Lago Vivi, Puerto Rico Weather Forecast Discussion

883
FXCA62 TJSJ 190906
AFDSJU
Area Forecast Discussion
National Weather Service San Juan PR
506 AM AST Fri Sep 19 2025
.KEY MESSAGES...
* Abundant moisture and an induce surface trough will keep the
  flood risk elevated today across north and northwest areas of
  Puerto Rico, where strong afternoon thunderstorms are likely.
* Across the U.S. Virgin Islands, a showery pattern will persist
  today.
* A warming trend is anticipated over the next few days. A Heat
  Advisory is in effect today from 10 AM to 5 PM AST across urban
  and coastal areas of Puerto Rico, Vieques, Culebra, and the
  U.S. Virgin Islands.
* A moderate risk of rip currents persist along the northwestern
  beaches of Puerto Rico, while a low risk elsewhere. Although
  Tropical Storm Gabrielle is forecast to move well northeast of
  the region this weekend, swells generated by the storm will
  increase the potential for life-threatening rip currents across
  the north and east facing beaches of the islands.
* A wet and unstable pattern is possible by the middle part of
  next week onward.
&&
.SHORT TERM...Today through Sunday...
Persistent showers and thunderstorms, supported by a nearby upper-
level trough and an associated surface trough, developed overnight
across the Caribbean waters, Mona Passage, and the coastal waters of
eastern Puerto Rico and the U.S. Virgin Islands. Radar estimates
indicated significant rainfall offshore, with 12 inches observed
along southeastern coastal areas. A Flood Advisory was issued there,
as soils were already saturated from up to 4 inches of rain reported
during the past 24 hours. Meanwhile, cooler overnight lows finally
returned across all surface observing stations, dropping below 80F.
Today, with less cloud cover and stronger heating, conditions will
favor increased locally induced showers and thunderstorms, supported
by well-above-normal PWAT values (~2.1 inches) and marginal
instability. Periods of moderate to heavy rainfall are likely this
morning across southern Puerto Rico, the U.S. Virgin Islands, and
nearby islands. With weak background winds under 10 knots, sea-
breeze and local effects will dominate, allowing afternoon storms to
first develop over the interior before spreading westward and
northward through outflows and weak southerly steering flow. These
slow-moving storms may bring frequent lightning, gusty winds, and
prolonged moderate to heavy rainfall leading to flooding, especially
in saturated and urban areas. The U.S. Virgin Islands will remain
less active overall, though similar hazards are possible. Meanwhile,
dangerous and potentially life-threatening heat will persist, with a
Heat Advisory in effect for urban and coastal areas.
A wet pattern will continue through Saturday evening, with typical
moisture lingering into Sunday. Clearer mornings over the next few
days will allow stronger heating, increasing dangerous heat risks
while also fueling afternoon storm development. Under weak winds,
showers and thunderstorms will form over the interior and spread
toward northern and western Puerto Rico as trade winds, outflows,
and sea breezes interact. By Saturday, southeasterly flow will shift
the strongest afternoon storms into the northwest, then farther west
by Sunday as winds turn more easterly. These slow-moving storms
could bring localized flooding, isolated flash floods, and
landslides. The U.S. Virgin Islands will remain comparatively drier,
but dangerous and potentially life-threatening heat will persist
across the region, with even higher values expected as southerly
winds enhance moisture transport.
.LONG TERM...Monday through Friday...
At the start of the upcoming workweek, light winds will prevail,
gradually shifting from southeasterly to easterly throughout the
day. This shift will be driven by a surface high-pressure system
dominating the Central Atlantic. Based on the latest model
guidance, Monday is expected to have the lowest precipitable water
content of the forecast period, with values ranging from slightly
below average to near seasonal levels. Additionally, drier air is
anticipated in the mid to upper levels of the atmosphere compared
to recent days. Temperature-wise, forecast data indicates that
925 mb temperatures will remain nearly two standard deviations
above normal on Monday, followed by a gradual cooling trend. Under
these conditions, expect morning showers over windward areas of
the islands, with afternoon showers and isolated strong
thunderstorms developing across interior and western Puerto Rico.
These will be driven by diurnal heating, available moisture, and
local effects.
By Tuesday, a transition to a wetter and more unstable pattern is
expected to evolve. An approaching wind surge will increase wind
speeds and moisture levels, leading to higher chances of rainfall
through the remainder of the forecast period. Model guidance
suggests a rapid rise in PWAT values, peaking around 2.0 to 2.25
inches, well above climatological norms for this time of year.
Around Wednesday into Thursday, a tropical wave currently being
monitored by the National Hurricane Center is forecast to approach
the region, further enhancing the wet pattern. As of now, the
wave is located just off the west coast of Africa, with a low
formation chance in the next 48 hours (near 0%) and a 20% chance
over the next 7 days. Aloft, some troughiness should be present
combined with low level southeasterly flow driven by the Atlantic
surface high pressure system. This will continue to advect
tropical moisture into the region through Friday maintaining the
wet pattern. Expect and elevated flood risk each day, likely
including rapid river rises and landslides. Current model guidance
suggests the system may develop after moving away from our area.
We will continue to monitor this evolving situation closely and
provide updates as necessary.
&&
.AVIATION...
(06Z TAFs)
Slow-moving SHRA/TSRA will affect the region over the next 24
hours, producing periods of MVFR and brief IFR conditions.
Activity will mainly impact TJSJ and USVI terminals, with
SHRA/TSRA also expected at TJPS and TJBQ between 19/17Z and
19/22Z. Winds will remain light, allowing sea-breeze circulations
to dominate, with speeds generally calm to 810 kt before easing
again to light/variable after 19/22Z. Gusty and erratic winds are
likely in and near SHRA/TSRA.
&&
.MARINE...
Gentle to light southeasterly winds will prevail today and through
the weekend. High moisture content and a weak induced trough will
continue to result in frequent showers and thunderstorms over the
local waters through tonight. A long period northeasterly swell
generated by Tropical Storm Gabrielle, currently moving over the
Central Atlantic, will reach the local waters and passages during
the weekend. However, seas are forecast to remain below Small Craft
Advisory criteria.
&&
.BEACH FORECAST...
Today, a moderate risk of rip currents persists along the
northwestern beaches of Puerto Rico, while a low risk is expected
elsewhere. However, even where the risk is low, life-threatening
rip currents can still occur, especially near groins, jetties,
reefs, and piers.
This brief improvement will not last long. The moderate risk is
expected to return tomorrow, Saturday, and continue into early
next week, driven by swells generated by Tropical Storm Gabrielle.
Beachgoers are urged to exercise caution at all times, as a
moderate risk means life-threatening rip currents are possible
within the surf zone.
Additional beach hazards today include strong afternoon thunderstorms,
especially along northwestern beaches, and elevated heat risk
during peak heat hours, typically from 10 AM to 5 PM. Stay well
hydrated, and remember, if you hear thunder, seek shelter in a
safe indoor location immediately.
&&
.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001>005-007-008-010>013.
VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.
AM...None.
&&
$$
SHORT TERM/AVIATION...ICP
LONG TERM/MARINE/BEACH FORECAST...YZR

NWS SJU Office Area Forecast Discussion