Dorado, Puerto Rico Weather Forecast Discussion

481
FXCA62 TJSJ 231915
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
315 PM AST Tue Sep 23 2025
.KEY MESSAGES...
* The Heat Advisory remains in effect for the urban and coastal
  areas of the US Virgin Islands and Puerto Rico, as well as the
  eastern interior valleys, through this afternoon at 4 PM AST.
* A robust tropical wave (AL94L) is expected to increase the
  flooding risk in the US Virgin Islands and Puerto Rico through
  the remainder of the workweek, particularly on Wednesday and
  Thursday.
* Squally weather will result in gusty winds and periods of
  downpours over the US Virgin Islands and PR at times.
* There is a slight to moderate chance that the wet pattern could
  extend into the upcoming weekend.
&&
.SHORT TERM...TONIGHT THROUGH THURSDAY...
An induced perturbation brought a line of showers across the
northeast portion of PR and the US Virgin Islands early this
morning, with above-normal moisture content. Then, a mixture of
sunshine and clouds promoted above-normal maximum temperatures
across the region, with low 90s along the coastal sections and
upper 80s along the mountains. Later in the day, the combination
of the available moisture and diurnal heating promoted hot heat
indices, for which the Heat Advisory was in effect through 4 PM
AST. Winds were mainly from the northeast at 10 to 15 mph, with
higher gusts and variations due to sea breezes. Afternoon
convection developed over St. Thomas and St John, as well as St
Croix, producing ponding of water in roads and poorly drained
areas. Also, convection formed across the southern slopes and
southwest quadrant of PR, where ponding of waters also developed.
Convection will continue to impact the interior and southwest
quadrant of PR, where urban and small stream flooding will remain
possible through this evening. Additionally, squally weather will
occasionally affect the US Virgin Islands and eastern PR during
that time.
Model guidance and satellite imagery indicate a tropical wave
interacting with an upper-level low on Wednesday and Thursday. The
tropical wave will first affect the Virgin Islands with the
highest chances for heavy showers and thunderstorms late tonight
and early Wednesday morning, before spreading into Puerto Rico,
impacting the east, the San Juan metro area, the interior, and the
west later in the day.
The main threat of flooding is expected to occur mid-morning
Wednesday into early Thursday. The incoming wave is expected to be
stronger, likely increasing the coverage and intensity of showers
and thunderstorms across both Puerto Rico and the US Virgin
Islands. There is a low chance of widespread flash flooding;
however, local guidance suggests that favorable weather conditions
may lead to enhanced rainfall periods.
.LONG TERM...Friday through Tuesday...
The unsettled and wet pattern will persist into the weekend and
through early next week as the vigorous tropical wave lifts north of
the region, while another trailing wave approaches from the
southeast and also tracks north. This evolving setup will induce a
deep-layer southerly flow over the local area, favoring the
continuous transport of abundant tropical moisture across the
islands. Precipitable water values will generally remain elevated,
exceeding 2.0 inches through Monday, supporting frequent rounds of
showers and thunderstorms.
Given the saturated soils and elevated streamflows from the passage
of the tropical wave in previous days, even modest rainfall could
quickly lead to urban and small stream flooding. The risk of more
significant flooding, including flash flooding, rapid river rises,
and landslides in steep terrain, cannot be ruled out during this
period. Daily shower and thunderstorm activity will be driven by
strong instability, diurnal heating, and sea breeze convergence,
with the strongest storms capable of producing frequent lightning,
gusty winds, and locally heavy downpours. Through the weekend,
shower and thunderstorm activity will favor the southern and eastern
portions of Puerto Rico, Vieques, Culebra, and the U.S. Virgin
Islands during night and morning hours, while afternoon convection
develops across the interior and northwestern Puerto Rico. The
prevailing southerly flow may further enhance heavy rainfall-
producing storms along the northern slopes, particularly where
upslope convergence interacts with abundant moisture.
By Monday, the trailing wave will keep the atmosphere moist and
unstable, prolonging the flooding risk. While deterministic guidance
continues to highlight favorable conditions for heavy rainfall-
producing thunderstorms, Saharan dust may remain intermittently
embedded within the moisture plume. This could limit convection
locally or modulate rainfall rates, adding variability to
precipitation totals across the forecast area.
It is not until Tuesday that a drier air mass is expected to filter
into the region. Precipitable water values are forecast to drop from
near 2.2 inches to below 1.6 inches, signaling a gradual improvement
in conditions. Despite the drying trend, isolated convection may
linger, especially during the afternoon hours, but the overall
flooding risk should decrease compared to previous days.
In addition to the rainfall hazards, a daily heat threat will
persist through the forecast period. Deep-layer southerly winds will
continue to transport warmer-than-normal air, while excessive low-
level moisture will elevate heat indices to sweltering levels.
Extreme Heat Warnings and Heat Advisories are likely to be issued
for coastal and urban areas across Puerto Rico, Vieques, Culebra,
and the U.S. Virgin Islands.
&&
.AVIATION...
(18Z TAFs)
Mostly VFR conditions will prevail across all TAF sites. However,
SHRA/TSRA will impact the terminals at times, especially at
IST/ISX and the vicinity of JPS. With this activity, periods of
MVFR are possible. Winds will remain from the E-NE thru 23/23z
at 10-15 kt with higher gusts and sea breeze variations. Winds
will become calm to light/vrb, after 23/22Z. But a tropical wave
moving near the region will deteriorate the conditions, first
across the USVI, spreading to PR from 24/06z onward.
&&
.MARINE...
An induced surface trough will maintain moderate easterly winds
across the region today. Late tonight into tomorrow Wednesday, a
vigorous tropical wave will move into the islands, increasing the
potential for thunderstorms and leading to localized hazardous
marine conditions. These storms may produce frequent lightning
over exposed waters. In addition, an increase in surface winds is
forecast from Wednesday onward, particularly across the offshore
Atlantic waters, where small craft operators should exercise
caution. Seas close to 7 feet over northeastern offshore Atlantic
waters are possible were winds will be highest.
&&
.BEACH FORECAST...
Attention beachgoers: Exercise caution on the exposed north- and
east-facing beaches in Puerto Rico and the US Virgin Islands, as
there is a moderate risk of rip currents through at least Friday.
Additionally, thunderstorms may develop. If you hear thunder,
please seek shelter by going indoors or finding a safe location.
&&
.HYDROLOGY...
Unsettled weather conditions are expected to persist through much
of the workweek, leading to an elevated flooding threat for both
Puerto Rico and the US Virgin Islands.
The main threats for Puerto Rico are Urban and isolated flash
flooding, landslides, river surges, and some rivers potentially
reaching flood stage. Rainfall accumulations for the event are
expected to range from 4 to 6 inches across the eastern third of
PR, including Vieques and Culebra, and the northwest quadrant,
particularly on Wednesday and Thursday. The best chance for the
flooding threat will be Wednesday through Thursday, when more
vigorous tropical wave activity and an upper-level low will
increase rainfall and thunderstorm coverage.
The primary threats to the US Virgin Islands include water surges
along drain gutters, squally weather with gusty winds, ponding of
water on roads/low-lying areas, urban and isolated flash
flooding, and possible landslides. Expect rainfall amounts of
around 2 to 4 inches, with isolated higher amounts of up to 6
inches. The best chance of flooding is between Wednesday and
Thursday, with stronger showers and thunderstorms.
Thus, we encourage residents and visitors to actively monitor NWS
flood watches and warnings, as additional rainfall on already
saturated soils will heighten vulnerability to flash flooding and
landslides. For further details, see the Hydrologic Outlook
(ESFSJU) issued by WFO-San Juan.
&&
.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.
&&
$$
MORNING CREW...CAM/MNG
EVENING...ICP/YZR

NWS SJU Office Area Forecast Discussion