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706
FXCA62 TJSJ 311851
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
251 PM AST Fri Jul 31 2026

...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER...

.KEY MESSAGES...
Issued at 245 PM AST Fri Jul 31 2026

- Showers and thunderstorm will continue to affect the western
  half of Puerto Rico due to the passage of an easterly
  disturbance. Expect ponding of water over roads.

- A tropical wave will move across the region on Saturday,
  increasing the chance of showers and thunderstorms through the
  weekend, which may result in localized flooding and disruptions
  to outdoor activities.

- Low concentrations of Saharan dust will persist through Sunday
  night before increasing to moderate levels on Monday, which may
  reduce air quality and visibility while aggravating respiratory
  conditions for sensitive individuals.

- Hot conditions will return by Sunday across Puerto Rico and the
  U.S. Virgin Islands, increasing the risk of heat-related
  illnesses, especially for vulnerable populations, outdoor
  workers, and anyone spending extended periods outdoors.

&&

.Short Term(This evening through Sunday)...
Issued at 245 PM AST Fri Jul 31 2026

Variable conditions prevailed this morning and through the early
afternoon as a wind perturbation increased moisture from the
surface to at least 700 mb. Combined with the influence of an
upper-level low west of the region, these conditions have produced
passing showers across windward areas and a greater coverage of
afternoon convection across north-central and western Puerto Rico.
Based on radar estimates, maximum rainfall amounts of 0.30 to
0.50 inches fell earlier today. Around 1 PM AST, a few thunderstorms
developed, producing localized rainfall totals of 1 to 2 inches
across north-central and western municipalities. These estimates
are based on data available through 130 PM AST, with additional
rainfall still occurring. Daytime high temperatures ranged from
the mid 80s to the low 90s, with heat indices reaching between
100 and 110F across localized, typically warmer areas. The latest
NASA GEOS Dust Aerosol Optical Depth (AOD) analysis suggests that
only traces of Saharan dust are present today, and similar
conditions are expected to persist through the forecast period,
providing a temporary break from more significant dust
concentrations.

At the surface, high pressure over the central Atlantic is
maintaining an easterly wind flow. For the remainder of the
afternoon, scattered to numerous showers and isolated
thunderstorms are expected to continue, particularly across the
western interior and western Puerto Rico. A limited to elevated
flood risk remains in effect across western Puerto Rico due to the
potential for ponding of water on roads and in poorly drained
areas, as well as localized urban and small stream flooding.
Tonight through early Saturday, the latest model guidance suggests
a drying trend before the arrival of the next tropical wave.

The likelihood of rainfall will increase over the weekend, with
the greatest potential expected on Saturday and possibly extending
into Sunday. By late Saturday morning, moisture associated with a
tropical wave is forecast to move into the region, accompanied by
lighter easterly to southeasterly winds. Precipitable water
(PWAT) values are expected to rise above normal, exceeding 2
inches on Saturday, with deeper moisture extending into the mid
and upper levels of the atmosphere, according to the latest GFS
cross-section analysis. At the same time, 500-mb temperatures are
forecast to cool to around -7C, creating a more favorable
environment for thunderstorm development. A limited to elevated
flood risk will remain in place, especially since the lighter
steering flow may allow showers and thunderstorms to move more
slowly, resulting in higher rainfall accumulations. Modest
moisture will persist, while the upper-level low lingers west of
Puerto Rico on Sunday. As a result, afternoon showers and
thunderstorms are expected to continue, with the greatest flooding
threat focused across the interior, western, and northwestern
portions of Puerto Rico, as well as possibly in areas downwind of
El Yunque.

In addition, 925-mb temperatures are forecast to remain near to
slightly above normal on Sunday, which may increase the likelihood
of heat-related products being issued. Continue to monitor the
forecast for updates as confidence increases and forecast uncertainty
decreases.

&&

.Long Term(Monday through Friday)...
Issued at 405 AM AST Fri Jul 31 2026

Mainly drier and warm-to-hot conditions are expected in the long-
term forecast, with the best chance of flooding on Monday and
Wednesday. The CWA should continue under an ESE-SE wind pattern
due to surface high pressure over the Central Atlantic. As
mentioned in the previous discussion, Monday should have been a
transition day due to lingering moisture from a tropical wave and
an upper-level low, with conditions becoming drier than normal
during the morning hours as a dense plume of SAL with moderate
concentrations reaches the local area, bringing hazy skies,
reduced visibility, and deteriorated air quality. From the latest
model guidance, PWAT values should plummet to below climatological
normal (most likely between 1.4 and 1.6 inches), with a dry slot
in the mid levels (below 40 %). Additionally, the Grand Ensemble
shows members tending to drier conditions on both Monday and
Tuesday, with low variability (PWAT difference of 0.2 - 0.3
inches). Nevertheless, Saharan dust concentrations may lower and
low-level moisture content may increase on Wednesday due to the
approach of an induced trough that should remain north of the CWA.
From the latest model guidance, PWAT values may increase to
seasonal values (between 1.5 and 1.7 inches), though theres
variability between ensemble models, with ENS tending to wetter
conditions compared to GEFS members. In terms of instability, some
lingering troughiness may aid deep convection near the CWA, with
cool mid-level temperatures (500 mb temperatures around -6 degrees
Celsius). Additionally, the Galvez-Davison Index (GDI) suggests
the potential of isolated thunderstorms due to afternoon
convection, particularly over western portions of Puerto Rico.
Taking into consideration well-above-normal temperatures (925 mb
temperatures above the 75th percentile), combined with local
effects and sea breeze convergence, afternoon convection could
still be expected over western Puerto Rico, though they may become
short- lived with low precipitation efficiency. Rainfall
accumulations may lead to ponding of water over roadways and
poorly drained areas, particularly on Monday and Wednesday. Drier-
than-normal conditions should return on Thursday, with limited
shower activity and no flooding or lightning threat.

As mentioned in previous discussions, model guidance continues to
suggest warmer-than-normal conditions across the islands,
particularly over urban and low-lying areas. Taking into account
the available moisture and the influence of SAL, maximum
temperatures could reach the low and mid 90s while heat indices
will likely be between the mid 100s and low 110s, affecting anyone
without effective cooling and/or exposed to prolonged sun
exposure, including sensitive groups. Therefore, the heat threat
will remain elevated to significant throughout the next workweek.

&&

.AVIATION...
(18Z TAFS)
Issued at 245 PM AST Fri Jul 31 2026

Mainly VFR conditions are present. TJBQ have the best chance to
observe TSRA/VCSH thru 31/20-21Z. Visibilities should improve as
just traces of Saharan dust will be present. The arrival of
moisture from a tropical wave will increase the chance of showers
from 01/14Z onward. MVFR conds will be more likely. Easterly
winds, with gusts near 23 to 28 kts thru 31/23Z, then around 10
kts or less overnight. Winds increasing again after 01/13-14Z.

&&

.MARINE...
Issued at 245 PM AST Fri Jul 31 2026

Moderate to fresh trades will prevail through next week as a
surface high pressure continues to build over the central
Atlantic. Choppy seas up to 6 feet will persist through Saturday
across most local waters and passages. A tropical wave will move
across the regional waters Saturday into Sunday, increasing shower
and thunderstorm activity. Saharan dust concentrations will
remain low through Monday.

&&

.BEACH FORECAST...
Issued at 245 PM AST Fri Jul 31 2026

A moderate risk of rip currents will persist across the northwest
to southeast beaches of Puerto Rico, Vieques, Culebra, and St.
Croix through Saturday. A low risk is expected across the western
to southern beaches of PR, and most beaches of St. Thomas and St.
John through the weekend. Nevertheless, beachgoers should be aware
that life-threatening rip currents often occur in the vicinity of
groins, jetties, reefs, and piers even when the risk is low. By
Tuesday, the risk will become low across all coastal areas.

&&

.FIRE WEATHER...
Issued at 245 PM AST Fri Jul 31 2026

Minimum relative humidity values have remained near or above
critical thresholds today. Influx of moisture during the weekend
could aid in improving overall fire weather conditions. However,
critically dry fuels persist across southern Puerto Rico and St.
Croix, with Keetch-Byram Drought Index (KBDI) values remaining
well above the critical Threshold. East to east-southeast winds
are expected to remain above or near the critical levels through
at least today, slightly which could promote the rapid spread of
any fire that develops. Residents and visitors should avoid
outdoor burning and exercise caution with activities that could
generate sparks or open flames, especially across areas with
critically dry vegetation.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION...YZR
LONG TERM...MNG
MARINE/BEACH...MMC

NWS SJU Office Area Forecast Discussion