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906
FXCA62 TJSJ 201821
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
221 PM AST Tue Oct 20 2026

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

.KEY MESSAGES...
Issued at 210 PM AST Tue Oct 20 2026

* Afternoon thunderstorms will persist through this evening
  producing lightning, locally heavy rainfall, and urban or small-
  stream flooding across northern Puerto Rico. Activity should
  decrease by Wednesday before a surge of tropical moisture
  brings more widespread showers and renewed flooding concerns
  Thursday into the weekend.

- Heat Threat (Level 1): A Level 1 heat threat is expected across
  most of the region throughout the week. Warm temperatures and
  elevated heat indices may primarily affect individuals who are
  highly sensitive to heat, particularly those spending extended
  periods outdoors without adequate hydration or effective
  cooling.

- High Surf Advisory will be in effect through 6 PM AST this
  evening. However, hazardous coastal conditions will persist,
  with a high risk of rip currents along northern exposed beaches
  due to incoming northerly swells. Therefore, a high risk of rip
  currents will continue across the northern exposed beaches for
  the rest of the week.

&&

.Short Term(This evening through Thursday)...
Issued at 210 PM AST Tue Oct 20 2026

Relatively quiet and typical weather conditions prevailed this
morning, with a few towering cumulus clouds and passing showers
mainly over the surrounding waters. However, afternoon convection
has already developed across northern Puerto Rico, prompting the
issuance of a Special Weather Statement for several north-central
municipalities. The 20/12Z TJSJ sounding indicated sufficient
moisture and instability to support this activity, with precipitable
water near 1.84 inches and seasonal low- to mid-level lapse rates.
Additionally, a surface-to-3 km mean wind of only 6 knots suggests
slow-moving convection. A weak pressure gradient continues to
maintain light southeast to southerly flow, allowing sea-breeze
convergence and local terrain effects to dominate convective
development.

For the remainder of the afternoon, showers and thunderstorms will
continue to affect portions of the interior and northern Puerto
Rico. Given the weak steering flow, slow-moving thunderstorms may
produce locally heavy rainfall, frequent lightning, and localized
urban and small-stream flooding, particularly where showers persist
over the same areas. Across the U.S. Virgin Islands, passing showers
and isolated thunderstorms should remain the predominant weather
feature, although most activity will remain over the surrounding
waters.

Wednesday continues to look like the most stable and least active
day of the short-term period. GFS guidance suggests precipitable
water values decreasing to around 1.4 to 1.5 inches, while 700-500
mb relative humidity drops to near 20 percent, well below
climatological normals. Although mid-level lapse rates will remain
near or slightly above seasonal values, the drier air aloft should
limit the coverage and persistence of deep convection. Nevertheless,
afternoon showers and isolated thunderstorms remain possible across
Puerto Rico due to local effects. Greater sunshine and reduced cloud
coverage may also make Wednesday the warmest day of the period,
particularly across coastal and urban areas.

A significant increase in tropical moisture remains on track for
late Wednesday into Thursday. Model guidance indicates precipitable
water values rising to around 2.2 to 2.3 inches, while 700-500 mb
relative humidity increases to around 75 to 85 percent, both
exceeding climatological normals. Despite weakening mid-level lapse
rates and slightly warmer temperatures aloft, the deeper moisture
should favor more widespread and persistent shower activity across
Puerto Rico and the U.S. Virgin Islands. Increasing cloud cover may
limit daytime heating and thunderstorm development in some areas;
however, local convergence and breaks in the clouds could still
support heavier convection. The primary concern by Thursday will
shift toward an increasing flooding threat, including urban and
small-stream flooding, rapid river rises, and isolated flash
flooding where persistent or repeated heavy rainfall develops. This
wetter pattern should continue into the latter part of the workweek.

&&

.Long Term(Friday through Tuesday)...
Issued at 145 AM AST Tue Oct 20 2026

No major changes were introduced to the long-term forecast. Wet
and unsettled weather conditions are expected across Puerto Rico
and the U.S. Virgin Islands through much of the weekend, with the
greatest potential for widespread showers and thunderstorms
Friday into Saturday, followed by gradually improving conditions
beginning Sunday.

A southeast to southerly steering flow will persist across the
region due to the interaction between a deep-layer low moving
across the eastern United States and surface high pressure over
the central Atlantic. This pattern will allow deep tropical
moisture to spread across the region, with precipitable water
values around or exceeding 2 inches. Latest model guidance also
indicates anomalously high mid-level relative humidity, favoring
enhanced shower and thunderstorm development and locally heavy
rainfall. Despite a building mid-level ridge over the eastern
Caribbean, daytime heating, sea breeze convergence, and local
topographic effects will support passing showers across windward
areas during the overnight and morning hours, followed by
scattered to numerous afternoon thunderstorms across interior,
northern, and western Puerto Rico. Unsettled conditions will
likely continue into Saturday, although increasing steering winds
should favor more progressive showers and thunderstorms. By
Sunday, a gradual reduction in atmospheric moisture is expected,
leading to decreasing shower coverage. However, weakening steering
winds late in the weekend could allow thunderstorms to move more
slowly, increasing the potential for localized rainfall
accumulations. The main hazards Friday through the weekend will
include lightning, urban and small-stream flooding, and isolated
flash flooding, particularly where repeated or persistent heavy
showers occur. Meanwhile, persistent southeast to southerly winds
will maintain hot and humid conditions, with the greatest heat
impacts expected across northern coastal and urban areas,
particularly for individuals spending extended periods outdoors
without adequate cooling or hydration.

A more noticeable improvement in weather conditions is forecast
for Monday into Tuesday as drier air spreads through the
atmospheric column and increasing subsidence associated with the
mid-level ridge limits convective development. This should favor a
drier and more stable weather pattern across the region early next
week.

&&

.AVIATION...
(18Z TAFS)
Issued at 210 PM AST Tue Oct 20 2026

SHRA/TSRA expected across the interior and northern PR thru 20/22Z.
VCTS at TJSJ/TJBQ thru early evening. VFR conds expected to prevail
overnight, with VCSH across TJSJ and USVI terminals. Winds mainly
ESE-SE at 8-11 kt, becoming VRB or calm overnight. E-SE winds at
10- 13 kt expected after 21/13Z.

&&

.MARINE...
Issued at 210 PM AST Tue Oct 20 2026

A long-period northerly swell continues to spread across the local
Atlantic waters and passages, resulting in hazardous marine
conditions. Recent observations from the San Juan buoy indicated
seas up to 6 feet with a dominant period of 13 seconds, while
buoy 41043, located approximately 170 nautical miles north-
northeast of San Juan, reported seas near 6 feet with a period of
13 seconds around 11 AM AST. This additional pulse of northerly
swell will continue to propagate across the regional waters,
maintaining seas up to 6 feet, particularly across the offshore
Atlantic waters and northern passages. Although a gradual
improvement is anticipated, another northerly swell is expected to
arrive late Thursday and persist into the upcoming weekend,
maintaining elevated seas and hazardous conditions for small
craft.

&&

.BEACH FORECAST...
Issued at 210 PM AST Tue Oct 20 2026

A long-period northerly swell continues to generate large breaking
waves along the northern exposed beaches of Puerto Rico, Culebra,
Vieques, and the U.S. Virgin Islands. Recent observations from the
San Juan buoy indicated seas up to 5 feet with a dominant period of
13 seconds, supporting breaking waves up to 10 feet. These hazardous
surf conditions are expected to persist through at least 6 PM AST.
Therefore, a High Surf Advisory remains in effect until 6 PM AST.

Additionally, a new pulse of northerly swell was observed around 11
AM AST at buoy 41043, located approximately 170 nautical miles north-
northeast of San Juan, with significant wave heights of 5.9 feet and
a dominant period of 13 seconds. This additional swell energy may
maintain borderline High Surf Advisory conditions through the
overnight hours. Although surf conditions are expected to gradually
improve, hazardous marine and beach conditions will likely persist
as another northerly swell arrives late Thursday and continues into
the upcoming weekend. As a result, a high risk of rip currents will
remain across all northern exposed coastal areas, posing life-
threatening conditions for swimmers and beachgoers.

&&

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

     High Surf Advisory until 6 PM AST this evening for PRZ001-002-
     005-008-010.

VI...High Rip Current Risk from 6 PM AST this evening through
     Wednesday afternoon for VIZ001.

AM...None.
&&

$$

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

NWS SJU Office Area Forecast Discussion