054
FXCA62 TJSJ 121747
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
147 PM AST Sat Sep 12 2026

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST...

.KEY MESSAGES...
Issued at 131 PM AST Sat Sep 12 2026

- High concentrations of Saharan dust will persist across the
  region today, promoting hazy skies, reduced visibility, and poor
  air quality. These conditions will gradually improve from early
  Sunday onwards.

- A Heat Advisory remains in effect for Puerto Rico, Vieques,
  Culebra, and the U.S. Virgin Islands through 5 PM AST. Elevated
  heat risks will continue over the next several days.

- Shower and thunderstorm activity will increase Sunday into
  Monday, particularly across interior and western Puerto Rico, as
  an upper- level low pressure system moves over the area,
  bringing a limited-to- elevated flood risk.

- A moderate risk of rip currents continues through Sunday night
  for northern, eastern, and southeastern beaches of Puerto Rico,
  as well as Vieques, Culebra, and St. Croix. Isolated rip
  currents remain possible even in low-risk areas.

&&

.Short Term(This evening through Monday)...
Issued at 131 PM AST Sat Sep 12 2026

Very hazy skies prevailed across the local islands during the
morning hours as Saharan dust remained entrenched across the region.
Visibility was reduced to as low as 5SM at times, while no
measurable rainfall was reported. Although lower moisture compared
to yesterday has resulted in somewhat lower heat index values, very
warm temperatures continue to support another hot day. Saharan dust
also limited nighttime cooling, allowing temperatures to start the
day warmer than usual. A Heat Advisory remains in effect for Puerto
Rico, Vieques, Culebra, and the U.S. Virgin Islands through 5 PM AST
today. The 12/12Z TJSJ sounding continues to show a dry and stable
environment, with a pronounced low-level inversion and limited mid-
level moisture.

For the remainder of today, hazy conditions will persist, with
gradual improvement expected tonight as the Saharan dust begins to
move away. An upper-level trough/low west to west-northwest of the
local islands places Puerto Rico within a generally favorable region
for divergence aloft; however, limited moisture remains the main
constraint for convection. A few showers and perhaps an isolated
thunderstorm may develop across western-interior and southwestern
Puerto Rico this afternoon, but activity should remain isolated and
short-lived, with only minor ponding possible in poor drainage areas.

Conditions will become gradually more favorable on Sunday as the
Saharan Air Layer exits and moisture recovers toward seasonal
levels. The upper-level low will remain west-northwest of the area,
maintaining favorable support aloft, with 500 mb temperatures near -
7 degrees Celsius. Passing showers are expected across the USVI and
eastern Puerto Rico during the overnight and morning hours, followed
by more numerous showers and isolated thunderstorms over the
interior and western portions of Puerto Rico during the afternoon.
Localized ponding will be possible in the heaviest activity.

A more favorable convective environment is expected Monday as a low-
level perturbation approaches from the southeast and moisture
deepens further. Precipitable water values are forecast to increase
into the 1.6 to 1.8 inch range, with improving mid-level humidity.
The upper-level low will remain favorably positioned west of the
area, with divergence aloft and 500 mb temperatures generally
between -7 and -8 degrees Celsius. Scattered showers and
thunderstorms are expected, particularly across the interior and
western portions of Puerto Rico during the afternoon. Some
thunderstorms could become strong, producing gusty winds and
frequent lightning. Weaker steering winds may allow slower-moving
convection and locally higher rainfall accumulations, increasing the
potential for urban and small-stream flooding. However, ensemble
guidance remains relatively conservative with rainfall amounts,
favoring a localized rather than widespread flooding threat.

By Monday night, model guidance suggests that the upper-level trough
may deepen further, with an increasing signal for organized
convection or an MCS to develop over the Caribbean waters. If this
feature materializes, activity may lift north to northwest toward
the local area, and its evolution will need to be monitored for any
approach toward the southern and coastal portions of Puerto Rico.

Hot and humid conditions will continue through the period, with
increasing moisture Sunday and Monday likely to produce higher heat
indices, especially across lower elevations and urban/coastal areas
that receive little rainfall.

&&

.Long Term(Tuesday through next Friday)...
Issued at 427 AM AST Sat Sep 12 2026

Some changes were introduced to the long-term forecast, with Tuesday
and Thursday having more flooding potential. A surface high pressure
across the Central Atlantic will continue to promote easterly-
southeasterly breezy winds for most of the period. A transition to a
wetter pattern is still expected for the long term period. By
Tuesday, moisture associated with a tropical wave combined with the
departing upper-level low near Hispaniola will result in PWAT values
up to 2.00 inches, near-above climatological normal. Additionally,
the vicinity of the upper-level low will likely lead to cooler-than-
normal mid-level temperatures (500 mb temperatures between -7 and -8
degrees Celsius) and divergence aloft, allowing cloud growth and
enhancing instability. By Wednesday, lingering moisture from these
synoptic features, daytime heating, and local effects could enhance
convection, especially over western and interior Puerto Rico in the
afternoon. However, a shallow drier air slot could be present on the
low-to-mid levels (700 - 850 mb), leading to limited shower and
thunderstorm activity. By Thursday morning, the latest model
guidance continues to suggest a tropical wave approaching the CWA,
pooling deep tropical moisture (PWAT values to 2.20 inches).
Additionally, the latest model solution suggests enhanced areas of
vorticity associated with the wave, increasing the likelihood of an
advective weather pattern across windward sections. Lingering
moisture on Friday will enhance convection, especially over western
and interior portions of Puerto Rico in the afternoon.

With these conditions expected for the long term, both Tuesday and
Thursday possess the highest potential for showers and thunderstorms
across Puerto Rico and the US Virgin Islands. Rainfall accumulations
could lead to urban and small stream flooding, particularly across
interior and western Puerto Rico, with a limited flooding risk
across the US Virgin Islands and eastern Puerto Rico on Thursday.
Meanwhile, on Friday, afternoon convection could lead to ponding of
water over roadways and poor drainage areas.

As mentioned in the previous discussion, warmer than normal
conditions could still be expected in the long-term forecast, as
model guidance suggest seasonal to above normal 925 mb temperatures.
Heat indices during peak hours are very likely to exceed 100 degrees
Fahrenheit, particularly across urban and low-lying areas.
Therefore, an elevated to significant heat threat will prevail
throughout the period.

&&

.AVIATION...
(18Z TAFS)
Issued at 131 PM AST Sat Sep 12 2026

Mainly VFR conds expected at all terminals thru the next 24 hrs. HZ
due to Saharan dust will maintain VSBY around 5-6SM thru tonight,
improving Sun morning. VCSH possible at TJSJ aft 13/13z, with VCTS
at TJPS/TJBQ aft 13/17z. E-ESE winds 12-18 kt with gusts 20-26 kt
this aft, becoming light overnight, then increasing again to 13-17
kt with gusts up to 25 kt aft 13/13Z.

&&

.MARINE...
Issued at 131 PM AST Sat Sep 12 2026

A pressure gradient between a weak surface high to the north and the
Colombian low will maintain moderate to fresh trade winds through
the rest of the weekend, primarily across Caribbean waters and
passages. Expect choppy seas of 4 to 5 feet, with occasional seas up
to 6 feet; small craft should exercise caution. Saharan dust will
persist through early Sunday, limiting shower activity and reducing
visibility. Moisture levels will increase by Monday, bringing an
increased chance of showers.

&&

.BEACH FORECAST...
Issued at 131 PM AST Sat Sep 12 2026

High concentrations of Saharan dust will continue to promote hazy
skies, reduced visibility, and poor air quality through early
Sunday, with elevated heat remaining a concern over the coming days.
Beachgoers spending time outdoors should stay hydrated and limit
prolonged sun exposure.

A moderate risk of rip currents will persist through Sunday night
across the northwest to southeastern beaches of Puerto Rico, as well
as Culebra, Vieques, and the U.S. Virgin Islands. While life-
threatening rip currents remain possible in these areas, protected
and western-facing beaches will see a lower risk. However, remember
that rip currents can still form near groins, jetties, reefs, and
piers. A low risk will return by early Monday through mid-week.

Beginning Sunday, afternoon thunderstorms are expected to develop,
primarily over western Puerto Rico. Beachgoers should remain alert
for lightning and rapidly changing weather conditions, seeking
shelter immediately if storms approach.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...

PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.

&&

$$

DAYTIME SHIFT...CVB/GRS
EVENING SHIFT...CAM/MNG