758
FXCA62 TJSJ 061744
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
144 PM AST Mon Oct 6 2025

.KEY MESSAGES...

* Travel disruptions and urban or small-stream flooding possible
  with isolated flash flooding not ruled out. Greatest impacts
  each afternoon shifting from eastern and north-central to
  northwestern areas on Tuesday, western areas Thursday, southern
  areas Friday, and eastern areas over the weekend.

* Increased risk of heat-related illness especially for
  vulnerable groups and outdoor workers. Heat impacts rise Tuesday
  into Wednesday and persist through the weekend with greatest
  effects in coastal and urban areas.

* For the US Virgin Islands, minor flooding and lightning possible
  through Tuesday night then returning Friday into the weekend.
  Elevated heat will affect outdoor activities every day.


&&

.SHORT TERM...Tonight through Wednesday...

Since early this morning, northeasterly steering winds directed
scattered showers and isolated thunderstorms along coastal areas
and inland, with streamers also developing from the U.S. Virgin
Islands and nearby islands. Around midday, diurnally driven
convection developed across the Cordillera Central and southern
sections of Puerto Rico. Rainfall estimates ranged from a quarter
of an inch in northeast Puerto Rico this morning to around 2
inches from afternoon convection. Winds, light to calm over land
this morning, increased to 1015 mph with sea-breeze effects by
early afternoon. Highs peaked in the low 90s, with heat indices in
the upper 100s to near 110F in isolated areas. No weather alerts
have been issued today.

Tonight, local weather will be influenced by two key features a
persistent upper-level low just west of Puerto Rico, maintaining
typically cool mid-level temperatures (-6.1 to -5.8 degrees C)
with a weak to absent trade wind cap, and an approaching tropical
wave, whose axis is crossing the Lesser Antilles. This wave will
bring abundant tropical moisture, with precipitable water values
soaring around 2.0 to 2.2 inches, well above normal for this time
of year. A moderate breeze near 13 to 18 mph will prevail, though
winds will become light to calm and variable over land. Remnants
of diurnally driven convection may linger across interior Puerto
Rico through mid-evening, with showers and thunderstorms gradually
shifting overnight into eastern and northern Puerto Rico, the US
Virgin Islands, and adjacent islands. Hazards tonight remain
limited, with localized heavy rainfall capable of minor flooding
and occasional lightning. Northeasterly flow, combined with cloud
cover and wet ground, will allow for slightly cooler overnight
lows.

From Tuesday into Wednesday, the tropical wave will dominate the
local weather pattern as it streams across the region. Abundant
moisture and instability will likely support increased showers and
thunderstorms on Tuesday, before conditions gradually begin to
improve Wednesday as ridging strengthens aloft and drier air
filters in. Winds will shift from northeast on Tuesday to
southeast by Wednesday, steering diurnally driven convection
southwestward Tuesday afternoon, with activity steered northward
and becoming more localized by midweek. As a result, hazards will
peak Tuesday, with a higher risk of flooding rainfall, frequent
lightning, and even localized severe weather from slow-moving
showers and thunderstorms. By Wednesday, flooding and lightning
risks will decrease, but temperatures will rise sharply under
southeasterly flow, elevating heat concerns. Overall, the main
hazards will transition from flooding and storms early in the
period to heat-related risks by midweek.

.LONG TERM...Thursday through Monday...

From previous discussion issued at 516 AM AST Mon Oct 6 2025

The long-term forecast depends on the development of a tropical wave
located southwest of the Cabo Verde Islands. The National Hurricane
Center (NHC) continues to monitor this wave, assigning it a high
(70%) chance of tropical cyclone formation within the next seven
days as it moves quickly across the central Atlantic.

Regardless of its development, the latest model guidance suggests a
track that will pass to the northeast of the Lesser Antilles by the
end of the workweek. Moisture associated with this system is
expected to reach the northeast Caribbean by early Friday,
increasing the potential for showers and thunderstorms across the
regional waters and portions of Puerto Rico and the U.S. Virgin
Islands.

By Saturday, as the system moves just north of the area, winds will
shift from the south. This southerly flow will steer convection
inland, focusing showers and thunderstorms over the northern
portions of Puerto Rico and the U.S. Virgin Islands. By Monday,
winds will become from the southeast as the trailing edge of the
system moves over the islands, further enhancing the potential for
showers and thunderstorms across Puerto Rico and the U.S. Virgin
Islands.

Uncertainty remains high regarding specific rainfall amounts and any
potential local threats. Residents and visitors are therefore urged
to monitor the progress of this system closely over the coming
days.

&&

.AVIATION...
(18Z TAF)

Diurnally driven SHRA/TSRA will cont thru 06/23Z, producing MTN
OBSCN and brief MVFR/IFR conds, mainly at TJPS, with VCSH/VCTS
psbl at otr PR TAF sites. Aft 06/23Z, VCSH/VCTS will cont N PR
terminals and USVI, occnlly mv closer to terminals. SFC winds NE
0812 kt with STRONG SEA-BREEZE VARIATIONS, bcmg LGT/VRB aft
06/23Z, xcp gusty/vrb nr SHRA/TSRA. Winds gradually veer E-SE on
Tue.

&&

.MARINE...

A long-period northeasterly swell will continue to spread across
the local Atlantic waters and passages through midweek,
maintaining hazardous conditions for small craft. Small Craft
Advisories remain in effect through midday Tuesday. In addition, a
tropical wave will influence the region through midweek,
increasing the frequency of showers and thunderstorms over the
regional  waters, especially during the afternoon hours, with
activity further enhanced near coastal areas by diurnally driven
convection.

&&

.BEACH FORECAST...

Although dissipating, a long-period northeasterly swell will
continue to produce life-threatening beach conditions through at
least Tuesday, mainly along Atlantic-exposed beaches. A High Surf
Advisory remains in effect for northern Puerto Rico through late
tonight. Meanwhile, a High Risk of Rip Currents will persist
through at least Tuesday afternoon for Vieques and the northern US
Virgin Islands, and through late Tuesday night along the north-
facing coasts of Puerto Rico and Culebra.. A moderate risk of rip
currents is expected to persist from midweek through the upcoming
weekend.

Potential impacts include large breaking waves that may wash over
jetties and sweep people or pets onto rocks, localized beach and
dune erosion, minor coastal flooding in the most vulnerable areas,
dangerous swimming conditions due to life-threatening rip
currents, and hazardous seas for small craft. Residents and
visitors are strongly urged to monitor the latest beach forecast,
follow the beach flag warning system, and stay out of the water,
as conditions will remain dangerous at least through Tuesday.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Tuesday night for PRZ001-002-
     005-008-010-012.

     High Surf Advisory until 6 AM AST Tuesday for PRZ001-002-005-008-
     010-012.

     High Rip Current Risk through Tuesday afternoon for PRZ013.

VI...High Rip Current Risk through late Tuesday night for VIZ002.

     High Surf Advisory until 6 AM AST Tuesday for VIZ001-002.

     High Rip Current Risk through Tuesday afternoon for VIZ001.

AM...Small Craft Advisory until noon AST Tuesday for AMZ711-712-741-
     742.

     Small Craft Advisory until 6 AM AST Tuesday for AMZ716-723.

&&

$$

MORNING...ICP/MMC
EVENING...LIS/MNG