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648
FXCA62 TJSJ 011723
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
123 PM AST Thu Oct 1 2026

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

.KEY MESSAGES...
Issued at 123 PM AST Thu Oct 1 2026

* A disturbance will aid in the formation of showers and t-storms
  this afternoon, mainly over interior to western Puerto Rico,
  while streamers, and showers and t-storms from the Caribbean
  Waters, move over the southern and eastern regions.

* Lingering moisture and more favorable conditions will result in
  stronger t-storms Friday and Saturday. This will sustain a
  flooding risk, especially over interior to western Puerto Rico
  where an elevated flooding risk will persist each afternoon.

* Heat risks will also persist through the rest of the week,
  possibly becoming elevated tomorrow, Friday, particularly
  across coastal urban areas of northern and western Puerto Rico
  and areas of the U.S. Virgin Islands.

* A moderate risk of rip currents will continue along northern,
  and eastern beaches of Puerto Rico, as well as across portions
  of the U.S. Virgin Islands.

&&

.Short Term(This evening through Saturday)...
Issued at 123 PM AST Thu Oct 1 2026

A surface-induced disturbance filtered into the US Virgin Islands
and Puerto Rico overnight bringing from average-to-above average
precipitable water values to the region. The bulk of this moisture
remains situated to the southeast of the islands and will continue
to move across the region during the day. The associated moisture
combined with breezy conditions has already lead to the development
of advective showers, affecting portions of the US Virgin Islands,
windward waters and eastern Puerto Rico. Radar estimates indicate
that municipalities like Yabucoa, Maunabo, Patillas, Naguabo,
Fajardo and Humacao have already received 0.5-1 inches of rain as
convection continues to progress over the region. Lingering moisture
from the disturbance will persist over the CWA until Saturday
promoting the development of passing showers and localized
thunderstorms. A couple of high and low pressure systems located
over the west and central Atlantic Ocean are setting the steering
flow to be from the ESE until the end of the short term period. This
is expected to promote breezy-to-locally windy conditions until
Saturday which will serve as a drivers for advective showers during
the mornings and streamers from el Yunque and the smaller islands
during the afternoons.

Model guidance suggests that today will be characterized by a
moister vertical profile during the afternoon. Additionally, while
the day began with warmer than average mid-tropospheric
temperatures, these will begin to decrease into the afternoon
leading to a steepening of the 700-500 mb lapse rates. While an
upper-level low pressure system continues to promote upper-level
convergence near the islands, the aforementioned features combined
with the enhanced moisture from the disturbance and breezy wind
conditions will lead to streamers affecting the US Virgin Islands
and eastern Puerto Rico this afternoon with showers and
thunderstorms and showers over the Caribbean waters also affecting
the southeast and south portions of Puerto Rico. In addition,
coupled with local effects, the enhanced moisture availability will
also lead to the development of convection across the interior,
western interior, and western Puerto Rico during the afternoon.
While the showers might move at a moderate pace, they might be
recurrent over some regions. Thus, there is an elevated flooding
risk wherever frequent downpours do occur. This means that flooding
in urban areas, roads, small streams, and washes along with a low
chance for isolated flash floods is possible.

With cooling aloft, steepening of the mid-tropospheric lapse rates,
lingering moisture from the surface-induced disturbance and the
departure of the upper-level low pressure system, Friday promises to
be the most active day of the short term period. A similar pattern
will persist with advective showers during the morning and streamers
from El Yunque and the smaller islands affecting the US Virgin
Islands and eastern Puerto Rico during the afternoons. Whereas,
rainfall is expected to affect the western and northwestern region
of Puerto Rico during the afternoon. The flooding risk remains
elevated with a focus on the norwestern portion of Puerto Rico.

For Saturday, there is some disagreement in the models, with some
suggesting a moister vertical column and others suggesting drier mid-
levels. Nonetheless, lingering moisture, breezy ESE conditions,
cooling aloft, steepening of the mid-tropospheric lapse rates, and
local effects will favor the development of convection. By Saturday,
the focus of the rainfall will be on the western interior and
western Puerto Rico during the afternoon. Model guidance suggest
that winds will back late afternoon into the evening. Please
continue to monitor the forecast, an earlier change in the steering
flow can affect the location of convection.

With and increase in precipitable water and low-level temperature
values above normal values for the season the heat risk is expected
to be limited for today and Saturday, with an elevated risk expected
by Friday. These levels of heat may affect individuals sensitive to
heat, without effective cooling and adequate hydration. Citizens and
visitors are invited to remain alert of the changing weather
conditions.

&&

.Long Term(Sunday through Thursday)...
Issued at 353 AM AST Thu Oct 1 2026

A variable and seasonal pattern will persist for the first part of
the long term. On Sunday, broad surface high pressure will remain in
place across the central Atlantic through Sunday into Monday, while
its combines with an induce disturbance over the central Atlantic,
maintaining an easterly wind flow within the 03 km layer across
Puerto Rico and the U.S. Virgin Islands. Aloft, a cut-off low
anchored over the western Atlantic and Caribbean, just north of
eastern Cuba, will enhance southerly winds at 250 mb, favoring the
development of an extensive upper-level cloud deck over the islands.
The influence of this upper-level low will also be evident at 500
mb, where southerly flow and slightly colder temperatures, near -7C
to -8C, are expected. Despite the colder temperatures aloft,
available moisture will remain limited, with PWAT values generally
near climatological normals and most of the moisture confined to the
lower levels. Therefore, although periods of upper-level cloudiness
will persist, especially into Monday, the overall weather pattern
should remain relatively stable with limited deep moisture available
for significant convective development. On Monday, similar weather
conditions will persist with possibly less upper cloudiness as the
advecting upper-level moisture pattern with the high clouds moves
out of the region. As a result, warmer temperatures and heat indices
are possible, with a limited heat threat along the islands, with
some isolated areas that have an elevated heat threat, especially in
the coastal areas and urbanized areas.

On Tuesday, the mid-level ridge will still be present, marginally
limiting deep convection. However, lapse rates returning to seasonal
values signal a better chance of typical afternoon showers and
thunderstorms driven by an easterly flow. This pattern should allow
afternoon convection to move over the interior and then over western
portions of Puerto Rico. A tropical wave will reach the islands by
Tuesday evening, increasing the PWAT values to above-seasonal levels
through Wednesday. Therefore, Wednesday is forecast to be the more
active day in terms of shower and thunderstorm activity, supported
by mid-level lapse rates at above-seasonal levels (exceeding 6.0
C/km). Additionally, a more east-southeasterly flow could drive
further activity over northwestern PR. As a result, for Tuesday into
Thursday there is a high chance of observer a limited to elevated
flooding threat especially over western sections of Puerto Rico.

Overall, the primary hazards for the long-term period will remain a
limited-to-elevated heat risk and a limited risk of excessive
rainfall from Sunday through Tuesday. As a result, urban and small-
stream flooding is possible over roadways and poorly drained areas,
and flash flooding cannot be ruled out. For those sensitive to the
heat, staying hydrated and avoiding outdoor activities during peak
heating hours is highly recommended.

&&

.AVIATION...
(18Z TAFS)
Issued at 123 PM AST Thu Oct 1 2026

Mainly VFR conditions to prevail during the period. Developing
VCTS over the region will affect mainly TJBQ/TJSJ/TJPS/TJBQ. Tempo
MVFR conditions due to -TSRA are more likely at TJBQ btw
01/18-22Z. VCSH/-SHRA also affecting terminals during the
afternoon, affecting more the eastern region overnight. ESE winds
up to 15 to 20 kts, with higher gusts and sea breeze variations
thru 01/21Z. Decreasing after 01/23Z, to up to around 10 kts with
land breezes, before increasing again after 02/13Z.

&&

.MARINE...
Issued at 123 PM AST Thu Oct 1 2026

A surface high pressure north of the region will maintain moderate
to locally fresh easterly trade winds through at least Friday,
producing choppy conditions across the Atlantic and Caribbean waters
as well as local passages. Winds will shift to mainly moderate over
the weekend and into early next week, keeping seas moderately
choppy. Meanwhile, a passing disturbance today will increase showers
and thunderstorms across the waters, creating locally hazardous
conditions for small craft, including gusty winds, higher seas,
lightning, and reduced visibility, with lingering moisture
maintaining unsettled weather into the early weekend.

&&

.BEACH FORECAST...
Issued at 123 PM AST Thu Oct 1 2026

A moderate risk of rip currents will persist across most exposed
north- and east-facing beaches of Puerto Rico and the U.S. Virgin
Islands through at least Sunday, before diminishing to low across
all beaches Monday into Tuesday. Beachgoers should remain cautious,
as dangerous rip currents can still occur near piers, jetties,
groins, and rocky areas, always swim near a lifeguard and follow
beach patrol guidance and posted warning flags. Additionally,
afternoon thunderstorms may develop, particularly along western and
northwestern Puerto Rico; if you hear thunder, immediately leave the
water and seek shelter inside a substantial building or enclosed
vehicle.

&&

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

$$

SHORT TERM...RVM
LONG TERM...JCP/LIS
KEY MESSAGES/AVIATION...MRR
MARINE/BEACH...GRS

NWS SJU Office Area Forecast Discussion