Urb Ramirez De Arellano, Puerto Rico Weather Forecast Discussion

870
FXCA62 TJSJ 021904
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
304 PM AST Thu Oct 2 2025
.KEY MESSAGES...
* A Heat Advisory 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 the rest of
  today and over the coming days.
* Showers and thunderstorms along interior and northern portions
  of Puerto Rico will increase flooding and lightning risk during
  the next hours.
* Across the U.S. Virgin Islands, occasional passing showers
  during the overnight and Friday morning hours.
* A strong, long-period northerly swell will create hazardous
  marine and beach conditions across the islands through at least
  early next week. Small Craft Advisories, Rip Current Statements,
  and High Surf Advisories are in effect. More details on timing
  and location can be found at the end of the following
  discussion.
&&
.SHORT TERM...Tonight through Saturday...
Below normal to slightly below normal precipitable water (PWAT)
values are being observed over the islands under a light
southeasterly steering flow. Current satellite derived PWAT values
indicate 1.3 (southern PR) to 1.6 inches over Puerto Rico, around 1.
58 inches over Culebra and St. Croix, around 1.53 inches over
Vieques and around 1.65 inches over St. Thomas and St. John.
Moisture content is expected to continue below normal to normal
values on Friday and more normal values on Saturday. This increase
in moisture is due to both a weak tropical wave moving mainly south
of the islands and a frontal boundary lingering north of the
islands. These systems will also increase the potential for more
frequent passing showers and showers and t-storms during the
afternoon under a mainly SE steering flow. Before that however,
current light southeasterly flow will back to become more
northeasterly on Friday, a a col area is northeast of the region,
and then veer to become southeasterly again by Saturday. Shower and
t-storm activity will continue to develop for the rest of this
afternoon and evening over mainly around the Cordillera, with radar
detecting activity over the NW interior. Showers are also developing
at the USVI and under light steering winds will continue to linger
at or around the islands this evening. Diurnal heating and local
effects will result in afternoon showers and t-storms along the
interior each afternoon, steered by the weak above mentioned flow
and affecting similar areas on Saturday (with increased coverage due
to the wave) and possibly more interior to southern areas tomorrow,
Friday. Lighter steering flow tomorrow will also help this
convective activity linger longer and prompt at least a limited
flooding risk. 925 mb wind speeds will gradually increase as the
period continues. Passing showers continue to be forecast across
windward sectors each overnight and day. Lines of showers can also
develop downwind of the local islands during each afternoon. Minimum
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 normal to slightly above normal on Friday.
.LONG TERM...Sunday through Thursday...
.PREV DISCUSSION... /issued 452 AM AST Thu Oct 2 2025/
No major changes were introduced to the long-term forecast. A
transition to an unstable and wetter pattern is still anticipated by
the beginning of the workweek, with gradual improvement by
Wednesday. Global models continue to suggest the approach of an
upper-level trough into the CWA by Sunday night. From the
deterministic guidance of the GFS and ECMWF, the tendency towards
wetter conditions is increasing, as Precipitable Water (PWAT) values
may range between 2.0 and 2.2 inches, above the climatological
normal. The latest model solutions also suggest a rise in relative
humidity content in the low and mid levels as well (> 60 %, up to 80
- 90%) during Monday and Tuesday. In terms of instability, the
presence of this upper feature should bring colder than normal 500
mb temperatures (down to -8 degrees Celsius), favorable for the
development of strong thunderstorms. The latest Galvez-Davison Index
(GDI) tool keeps suggesting the potential of isolated to scattered
thunderstorms across the CWA, higher on Monday and Tuesday. As
mentioned in the previous discussion, winds will become lighter late
Monday night and Tuesday, meaning that theres a high chance of
stationary showers and thunderstorms. Given the expected conditions,
the potential of flooding and lightning will increase, particularly
over mountain ranges and eastern Puerto Rico, including Vieques and
Culebra, and the U.S. Virgin Islands.
Conditions should gradually improve by Wednesday, as a drier air
mass may filter into the region.  Although PWAT may drop to seasonal
values (1.5 - 1.7 inches), the available moisture combined with
diurnal heating and local effects could trigger convection activity
in the afternoon, affecting mainly portions of interior and western
Puerto Rico, including the San Juan Metropolitan Area. Nevertheless,
the likeliest scenario could be mostly ponding of water over
roadways, urban, and poorly drained areas.
The National Hurricane Center is monitoring a tropical wave expected
to move off the coast of Africa in the next few days, with a 20%
chance of cyclonic formation in the next 7 days. The latest model
solutions suggest an increase in moisture content due to this
tropical wave approaching the Caribbean Basin by late Thursday
night, but the variability between them is high, introducing
uncertainty to the forecast. Hence, well keep monitoring the
development of this system.
&&
.AVIATION...
(18Z TAFS)
Mainly VFR conditions expected. Afternoon VCSH will affect most
terminals with VCTS forecast for TJBQ, TJPS and TJPS until around
23Z. Light SSE winds will continue, dominated by sea breeze
variations. Winds will be light and variable with land breezes after
01/23Z, gradually gaining a northerly component overnight picking up
again as light NE flow with sea breeze variations after 03/13Z.
&&
.MARINE...
Light to gentle southeasterly winds will continue for the next few
days due to a col area near the region. A strong, long-period
north to northwesterly swell spreading across the Atlantic waters
will create hazardous marine conditions across the islands through
at least early next week. A Small Craft Advisory is in effect
across the Atlantic Waters, northwest PR, and Mona Passage from
midnight tonight to 4 AM AST Monday due to seas around 7 to 10
feet expected. Inexperienced mariners, especially those operating
smaller vessels, should avoid navigating in hazardous conditions.
Additionally, a weak tropical wave is expected to move over the
area this weekend, while a frontal boundary will remain positioned
to the north over the Atlantic.
&&
.BEACH FORECAST...
A strong, long-period northerly swell will create hazardous beach
conditions across the islands through at least early next week. A
high risk of rip currents is in effect from 2 PM AST through at
least late Monday night across beaches along the west, north, and
east coasts of Puerto Rico, as well as in Culebra and St. Thomas.
Additional areas may be added in the coming days. Also, a High
Surf Advisory is in effect for these same areas from 6 AM AST
tomorrow, Friday, through, at least 6 AM AST Monday, due to
breaking waves above 10 feet. This conditions will likely produce
localized beach erosion and dangerous swimming conditions.
Citizens and visitors are encouraged to continue monitoring the
beach forecast and heed the advice of the flag warning system.
Beachgoers, do not risk your life, is better to stay out of the
water!
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.
High Rip Current Risk through late Monday night for PRZ001-002-
     005-008-010-012.
High Surf Advisory from 6 AM Friday to 6 AM AST Monday for
     PRZ001-002-005-008-010-012.
VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.
High Rip Current Risk through late Monday night for VIZ001.
High Surf Advisory from 6 AM Friday to 6 AM AST Monday for
     VIZ001.
AM...Small Craft Advisory from midnight tonight to 4 AM AST Monday
     for AMZ711.
Small Craft Advisory from noon Friday to 4 AM AST Monday for
     AMZ712-716-741-742.
&&
$$
SHORT TERM/AVIATION...MRR
MARINE/BEACH FORECAST...YZR
LONG TERM....MNG

NWS SJU Office Area Forecast Discussion