the islands developed mostly over the regional waters,
particularly the Caribbean waters. Some of these showers moved
inland, but rainfall accumulations were minimal as they moved
through the area fairly quickly. Minimum temperatures were in the
mid to upper 70s across the lower elevations of Puerto Rico and in
the low 80s across the U.S. Virgin Islands. Gusty wind conditions
were reported, with San Juan weather stations recording wind gusts
up to 30 mph throughout the night. These breezy conditions will
persist today through midweek, as the pressure gradient between the
Bermuda High and lower pressure over the central Caribbean
strengthens during the period. Shower activity will continue to
stream westward across Puerto Rico as the wave gradually exits the
region throughout the day.

Thereafter, a broad area of reduced humidity and stable air is
expected to move in from the east. However, residual moisture from
the tropical wave, combined with marginal instability from an upper-
level low northwest of the forecast area, along with daytime heating
and sea breeze convergence, will lead to the development of showers
and thunderstorms over portions of central and western Puerto Rico
this afternoon. These thunderstorms could produce gusty winds and
locally heavy rainfall, which may result in urban and small-stream
flooding.

Increased cloud cover over the islands may help limit the heat risk
somewhat today. However, any clearing of clouds could result in
elevated heat index values as high temperatures combine with
elevated dewpoints leading to a limited to locally moderate risk of
excessive heat across most coastal areas. We will continue to
monitor weather conditions in case a Heat Advisory becomes necessary
due to clearing skies during the peak heating hours of the day.

A Saharan Air Layer and a building mid-level ridge will result in
stable weather conditions across the islands beginning this evening
and continuing through the remainder of the forecast period. These
conditions will likely promote a strong inversion cap, primarily
near the 850 mb level, which will inhibit vertical mixing and
suppress shower development for the most part. Low to moderate
concentrations of suspended Saharan dust will lead to hazy skies,
reduced visibility, and deteriorating air quality,  potentially
affecting vulnerable and sensitive groups. Although the upper-level
ridge and Saharan Air Layer will suppress widespread rainfall
activity, local effects could still trigger isolated to scattered
afternoon convection on Monday and Tuesday. Additionally, patches of
low-level clouds carried by the trade winds may bring brief passing
showers to windward areas, particularly on Tuesday.

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

Surges of moisture will result in increases in precipitable water
(PWAT) content on Wednesday as they are steered towards the islands,
but overall PWAT values will be very variable as surrounding drier
air is also present. Available moisture can reach up to around 700
mb, contrasting the more pronounced cap that will be present to
end the short term period. The diurnal shower pattern can still be
limited on Wednesday and for most of Thursday, with passing
morning showers and afternoon showers and isolated thunderstorms
over western PR, where a limited flooding risk will remain. By
Thursday through late Friday, a gradual increase in PWAT is
forecast, especially on Friday as values reach above 2 inches.
Current model guidance, has most if not all of the region with 2
inches or more of PWAT. Available moisture at this time will also
be present in the mid to upper levels. Late Thursday through
Friday is expected to be the wettest period, as a tropical wave
approaches the islands and a retrograding mid- to upper-level
trough is also nearby. This will promote a boost in the diurnal
pattern and overall increased shower and t-storm activity over the
region, a limited to elevated flooding risk will remain. PWAT
values will quickly drop to below normal by Saturday before
bouncing back to around 1.90 in by Sunday as moisture remains of
the wave are circled back towards the islands by the surface high.
This surface high over the Atlantic will also promote up to
breezy east to east-southeast steering flow during the period. 925
mb temperatures will be above normal with a limited to possibly
elevated heat risk each afternoon. The presence of saharan dust,
increasing as the period continues, can also serve to limit
nighttime cooling.

&&

.AVIATION...
(06Z TAFS)

VFR conds will persist through much of the forecast. However, a
tropical wave will increase SHRA/TSRA across the local flying areas,
affecting the terminals at times. SHRA/TSRA will impact TISX through
13Z, spreading into PR late this afternoon into the evening causing
brief MVFR conditions. SHRA/TSRA will dissipate or move away aft 22Z
across TAF sites. Low to moderate concentrations of Saharan dust
will result in HZ skies, with visibility occasionally reduced to 5-6
SM. Winds will continue from ESE at around 15-20 kt but gusty near
SHRA/TSRA and with sea breeze.


&&

.MARINE...

Mariners can expect choppy seas across the local Atlantic and
Caribbean waters due persistent moderate to fresh easterly winds.
A tropical wave is expected to move across the eastern Caribbean
today, Sunday, increasing the potential for showers and
thunderstorms across the region. Another pulse of drier air with
suspended Saharan Dust Particles will follow, maintaining hazy skies
through Tuesday.


&&

.BEACH FORECAST...

A moderate risk of rip currents (life-threatening rip currents
are possible in the surf zone) will be in effect for most
northern and southeastern beaches of Puerto Rico, as well as for
beaches of Culebra, Vieques, southern St. John, southern St.
Thomas and all but western St. Croix. This moderate risk will
spread to areas of southwestern Puerto Rico tonight. Although
other areas have a low risk of rip currents, keep in mind that
life-threatening rip currents often occur in the vicinity of
groins, jetties, reefs, and piers. Up to a moderate risk of rip
currents will remain for most of the period.

&&

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

$$

SHORT TERM...CVB
LONG TERM....MRR