726
FXCA62 TJSJ 180934
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
534 AM AST Sat Oct 18 2025

...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST...

.KEY MESSAGES...

* For Puerto Rico, there is a limited to elevated flood threat,
  due to afternoon thunderstorms throughout the weekend.

* For the U.S. Virgin Islands, a limited flooding risk and
  lightning activity across the islands from time to time.

* Hazardous marine and coastal conditions due to dangerous
  breaking waves and life-threatening rip currents from NOW into
  the upcoming workweek for the north-facing beaches of PR and
  the USVI.

&&

.SHORT TERM...Today through Monday...
Issued at 523 AM AST Sat Oct 18 2025

The remnants of a frontal boundary interacting with deep tropical
moisture continue to promote heavy showers and strong thunderstorms
across portions of the local area. Overnight, a strong thunderstorm
developed over St. Thomas, prompting the issuance of a Special
Weather Statement and a Flash Flood Warning. Radar estimates
indicated rainfall totals up to 2.5 inches over the island.
Elsewhere, scattered thunderstorms developed over the Atlantic and
Caribbean waters and Mona Passage, producing frequent lightning and
locally heavy rainfall, resulting in Marine Weather Statements and
Special Marine Warnings.

Today, southerly low-level winds will favor afternoon convection
developing over the interior and moving northward toward the north-
central municipalities of Puerto Rico. The light steering flow will
support slow-moving thunderstorms, enhancing the potential for urban
and small-stream flooding. The most intense thunderstorms could
produce frequent lightning, gusty winds, and ponding of water on
roadways.

The GFS-sounding continues to depict precipitable water values
between 2.0 and 2.2 inches, steep 700-500 mb lapse rates, and 850-
700 mb relative humidity exceeding 70 percent, confirming a very
moist and unstable column consistent with the ongoing convective
activity.

By Sunday, winds will increase slightly and shift to the southeast
as the remnants of the front lift northward. A gradual drying trend
is expected as mid-level moisture decreases, but sufficient low-
level moisture will linger to support scattered afternoon showers
and isolated thunderstorms, mainly across the interior and
northwestern Puerto Rico. Although a tropical wave will move near
the region, its deepest moisture should remain south of the islands.
Even so, sea-breeze convergence, orographic lift, and favorable
upper-level dynamics (500 mb temperatures near 6C, moderate lapse
rates) will sustain occasional strong convection capable of
producing frequent lightning, gusty winds, and localized flooding.

Additionally, having the remnants of the front north of the area,
combined with the tropical wave passing south, could result in an
increase in low-level convergence. A similar weather pattern is
expected on Monday as another tropical wave passes south of the
forecast area, supporting another round of afternoon showers and
thunderstorms, particularly across the interior and western sections
of Puerto Rico. While the bulk of the moisture will remain south of
the islands, localized flooding, gusty winds, and dangerous cloud-to-
ground lightning will continue to be concerns.

&&

.LONG TERM...Tuesday through Friday...
Issued at 523 AM AST Sat Oct 18 2025

Tuesday is expected to be the driest and most stable day of the
long-term period, as model guidance indicates precipitable water (PWAT)
values decreasing to around or slightly below normal for this
time of year. This drier and more stable air mass will help
suppress widespread convective activity, leading to mostly fair
weather conditions. However, localized afternoon showers cannot be
ruled out, mainly across the interior and western sections of
Puerto Rico due to daytime heating and local effects. Flooding
potential remains minimal through this period.

Wednesday will still be a generally fair day, with conditions remaining
relatively stable during the morning hours. However, as the day
progresses, a gradual increase in moisture content and mid-level
humidity is expected, resulting in a higher potential for
afternoon showers across portions of the interior and western
Puerto Rico. This trend marks the beginning of a slow transition
toward a wetter pattern.

From Thursday onward, the forecast becomes increasingly dependent on
the evolution of a system currently being monitored by the
National Hurricane Center (NHC). At this time, the long-term
forecast leans toward the GFS solution, which depicts a wetter
pattern developing from Thursday into the weekend. The NHC
currently assigns this disturbance a 0 percent chance of
development within the next 48 hours and a 30 percent chance
within the next 7 days.

It is important to note that forecast confidence decreases
significantly after Thursday, as there remains high uncertainty and
notable discrepancies among model solutions regarding the
development and possible impact for Puerto Rico and the U.S.
Virgin Islands. Regardless of development, an increase in moisture
and instability could lead to wetter and potentially unsettled
conditions late in the week.

Residents and visitors are advised to monitor the latest forecasts
and updates from the National Weather Service and the National
Hurricane Center for any significant changes.

&&

.AVIATION...
(06Z TAFS)

SHRA/TSRA over regional waters will cont through early mrng, with
brief MVFR/IFR conds psbl near coastal terminals, particularly
ovr TIST/TISX. Aft 15Z, SHRA/TSRA will redevelop ovr the interior
and N- CNTRL PR due to sea breeze conv and ample low-lvl moisture.
Expect VCTS at TJSJ, TJBQ, and TJPS by late mrng thru aftn, with
brief TSRA and reduced VIS/CIGs psbl mainly btw 17-22Z. VFR conds
should prevail at most terminals ovrngt. Winds lgt/vrb bcmg S
10-12KT aft 14Z with higher gusts, diminishing aft 22Z.

&&

.MARINE...
Issued at 523 AM AST Sat Oct 18 2025

The nearby stationary boundary will continue to promote shower and
thunderstorm activity across the regional waters and passages
through at least today. A long period northerly swell, associated to
low pressure system over the northwestern Atlantic waters, is
expected to reach the Atlantic waters and local passages today into
Sunday through the upcoming workweek. As a result, hazardous marine
conditions are anticipated. Please, continue to monitor the forecast
updates over the coming days.

&&

.BEACH FORECAST...
Issued at 523 AM AST Sat Oct 18 2025

Due to the presence of an incoming long period northerly swell
a HIGH rip current risk was issued for the northern, western, and
eastern coast of PR and Culebra through the upcoming workweek. This
swell will create dangerous breaking waves and life- threatening
rip currents along the west, north, and eastern beaches of the
islands.

High surf conditions and minor coastal flooding are likely by
Sunday, and will possibly extend into the early part of next week.
Overall, dangerous swimming conditions, minor beach erosion, and
life-threatening rip currents are expected from this weekend into
early next week.

&&

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

VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION/MARINE...CVB
LONG TERM/MARINE/BEACH FORECAST...MMC