Urb Fullana, Puerto Rico Weather Forecast Discussion

723
FXCA62 TJSJ 011859
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
259 PM AST Wed Oct 1 2025
.KEY MESSAGES...
* Afternoon showers and thunderstorms are expected across the
  northwest quadrant of Puerto Rico today.
* Across the U.S. Virgin Islands, mainly fair weather conditions
  will persist with passing showers moving from time to time.
* An Extreme Heat Warning remains in effect for all urban and
  coastal areas of Puerto Rico and the U.S. Virgin Islands until 5
  PM AST. Stay hydrated and avoid prolonged sun exposure.
* A large, long-period northerly swell is expected to arrive by late
  Thursday night, promoting hazardous marine and beach conditions
  across the islands, persisting through at least early next week.
&&
.SHORT TERM...Tonight through Friday...
Light southeasterly steering flow continues to steer below normal
moisture over the area. Current satellite derived precipitable water
(PWAT) values indicate 1.3 (over southern PR) to 1.65 inches over
Puerto Rico. Around 1.65 inches over Vieques, around 1.7 inches over
Culebra and St. Croix, and around 1.75 inches (borderline normal
values) over St. Thomas and St. John. Some patches of moisture with
PWAT values around 1.8 inches will reach the area at times. Light
steering flow will veer to become more southerly tonight before
backing again to become more southeasterly on Thursday and then more
northeasterly on Friday to end the period. Shower and t-storm
activity will continue to develop for the rest of the afternoon and
evening over mainly the NW quadrant of PR, and also over northern PR
and around the Cordillera. Increasing 700 to 500 mb lapse rates and
decreasing 500 mb temperatures continue to be forecast for the rest
of the period due to a series of short-wave troughs along a ridge
north of the region. This along with diurnal heating and local
effects will still result in afternoon showers and t-storms along
the interior each afternoon, steered by the weak above mentioned
flow and affecting similar areas today and possibly more interior to
southern areas on Friday. Lighter flow will also help this
convective activity linger longer and prompting a limited flooding
risk. Passing showers continue to be forecast across windward
sectors each day. Lines of showers can also develop downwind of the
local islands during each afternoon. Low temperatures are forecast
in the mid to upper 70s along the coastal areas of the islands to
the mid to upper 60s over interior PR. Highs are forecast in the
upper 80s to low 90s across coastal areas of the islands. In general
925 mb temps are forecast to decrease during the period, still 1
standard deviation above normal values tomorrow and normal to
slightly above normal on Friday. Elevated heat risk is still
forecast for tomorrow with a Heat Advisory possibly being issued
with the next forecast package.
.LONG TERM...Saturday through Wednesday...
/FROM PREV DISCUSSION...Issued at 509 AM AST Wed Oct 1 2025/
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...
(18Z TAFS)
Mainly VFR conditions expected. Afternoon VCSH will affect most
terminals with VCTS to TS forecast mainly for TJBQ until around 23Z.
This can promote brief MVFR conditions. Winds will continue from the
SE up to 10 kts with sea breeze variations and higher gusts. Winds
will be light and variable after 01/23Z, gradually gaining a
southerly component and picking up again from the SE up to 10-12 kts
with sea breeze variations after 02/13Z.
&&
.MARINE...
Light to gentle southeasterly winds will continue for the next few
days across the region. A large long-period northerly swell will
promote hazardous marine conditions for small craft across the
Atlantic waters and local Passages by late Thursday night through
the upcoming weekend. A tropical wave approaching the Lesser
Antilles will increase the potential for showers and thunderstorm
activity across the regional waters and local passages from Saturday
onwards.
&&
.BEACH FORECAST...
There is a low risk of rip currents for all beaches of Puerto Rico
and the U.S. Virgin Islands. However, the coastal conditions will
begin to deteriorate by late Thursday night due to the arrival of a
large long-period swell. 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. Therefore, beachgoers are urged to continue
monitoring the beach forecast and heed the advice of the flag
warning system. Residents and visitors, please 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 until 5 PM AST this afternoon for
     PRZ001>005-007-008-010>013.
VI...Extreme Heat Warning until 5 PM AST this afternoon for VIZ001-
     002.
AM...None.
&&
$$
MRR/GRS

NWS SJU Office Area Forecast Discussion