FROM: NWS SJU
AREA: NWS SJU County Warning Area
SENT: 2026-07-27T07:05:00+00:00
Area Forecast Discussion: NWS SJU
000
FXCA62 TJSJ 270705
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
305 AM AST Mon Jul 27 2026
…New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER…
.KEY MESSAGES…
Updated at 305 AM AST Mon Jul 27 2026
* Dangerous heat will persist across Puerto Rico and the U.S.
Virgin Islands through much of the week. Heat Advisories remain
in effect today, and additional Heat Advisories or even Extreme
Heat Warnings may be needed on subsequent days, especially where
rainfall is limited.
* A tropical wave will increase the chance of numerous showers
and thunderstorms through Tuesday, elevating the risk of urban
and small-stream flooding, with isolated flash flooding possible
where heavy rainfall persists.
* A gradual decrease in moisture is expected by Wednesday,
reducing the flooding threat to a more localized level, mainly
across northwestern Puerto Rico during the afternoon.
* Drier air, accompanied by periodic pulses of Saharan dust, will
return during the second half of the week, reducing rainfall
coverage while maintaining hot and hazy conditions across Puerto
Rico and the U.S. Virgin Islands.
&&
.Short Term(Today through Wednesday)…
Issued at 305 AM AST Mon Jul 27 2026
Subsidence ahead of today’s tropical wave dominated overnight,
limiting rain activity across Puerto Rico and the US Virgin
Islands. Additionally, we observed another warmer-than-normal
night and early morning, with lows in the low 80s and upper 70s in
coastal and urban areas of the USVI and PR. However, the Doppler
Radar detected a few showers across the local waters, some of
which moved briefly across the USVI. GOES-19-derived Total
Precipitable Water (TPW) showed the leading edge of the tropical
wave approaching the region, as the Doppler detected those
showers. Winds were mainly from the east to east-southeast across
most local stations, at 5 to 10 mph, with land-breeze
fluctuations.
Increasing tropical moisture associated with an approaching
tropical wave, combined with another warmer-than-normal day, is
expected to lead to dangerous heat across PR and the USVI.
Therefore, a Heat Advisory remains in effect from 9 am to 5 pm AST
for all of PR and the USVI, where heat indices are likely to
reach hazardous levels, especially in areas that receive little or
no rainfall.
Throughout the morning, bands of showers associated with the
tropical wave will likely move across the USVI and portions of
eastern and southeastern PR, producing periods of moderate to
locally heavy rainfall. Low-level convergence and orographic
lifting may locally enhance this activity, while areas that remain
dry are expected to experience rapidly rising temperatures. As
the tropical wave axis continues moving westward through the day,
shower and thunderstorm coverage is forecast to increase across
the interior and western portions of Puerto Rico. There is a
medium to high chance that some storms will produce torrential
rainfall, frequent lightning, and gusty winds. Consequently, an
elevated risk of urban and small-stream flooding exists this
afternoon, with isolated flash flooding possible where heavier or
repeated rainfall occurs. Residents and visitors should continue
to monitor the forecast and remain prepared for additional short-
fused advisories, or warnings if hazardous conditions develop.
The tropical wave is forecast to linger across the region through
at least Tuesday as it interacts with an upper-level trough,
resulting in the wettest period of the short-term forecast.
Although the anticipated rainfall could provide beneficial
moisture to areas experiencing significant rainfall deficits over
the past several months, it will also increase the risk of
flooding, particularly in urban and poorly drained areas. As the
wave moves west-northwestward across the northeastern Caribbean,
deep tropical moisture and atmospheric instability are expected to
remain above normal through Tuesday, supporting numerous showers
and scattered thunderstorms. Precipitable water values will likely
remain near 2.0 inches, favoring efficient rainfall.
Consequently, there is a moderate to high likelihood of urban and
small-stream flooding through Tuesday, while localized flash
flooding will be possible where repeated rounds of heavy rainfall
or persistent convection occur. Stronger thunderstorms may also
produce frequent lightning and locally gusty winds.
By Wednesday afternoon, moisture is forecast to gradually decrease
as a drier air mass, with precipitable water values falling to
around 1.4 to 1.5 inches, overspreads the region. Periodic pulses
of Saharan dust are also expected to return during the second half
of the week, resulting in hazier skies and a lower probability of
widespread shower and thunderstorm activity.
&&
.Long Term(Thursday through Monday)…
Issued at 305 AM AST Mon Jul 27 2026
The latest model guidance continues to suggest the arrival of a
drier air mass across the region, dropping precipitable water values
below normal to around 1.30 to 1.40 inches on Thursday. Along with
this drier air, NASA GEOS-5 model guidance indicates low-to-moderate
concentrations of Saharan Dust arriving through the first half of
the period.
Strengthening surface high pressure over the central Atlantic will
create a tight pressure gradient, bringing breezy to locally windy
east-southeast winds through Saturday. Aloft, a mid-to-upper-level
ridge will promote stable conditions, limiting widespread shower
activity. However, the forecast remains on track for patches of low-
level moisture embedded in the trade winds to move across the area.
This will bring passing showers, particularly over the windward
portions of the islands during the morning hours. In the afternoons,
this moisture combined with daytime heating and local effects could
trigger shower activity along the interior and northwestern portions
of Puerto Rico.
By late Sunday into Monday, an approaching tropical wave will
increase moisture alongside the potential for widespread showers and
thunderstorms. Elevated temperatures will persist through the
period, driven by normal to above-normal temperatures at the 925 mb
level, with daytime highs reaching the upper 80s to low 90s across
coastal and urban areas. Residents and visitors are encouraged to
monitor forecasts and stay updated on changing conditions as the
week progresses.
&&
.AVIATION…
(06Z TAFS)
Issued at 305 AM AST Mon Jul 27 2026
VFR conds will prevail today, although passing SHRA may bring
brief MVFR cigs/vsbys to TIST/TISX and TJSJ after 27/06Z.
SHRA/TSRA coverage will increase after 27/16Z as a tropical wave
moves across the area, with TEMPO MVFR/IFR conds likely at TJBQ
and possible at TJSJ due to +SHRA/TSRA, reduced vsbys, mtn
obscurations, and gusty winds. ESE sfc winds 10-18 kt with gusts
to 28 kt, becoming light/vrb after 27/23Z. Stronger convection may
produce gusts above 30 kt and frequent lightning.
&&
.MARINE…
Issued at 305 AM AST Mon Jul 27 2026
A tropical wave will bring increasing showers and isolated
thunderstorms through Tuesday, producing locally higher winds and
seas, frequent lightning, and reduced visibility. However, in
general, expect moderate east to east-southeast winds today,
becoming moderate to locally fresh Tuesday into Wednesday. These
winds will maintain choppy seas across portions of the Atlantic
waters, offshore Caribbean waters, and local passages, with seas
of 6 feet limited to localized areas.
&&
.BEACH FORECAST…
Issued at 305 AM AST Mon Jul 27 2026
A moderate risk of rip currents continues across northern Puerto
Rico. As winds decrease early this week, the rip current risk
should gradually lessen, resulting in improving beach conditions
through midweek. However, life-threatening rip currents will
remain possible near reefs, jetties, groins, and piers. The rip
current risk is expected to increase again later in the workweek
as easterly winds strengthen. Beachgoers should also remain alert
for thunderstorms and heat.
&&
.FIRE WEATHER…
Issued at 305 AM AST Mon Jul 27 2026
Despite increasing moisture associated with a tropical wave
keeping minimum relative humidity values above critical
thresholds, critically dry fuels persist across southern Puerto
Rico and St. Croix. As a result, any fire that develops could
still spread through dry vegetation, particularly where locally
breezy winds occur. Residents and visitors are encouraged to avoid
outdoor burning and use caution with activities that could
generate sparks or open flames in areas with critically dry fuels.
&&
.SJU WATCHES/WARNINGS/ADVISORIES…
PR…Heat Advisory from 9 AM this morning to 5 PM AST this afternoon
for PRZ001>005-007-008-010>013.
VI…Heat Advisory from 9 AM this morning to 5 PM AST this afternoon
for VIZ001-002.
AM…None.
&&
$$
KEY MESSAGES/SHORT TERM/AVIATION/FIRE…CAM
LONG TERM….GRS
MARINE/BEACH FORECAST…ICP
Alert Expiration: 2026-07-27T15:05:00+00:00
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