Repto Vega Badillo, Puerto Rico Weather Forecast Discussion

756
FXCA62 TJSJ 010909
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
509 AM AST Wed Oct 1 2025
.KEY MESSAGES...
* Hot temperatures today will promote hazardous heat conditions
  across the islands. An Extreme Heat Warning is in effect for all
  urban and coastal areas of the islands from 10 AM AST through 5
  PM AST. Stay hydrated and avoid prolonged sun exposure.
* Showers and thunderstorms will increase the flooding and
  lightning threat over interior and northern portions of Puerto
  Rico this afternoon, while the U.S. Virgin Islands can expect
  occasional passing showers throughout the day.
* A large, long-period northerly swell is expected by late
  Thursday night, bringing hazardous marine and beach conditions
  across the islands in the upcoming weekend.
&&
.SHORT TERM...Today through Friday...
Calm weather conditions prevailed overnight with a few showers
moving across the local waters, and some making their way inland
across the US Virgin Islands and windward locations in PR. Winds
were mainly from the southeast, calm to light and variable. In
general, skies were mostly clear with some clouds moving through the
Virgin Islands. Low temperatures ranged from the mid- to upper 70s
along the coastal areas to the mid- to upper 60s across the
mountains.
An easterly perturbation will increase the available moisture across
the islands today, which, combined with a light southeasterly wind
flow and the above-normal maximum temperatures, will result in
extreme heating across the urban and coastal locations in the US
Virgin Islands and Puerto Rico. As a result, an Extreme Heat Warning
is in effect from 10 AM AST to 5 PM AST. The excessive heating will
then combine with the local effects, giving place to one or two
strong afternoon thunderstorms across the interior and northern
portions of PR. These thunderstorms will result in frequent cloud-to-
ground lightning, downpours, and gusty winds. The afternoon
convection is expected to dissipate by evening, leaving mostly clear
skies and calm weather conditions for the islanders. The weak
steering wind flow will allow the thunderstorms to remain almost
stationary, thus creating a slight risk of flooding rains,
especially if forming over areas with poorly drained conditions.
A similar weather pattern will prevail in the short term, consisting
of warm to hot indices during the maximum heating time of the day,
followed by afternoon convection across portions of PR, and a few
passing showers across the US Virgin Islands and windward locations
of PR overnight and early in the morning.
&&
.LONG TERM...Saturday through Wednesday...
The long-term forecast remains on track, with a transition to
wetter and unstable conditions for the first part of the
workweek. A tropical wave over Central Atlantic approaching the
Caribbean Basin is expected to arrive by Saturday, increasing
shower activity across the local islands but remaining limited due
drier air in the 700 - 300 mb layer. The latest model guidance
continues to suggest the approach of upper- level troughs over the
CWA by Saturday and lingering for the remainder of the period.
From the latest deterministic guidance of the GFS and ECMWF,
Precipitable Water (PWAT) values may increase to above normal
values (2.0 - 2.2 inches) by Monday through Wednesday.
Additionally, this could push relative humidities in the lower and
mid-levels to well above normal (around 80%). The presence of
these troughs should promote colder 500 mb temperatures (-7 and -8
degrees Celsius), favorable for deep convection activity. The
Galvez-Davison Index (GDI) also suggests the potential of isolated
to scattered thunderstorms on Monday and Tuesday, increasing the
lightning threat. Another factor that may influence the shower and
thunderstorm activity is that winds will once again drop by late
Monday night, meaning that any convection activity could remain
stationary and increase the flooding threat as well. At the
moment, expect an increase in shower and thunderstorm activity,
particularly over windward sections of the islands and deep
convection activity affecting mostly the mountain ranges of Puerto
Rico, gradually improving by Wednesday. Although the flooding and
lightning threat will remain limited to elevated during the long-
term forecast, confidence remains low due to variability between
ensemble members.
&&
.AVIATION...
(06z TAF)
VFR conditions will prevail today for all TAFs. However, a few TSRA
will develop during the afternoon (between 01/15-22z), which could
impact JSJ/IST/ISX or JBQ, creating brief MVFR conditions if those
TSRA move over them. Winds will prevail mainly from the SE-ESE at 5-
10 kt, with sea breeze variations after 01/13z, and gusty winds near
TSRA. Winds will turn calm to light and VRB aft 01/23z.
&&
.MARINE...
Light to gentle southeasterly winds will continue for the next few
days as a col area approaches the region. A tropical wave approaching
the Caribbean Basin should increase shower and thunderstorm activity
across the regional waters and local passages by Saturday. A large
long-period swell will promote hazardous marine conditions for small
craft by late Thursday night through the upcoming weekend.
&&
.BEACH FORECAST...
As anticipated, the risk of rip current became low for all beaches
of Puerto Rico and the U.S. Virgin Islands. Nevertheless,
a large, long-period swell is anticipated to arrive by late
Thursday night and persisting through at least Sunday. Life-
threatening rip currents are expected mainly along north-facing
beaches of Puerto Rico and the northern U.S. Virgin Islands. This
swell could also increase high surf potential as well, hence,
beachgoers should continue monitoring the beach forecast and heed
the advice of the flag warning system. Additionally, they should
stay weather alert due to the potential of showers and
thunderstorms moving over the western and northwestern coastal
areas of Puerto Rico in the afternoon.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Extreme Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for PRZ001>005-007-008-010>013.
VI...Extreme Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...CAM
LONG TERM...MNG
MARINE/BEACH FORECAST...CAM/MNG

NWS SJU Office Area Forecast Discussion