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172
FXCA62 TJSJ 261856
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
256 PM AST Fri Sep 26 2025

.KEY MESSAGES...

* Tropical moisture from AL94, currently located north of
  Hispaniola, is creating excessive heat indices reaching up to
  111 degrees Fahrenheit.

* A Heat Advisory is in effect through 4 PM AST for coastal and
  urban areas of Puerto Rico, including Culebra and Vieques. A
  Heat advisory is in effect for the US Virgin Islands due to warm
  to hot heat indices across the islands, with heat indices
  reaching up to 108 degrees.

* Thus, a heat advisory is in effect through this afternoon at 4
  PM AST. The hot weather pattern is expected to persist into the
  weekend. The risk of rip currents will increase from moderate to
  high during the weekend due to the arrival of swell energy
  generated by Humberto.

* The distant Humberto, which continues to move at a safe
  distance from PR and the USVI, could promote an unstable and
  moist weather pattern on Monday and Tuesday.

&&

.SHORT TERM...TONIGHT through SUNDAY...

The potential for flooding rains continues to decrease during the
day as Tropical Wave (AL94) moves away from Hispaniola. However,
the east-southeasterly wind flow, combined with available
moisture, results in hot heat indices between 102 and 111 degrees
Fahrenheit across the coastal and urban sections in PR and the
USVI, where advisories continue in effect through this afternoon
at 4 PM AST. The southerly winds also brought showers and
thunderstorms across the Caribbean Sea near St. Croix and
southeast PR, as well as near the northern US Virgin Islands.
Maximum temperatures ranged from the low to mid-90s, primarily
across St. Croix and west and south PR, to the mid-to-upper 80s
across the mountainous areas. Strong thunderstorms developed along
the north coast of PR, producing frequent cloud-to-ground
lightning, downpours, and gusty winds, while the US Virgin Islands
remained under clear skies during the afternoon.

Showers and thunderstorms will continue across northwest PR,
producing frequent cloud-to-ground lightning, downpours, and gusty
winds through this evening. At the same time, the USVI will
continue to experience warm to hot temperatures, with a mixture of
clouds and sunshine or clear skies by evening.

Humberto`s trajectory will promote a col near PR and the USVI on
Saturday and Sunday, thus expect calm to light southerly wind
flow. Sea breezes will form during the morning and afternoon
hours, giving way to afternoon convection over the region, mainly
along the Cordillera Central and north PR. The US Virgin Islands
are experiencing a few showers over the waters, slowly drifting
inland at times. A hot weather pattern will also affect the
region, and Excessive Heat Warnings or Heat Advisories will be
needed.

.LONG TERM...Monday through Friday....from previous discussion...

The long-term forecast remains on track for the first part of the
upcoming workweek. On Monday, the surface weather pattern across
Puerto Rico and the U.S. Virgin Islands will be influenced by
Hurricane Humberto, a tropical system located over the western
Atlantic. At the upper levels, a mid- to upper-level ridge will
dominate, providing stability over the islands. Under this
pattern, southeasterly winds will transport patches of fragmented
moisture. According to global model guidance (GFS), precipitable
water values for Monday are expected to range between 1.9 and 2
inches, supporting a variable pattern with warm conditions during
the morning, followed by convection and thunderstorms across
northwestern Puerto Rico. By Tuesday, surface moisture is expected
to decrease to near the 25th percentile as the moisture plume
shifts into the western Atlantic, leaving the islands under a much
drier pattern. This reduction will favor a more stable day with
extended periods of morning sunshine, contributing to warmer
conditions as also suggested by the 925 mb temperatures. Weaker
winds will further support a dry pattern due to the lack of a
significant sea breeze effect, particularly across western Puerto
Rico. Although surface conditions will generally be unfavorable
for widespread weather activity, isolated showers with
thunderstorms cannot be completely ruled out.

From Wednesday through Friday, 250 mb heights are forecast to
decrease, resulting in slight instability due to an upper-level
trough near the region. However, compared to yesterdays
solutions, todays guidance suggests less instability for
Thursday. At the surface, fragmented moisture will be steered
mainly by the prevailing winds associated with a strengthening
surface high over the central Atlantic as Hurricane Humberto
continues moving into the northwestern Atlantic. As this high
pressure builds, a weak point in the pressure gradient is expected
to develop, diminishing the 03 km surface winds.

&&

.AVIATION...
(18z TAFs)

Expect VFR conditions through the fcst period. JBQ can expect a
isolated TSRA reducing CIGs and VIS (likely to promote MVFR/IFR
conds thru 26/23z). Expect winds from the ESE-SE at 10-15 kt with
higher gusts and sea breeze variations, becoming light/var aft
26/23z. Winds will be dominated by sea breezes on the 27th. Aft
convection will develop tomorrow, which could impact JSJ or JBQ.

&&

.MARINE...

Gentle to light winds will persist due to the interaction between
Invest 94L and Humberto. Therefore, mariners can expect surface
winds up to 10 knots with seas up to 5 feet. An increase in seas
is forecast for the upcoming weekend as pulses of northerly swells
from Humberto moves across the local Atlantic waters,
deteriorating marine conditions across the region.

&&

.BEACH FORECAST...

Beachgoers on north- and east-facing beaches in Puerto Rico and
the US Virgin Islands are advised to exercise caution due to a
moderate risk of rip currents through Saturday afternoon. The risk
of rip currents will increase from moderate to high during the
weekend due to the arrival of swell energy generated by Humberto.

Additionally, thunderstorms in the area may produce lightning
(especially during the afternoons), so if you hear thunder, seek
shelter indoors or find a safe location.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 4 PM AST this afternoon for PRZ001>003-005-
     007-008-010>013.

VI...Heat Advisory until 4 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...CAM
LONG TERM....MNG
AVIATION...YZR

NWS SJU Office Area Forecast Discussion