National Weather Service Today's Weather Impact Levels

036
FXCA62 TJSJ 150905
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
505 AM AST Mon Sep 15 2025

.KEY MESSAGES...

* A trade wind perturbation today and a tropical wave combined
  with a TUTT low expected around mid week, will increase the
  frequency of showers and thunderstorms throughout much of the
  workweek. As a result, the risk of flooding will remain
  elevated. Additional hazards include rapid river rises,
  landslides, and frequent lightning.

* The Virgin Islands will also experience an increase in showers
  and thunderstorms, potentially leading to water surges in
  drainage guts, ponding on roadways, occasional lightning, and
  isolated landslides.

* Increasing heat risk is forecast again toward the end of the week
  and into the weekend. A limited heat risk is expected today and
  over the next few days.

* A moderate risk of rip currents is in effect for the northern
  and eastern beaches of Puerto Rico, as well as Culebra and St.
  Croix today. A low risk is expected in other areas. Similar
  conditions are likely to continue through most of the workweek.

&&

.SHORT TERM...Today through Wednesday...

Conditions were mostly tranquil during the night hours across the
local islands. Some clouds reached southeastern Puerto Rico, but
otherwise, it was mostly clear. A few showers formed in the vicinity
of the Virgin Islands, reaching St. Croix late in the overnight
hours. A low level perturbation, located just to the east of the
Virgin Islands will cross the local area today. So far, the most
recent precipitable water satellite-derived product indicate values
around 2.2 inches, which is just over one standard deviation
above normal. Once the moisture field reaches Puerto Rico this
afternoon, it will combine with strong diurnal heating to produce
showers and thunderstorms. The activity will be occasional across
the Virgin Islands, where water surges along guts can be
anticipated. For the interior, east, west, and the San Juan metro
area in Puerto Rico, there is a risk of flooding, landslides,
water surges along rivers, gusty winds and lightning strikes. By
the evening hours, as the disturbance departs, showers will become
less frequent, but will still move occasionally across the east
of Puerto Rico and over the Virgin Islands.

On Tuesday, drier air will filter in, but only at the mid-levels.
Moisture at the surface will be enough for an additional round of
afternoon showers, mostly as streamers coming out of St. Croix, St.
Thomas and Culebra, and from El Yunque into the interior. Another
cluster of showers and thunderstorms is anticipated for western
Puerto Rico.

By Wednesday, a stronger tropical wave will approach the region,
along with an upper level low staying north of the island. Shower
and thunderstorm are expected to increase in coverage and intensity
than today, with a higher risk of flooding. It is worth mentioning
that even though the rain is not expected to cover all areas, or to
be raining all day, those areas that have received heavy rain in the
last couple of days will be especially prone to urban and river
flooding, as well as landslides in steep terrain. Flash flooding
cannot be ruled out either. Also, it is important to consider the
risk of gusty winds and lightning strikes within the thunderstorm.
The users should avoid flood prone areas, and seek shelter if
thunder is heard.


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

By Thursday, unsettled weather conditions are expected to prevail
due to the combined influence of a tropical wave and a TUTT low
moving across Puerto Rico and the Virgin Islands. Latest model
guidance suggests precipitable water (PWAT) values will remain
above normal for this time of yearranging between 2.0 and 2.25
inches. These atmospheric features will maintain high rain chances
and enhance thunderstorm development, supported by 500 mb
temperatures near average for mid-September. As a result, urban
and small stream flooding is likely to persist daily, and some
rivers may reach flood stage. In the Virgin Islands, periods of
showers and thunderstorms are expected to cause water surges along
guts and drainage channels, ponding on roadways, and isolated
urban flooding. Additional hazards include landslides, frequent
lightning, and gusty winds.

By Friday, drier air is forecast to move into the region, decreasing
PWAT values to near normal levels, and dropping further to below
normal over the weekend and into early next week. Rainfall
activity will diminish, but localized afternoon convection,
particularly in northwestern Puerto Rico, will still be possible
due to local effects and diurnal heating. As cloud cover decreases
and rain subsides, winds will shift from the southeast, and 925 mb
temperatures are forecast to peak at nearly two standard deviations
above normal, especially on Friday and Saturday. This pattern
will elevate the heat risk across the region. Heat Advisories or
even Extreme Heat Warnings may be issued. Residents and visitors
are strongly advised to stay hydrated, avoid prolonged sun
exposure during peak heat hours, and take extra precautions,
particularly those without access to adequate cooling or hydration.
This forecast scenario assumes the current guidance remains
consistent for the tropical wave being monitored by the National
Hurricane Center, which currently has a 30% chance of formation in
the next 48 hours and an 80% chance over the next 7 days. If
development continues as expected, the system may track
northwestward toward the northeastern Caribbean. We will continue
to monitor this closely and provide updates throughout the
workweek.

&&

.AVIATION...
(06Z TAFs)

SHRA and TSRA will be on the increase today, reaching the USVI
terminals after 12Z. Brief periods of reduced VIS and low ceilings
is expected here. For the PR terminals, after 15Z, SHRA and TSRA
will cause mountain obscuration across the Cordillera Central, and
will likely result in longer periods of reduced VIS. Winds at FL050
will be at 15-23 kts, with stronger gusts.

&&

.MARINE...

A surface high pressure system located over the central and eastern
Atlantic, in combination with an approaching low-level perturbation,
will promote moderate to locally fresh east to southeasterly winds
today. Small craft are urged to exercise caution across the regional
waters. The trade wind perturbation that will cross the region
today, will also increase the likelihood of showers and thunderstorms
across the waters and local Caribbean passages. A stronger tropical
wave is expected to approach by midweek, enhancing shower
coverage.

&&

.BEACH FORECAST...

Today, there is a moderate risk of rip currents across the north and
east-facing beaches of Puerto Rico, Culebra, and St. Croix. Similar
conditions are expected to prevail most of the workweek so beachgoers
are advised to exercise caution at all times as a moderate risk
means life-threatherning rip current are possible in the surf
zone. A low risk should prevail elsewhere. Additional hazards
include afternoon thunderstorms. Remember: if you hear thunder,
seek shelter inside of a safer place immediately.

&&

.HYDROLOGY...

The disturbance that will move in today, and the tropical wave
coming in on Wednesday will maintain the risk of flooding elevated
for most of the area in Puerto Rico and the Virgin Islands. The
latest Root Soil Saturation product from PRAGWATER, and also from
the National Water Model shows saturated soils along the western
half of Puerto Rico, and also for the extreme east and southeast.
Also, the 7-day average streamflow from the USGS show that many
rivers are running near normal or well above normal. Therefore, it
is likely that any upcoming rain will quickly turn into runoff,
enhancing the risk of flooding, rapid river rises, and landslides.

For the Virgin Islands, periods of heavy rain will cause water
surges along drainage guts, and ponding of water on roadways.
Urban flooding could develop too, while landslides in areas of steep
terrain cannot be ruled out.

Although an elevated flood risk is shown everyday of the week, the
higher risk is on Wednesday and Thursday due to the above normal
moisture from the tropical wave and increased instability aloft from
an upper level low. Conditions will begin to improve by Friday.

&&

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

$$

SHORT TERM/AVIATION...ERG
LONG TERM/MARINE/BEACH FORECAST...YZR

NWS SJU Office Area Forecast Discussion