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431
FXCA62 TJSJ 080817
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
417 AM AST Sun Nov 8 2026

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

.KEY MESSAGES...
Issued at 352 AM AST Sun Nov 8 2026

* Slow-moving thunderstorms will keep the potential for urban and
  small-stream flooding risk this afternoon, especially across
  interior and eastern Puerto Rico, including the San Juan metro
  area.

* Scattered showers and thunderstorms between the U.S. Virgin
  Islands waters could produce periods of brief, but heavy
  rainfall over land areas today.

* A northerly swell will maintain a high risk of life-threatening
  rip currents along exposed northern beaches of Puerto Rico and
  portions of Culebra through Monday.

* By mid-week next week a stable weather pattern is expected to
  prevail across the region. However, diurnally induced afternoon
  thunderstorms are expected each day over the interior of PR.

&&

.Short Term(Today through Tuesday)...
Issued at 352 AM AST Sun Nov 8 2026

A broad, decaying frontal boundary trailing across the western
Atlantic will remain stalled well north of the region through
Tuesday, while a surface high pressure builds into the central
Atlantic. Between these systems, a plume of deep tropical moisture
trailing the broad surface low over the central Atlantic will
remain anchored over the northeastern Caribbean. Which is
supporting convective bands over the Caribbean and Atlantic water
under a deep southwesterly steering flow. Some of this activity is
expected to move over land areas this morning, particularly
across the U.S. Virgin Islands, and along western/southern coastal
areas of Puerto Rico. At the upper levels, weak ridges building
over the Caribbean basin and the tropical Atlantic will bring
periods of dry air intrusion and somewhat stable condition.
However, precipitable water (PWAT) content will fluctuate between
1.80 and 2.10 inches through the period, exceeding the 75th
percentile of climatology. Sounding analysis and model guidance
show saturated lower to middle levels, with 850-700 mb relative
humidity averaging 70 to 80 percent, elevated equivalent potential
temperatures, and near-normal lapse rates. Adding to the moist
environment is an exceptionally weak wind field, with 925 mb winds
remaining light to calm at 3 to 8 knots through Tuesday, falling
near the climatological minimum in response to an approaching col
area from the northwest.

That said, strong diurnal surface heating, sea breeze
convergence, and local orographic lifting will interact with this
moisture rich, weakly sheared environment each day. Expect
nighttime and early morning showers to brush windward coastal
areas of the islands, followed by strong convective development
over the Cordillera Central and eastern municipalities during the
afternoon. Because of light steering winds, convective cells will
remain nearly stationary or drift slowly, significantly increasing
local precipitation efficiency, with localized areas receiving
over 2.00 inches. Therefore, the risk of urban and small-stream
flooding, rapid rises along river tributaries, and localized
mudslides across steep terrain will remain elevated each
afternoon.

Boundary-layer temperatures at 925 mb will remain above normal,
fluctuating between 21.5 and 22.5 degrees Celsius. With surface
dewpoints holding in the mid-to-upper 70s, afternoon heat indices
are forecast to reach 100 to 106 degrees Fahrenheit along coastal
and urban areas of northern, western, and southern PR, as well as
the U.S. Virgin Islands, before afternoon convection begins each
day.

&&

.Long Term(Wednesday through Sunday)...
Issued at 352 AM AST Sun Nov 8 2026

The long term period will be mainly characterized by variable
conditions at the beginning, becoming slightly more stable by the
end of the week. At the synoptic level, a surface high pressure will
build across the western Atlantic by Wednesday, gradually shifting
eastward into the central Atlantic during the remainder of the
period. This will result in easterly winds by midweek, becoming east
to southeasterly from Thursday through at least Saturday, before
backing to the east once again on Sunday. Winds are expected to
follow an increasing trend. Compared to previous days, when winds
will remain light, they should increase to around 12 to 16 kt
overall during the long-term period.

By Wednesday and Thursday, the latest model guidance suggests the
influence of shortwave troughs near the Caribbean. Moisture content
across the region is expected to remain near seasonal levels, with
mean precipitable water values fluctuating between 1.7 and 1.9
inches. However, global models show some disagreement, with the GFS
indicating higher values and the ECMWF showing lower values.
Relative humidity in the 850 to 700 mb layer is expected to remain
generally near seasonal levels, while the 700 to 500 mb layer should
also maintain sufficient moisture to support cloud and precipitation
development. The passage of shortwave troughs during this period
could provide marginal dynamical support for upward motion. Combined
with daytime heating and local effects, these features could
contribute to periods of showers and possibly isolated
thunderstorms, particularly during the afternoons across northern to
northwestern portions of Puerto Rico. As a result, flooding will
remain in the forecast across these areas (Level 1 of 4), indicating
the potential for flooding in urban areas, along roads, and near
small streams.

From Friday onward, moisture conditions are expected to remain
relatively favorable, although a gradual decrease in mid-level
moisture may occur as a ridge builds across the region. Precipitable
water values are generally expected to remain near or below 1.9
inches, which is close to seasonal levels. However, the development
of a mid-level ridge should promote greater atmospheric subsidence
and potentially limit vertical development, despite the persistence
of moisture in the lower levels. Additionally, models suggest a
warming trend in 500 mb temperatures. Consequently, while isolated
to scattered showers may continue, particularly due to local
effects, precipitation activity could become more localized compared
to Wednesday and Thursday.

Throughout the period, current model guidance suggests that 925 mb
temperatures will remain near the 75th percentile or two standard
deviations above normal, based on climatology for the month of
November. Therefore, before rainfall develops, heat indices are
expected to be elevated, primarily affecting individuals who are
particularly sensitive to heat. Residents and visitors are
encouraged to remain hydrated and continue monitoring weather
conditions, as forecast updates and adjustments become available.

&&

.AVIATION...
(06Z TAFS)
Issued at 352 AM AST Sun Nov 8 2026

Mainly VFR conditions will prevail across the local terminals
through most of the period. However, early morning TSRA developing
in and around the USVI terminals and TJBQ will support mostly
VCTS, with possible tempo MVFR conditions at TISX. Afternoon
convection developing over the interior of PR will drift slowly
and erratic toward coastal areas, producing mtn obscurations and
VCTS at TJSJ/TJPS with tempo MVFR to brief IFR conds at TJSJ btw
08/18-21z. Surface winds will be light and variable, although a
W-SW component is expected to continue through the fcst period.

&&

.MARINE...
Issued at 352 AM AST Sun Nov 8 2026

A low-pressure northeast of the region will continue moving eastward
over the next few days, while high pressure builds across the
western Atlantic. Light to moderate westerly to southwesterly winds
today will become variable tonight, then gradually shift to a more
easterly direction early this week. A northerly to northeasterly
swell will peak today through tonight, maintaining hazardous
conditions near exposed Atlantic coastal waters. Although seas will
remain below Small Craft Advisory criteria, the longer-period swell
will produce locally rough conditions, particularly across the
offshore Atlantic waters. The northeasterly swell will gradually
subside on Monday, with improving marine conditions expected
thereafter. Showers and isolated thunderstorms will remain possible
across the regional waters through midweek.

&&

.BEACH FORECAST...
Issued at 352 AM AST Sun Nov 8 2026

A northerly to northeasterly swell will peak today through tonight,
producing breaking waves of 6 to 8 feet along exposed northern
and northeastern beaches of Puerto Rico. Some of this swell energy
will also reach the northwestern coastline, including the beaches
of Rincon and Aguada, where a high risk of life-threatening rip
currents is now in effect. A high rip current risk will also
continue along other exposed northern and northeastern beaches of
Puerto Rico, including Culebra, through at least Monday afternoon.
Although swell energy will gradually subside on Monday, hazardous
rip currents will likely persist as lingering swell continues to
affect the region.

Elsewhere, a moderate rip current risk will persist along other
exposed beaches, including portions of Vieques and the U.S. Virgin
Islands. Thunderstorms will pose an additional hazard, with isolated
thunderstorms possible through midweek. Avoid swimming at beaches
with a high rip current risk. If thunder is heard, leave the beach
and seek shelter in a substantial building or enclosed vehicle.

&&

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

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

$$

AVIATION/SHORT TERM...DSR
LONG TERM....YZR
BEACH/MARINE...CVB

NWS SJU Office Area Forecast Discussion