136
FXCA62 TJSJ 311704
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
104 PM AST Thu Jul 31 2025

.KEY MESSAGES...

* A Saharan Air Layer with minor to moderate dust concentrations
  will bring hazy skies and deteriorate air quality on Friday.
  However, afternoon thunderstorms are expected over western PR.

* A wetter pattern is expected over the weekend into early next
  week with the passage of a vigorous tropical wave, increasing
  the potential for flooding.

* Moderate to locally fresh winds will create choppy seas and a
  moderate rip current risk along north-and east-exposed beaches
  over the next few days.

&&

.SHORT TERM...Rest of today through Saturday...

...A Heat Advisory is in effect until 5 PM this afternoon for the
coastal municipalities and lower-elevations of eastern PR...

Mostly sunny to partly cloudy skies were observed during the morning
hours across the islands. Passing showers left minor rainfall
accumulations over land areas. Daytime temperatures were from the
upper 80s to low 90s across coastal areas. The wind was mainly
from the east around 15 to 20 mph with sea breeze variations and
higher gusts. Diurnally induced streamers and showers were
developing around noon downwind of the U.S. Virgin Islands and
across the interior areas of Puerto Rico.

For the rest of this afternoon, showers and thunderstorms will
continue to grow over western PR, leading to possible urban and
small stream flooding. Meanwhile, a weak Saharan Air Layer with
minor to moderate amounts of Saharan dust will filter through
Friday afternoon. Promoting hazy skies and somewhat more stable
conditions than today. However, diurnally induced afternoon
showers with isolated thunderstorms are still expected to develop
in the usual areas of western PR. Conditions will quickly
deteriorate from Friday evening into Saturday as a broad tropical
wave brings better tropical moisture across the northeastern
Caribbean. Therefore, expect periods of showers and isolated
thunderstorms to increase gradually, particularly during the
overnight and early morning hours of Saturday across the USVI,
and across the eastern half and northern sections of PR. The
threat for flooding, lightning strikes, and strong gusty winds
will increase with these thunderstorms.


.LONG TERM...Sunday through Thursday...

/From previous discussion issued at 400 AM AST Thu Jul 31 2025/

The wet and unsettled weather pattern is expected to continue
early next week through at least Wednesday, becoming variable on
Thursday. High moisture content associated with a vigorous
tropical wave should linger through at least Monday, enhancing
shower and thunderstorm activity across the area. Based on the 12z
deterministic guidance of the GFS and ECMWF, PWAT values are
likely (50 - 60%) to exceed the 75% percentile (2.0 - 2.2 inches),
with a low chance of reaching 2.4 inches. From the latest model
guidance, high moisture content in the low to mid levels will
increase lapse rates (700 mb) to above climatological values
(around 6.5 C/km). Additionally, an upper-level trough approaching
the region and lingering through Tuesday could cool mid-level
temperatures (between -7 and -8 Celsius), promote instability
aloft, and enhance deeper convection activity. From the latest
Galvez-Davison Index (GDI) run, there`s potential of observing
isolated to scattered thunderstorms across the CWA from Sunday
through Tuesday. Nevertheless, the latest NASA GEOS-5 GMAO Dust
Extinction product shows a dense Saharan Air Layer (SAL) that may
move over the local area by Monday. The interaction of these
particles with the tropical wave introduces uncertainty to the
forecast; nevertheless, the flooding risk will remain elevated on
Sunday through Wednesday.

By late Wednesday into Thursday, a drier air mass should filter
into the region, reducing PWAT to season values (1.5 - 1.7
inches). Nevertheless, patches of moisture may arrive from time to
time over the CWA and, combined with diurnal heating and local
effects, will likely enhance shower and thunderstorm activity,
mainly over northwestern Puerto Rico. Rainfall accumulations may
promote mostly ponding of water over roadways, urban, and poorly
drained areas, along with urban and small streams flooding.


&&

.AVIATION...

(18Z TAFS)

Mainly VFR conditions are expected across all terminals through
the forecast period. However, SHRA/TSRA in and around the western
interior may cause brief MVFR cigs at TJPS/TJBQ thru 31/22z.
Across the rest of the terminals, streamers and passing showers
will cause mostly VCSH and brief periods of -RA. The 31/12z TJSJ
sounding indicated east winds up to 19 kt blo FL050.


&&

.MARINE...

A surface high-pressure system over the central Atlantic, combined
with a vigorous tropical wave, will promote moderate to locally
fresh winds over the next few days, maintaining choppy marine
conditions across portions of the local waters. From Friday into the
weekend, the wave will increase the potential for thunderstorms and
contribute to localized hazardous marine conditions.


&&

.BEACH FORECAST...

Breezy conditions will also sustain a moderate rip current risk at
selected beaches of northwestern Puerto Rico, Culebra, and Saint
Croix. While this moderate risk is expected to spread across most
local beaches over the next few days, isolated stronger rip
currents may still occur in areas currently at low riskespecially
near piers, jetties, and channels. Swimming is safest at
lifeguarded beaches, and caution is advised for inexperienced
swimmers and anyone using flotation devices.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010-011.

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

$$

DSR/GRS