063
FXCA62 TJSJ 300912
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
512 AM AST Mon Jun 30 2025

.KEY MESSAGES...

* Moderate concentrations of Saharan Dust will continue to filter
  in, leading to hazy skies and deteriorated air quality through
  Monday, with lingering dust after. Another plume of Saharan Dust
  can reach the region to end the week.

* Today, a limited heat risk will be present for urban and lower
  elevation areas of the islands while the southern coastal plains
  will have elevated fire weather conditions.

* 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 Wednesday...

Since midnight, only minimal rainfall accumulations have been
detected across the islands. Minimum temperatures have ranged from
the upper 60s in the higher elevations of Puerto Rico to the upper
70s and warm low 80s at lower elevations across the islands. Current
satellite-derived precipitable water (PWAT) values indicate up to
around 1.4 inches over eastern Puerto Rico and the nearby islands,
gradually decreasing to about 1.30 inches over western Puerto Rico.

A mid-level ridge will build over the islands, promoting a dry air
layer between 850 and 700 mb. At the surface, high pressure over the
central Atlantic will maintain a tight pressure gradient across the
northern Caribbean, resulting in breezy wind conditions once again.
However, this pressure gradient is expected to gradually weaken by
midweek, leading to reduced breezy conditions through the rest of
the period. Saharan dust is embedded within the dry air layer,
contributing to hazy skies through at least Tuesday. The mid-level
ridge is also expected to hold through at least Tuesday, maintaining
a trade wind inversion around 850 mb. This will limit widespread
rainfall development, with only isolated afternoon convection
possible over western portions of Puerto Rico.

On Wednesday, the mid-level ridge will begin to erode as a mid-to-
upper-level low northeast of the forecast area deepens and
approaches the region. However, relative humidity in the 700-500 mb
layer will remain below normal for this time of year, which could
inhibit vertical mixing.  At lower levels, patches of moisture
embedded within the trade winds will move over the area, causing
precipitable water values to fluctuate between 1.15 and 1.80 inches,
ranging from well below to above normal. As a result, an increase in
shower activity may occur on Wednesday due to marginal instability,
although it will be closely tied to the availability of moisture.
Mid-level dry air may limit deep convection overall, but isolated
development cannot be ruled out during the afternoon over western
Puerto Rico.

A limited heat threat will be present across urban and coastal
areas, particularly during the late morning and early afternoon
hours. This will be more noticeable when humidity levels increase,
resulting in higher heat indices. While not expected to reach
advisory criteria, residents in these areas should still take
precautions, especially during prolonged outdoor activity.

.LONG TERM...Thursday through Monday...

Although Thursday will start with below normal concentrations of
precipitable water (PWAT), peripheral moisture from an approaching
tropical wave will increase PWAT to around 1.8 inches by Thursday
afternoon and evening. A surface high over the Atlantic will
promote up to breezy east to east-southeast steering flow through
Saturday, backing thereafter. The diurnal shower pattern will be
limited for most of Thursday, with passing morning showers and
afternoon showers and stronger showers over western PR. Available
moisture will remain below 850 mb on Thursday. However, deeper
moisture will reach the area on Friday as PWAT values increase to
2 inches or more. Therefore, Friday is expected to be the wettest
day, as a tropical wave crosses the islands and a retrograding
mid- to upper-level low also approaches. 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. Galvez-Davidson Index (GDI) values are forecasted to
reach +45 on Friday, meaning that thunderstorm and heavy rainfall
potential will remain. PWAT values will quickly drop to below
normal by Saturday before bouncing back to around 1.8 in by Sunday
through Monday as moisture remains of the wave are circled back
towards the islands by the surface high. The mid- to upper- level
low will continue to be near the islands during the weekend and
into early next week, promoting some instability. 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. This saharan dust can also serve as a limiting
factor for Friday`s tropical wave.

&&

.AVIATION...
(06Z TAFS)

Mainly VFR conditions are expected across the terminals during the
next 24 hours. Saharan dust will continue filter into the region.
Low to moderate concentrations of Saharan dust will result in HZ
skies, with visibility occasionally reduced to 5-6 SM. Winds will be
more ESE at up to around 15-20 kts with higher gusts, shifting to
ENE aft 22Z up to around 8-12 kts.

&&

.MARINE...

Mariners can expect choppy seas across the local Atlantic and
Caribbean waters due persistent moderate to fresh easterly winds.
Small Craft Should Exercise Caution over most local waters. A mass
of drier air with suspended Saharan Dust Particles will continue to
filter into the area, maintaining hazy skies through Tuesday. A
tropical wave is forecast to move over the local waters on Friday,
increasing shower and thunderstorm chances.

&&

.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, southwestern 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. 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. The low risk of risk of rip
currents will spread tomorrow to St. Thomas and St. John. Up to a
moderate risk of rip currents will remain for most of the period.

&&

.FIRE WEATHER...

Little to no rainfall is anticipated across the southern sections
of Puerto Rico. Dry air with Saharan Dust particulates is also
filtering into the region. Relative humidities can drop to near
44 to 50%. Wind speeds can reach around 16 to 21 mph with gusts
near 30 mph. KBDI values in Guanica and Cabo Rojo remains above
critical fire weather thresholds (661 and 628 respectively).
Therefore, weather conditions should be favorable for the spread
of wildfires along the Southern Coastal Plains of Puerto Rico.

&&

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

$$

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