National Weather Service Today's Weather Impact Levels

454
FXCA62 TJSJ 211858
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
258 PM AST Sat Mar 21 2026

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

.KEY MESSAGES...
Issued at 209 PM AST Sat Mar 21 2026

* For the rest of the afternoon, isolated showers may develop over
  interior Puerto Rico and drift northward with the southerly
  winds. None to limited flood threat.

* A more unsettled weather pattern is forecast to develop from
  late tonight into Sunday, as shower and thunderstorm activity
  associated with an approaching frontal boundary and its induced
  surface trough moves into the CWA. These conditions may lead to
  localized flooding, landslides in areas of steep terrain and
  lightning hazard.

* Residents and visitors are advised to monitor the forecast as
  conditions evolve and to avoid rivers and flood-prone areas,
  particularly on Sunday.

* A low to moderate risk of rip currents exists across the
  coastal waters of Puerto Rico and the U.S. Virgin Islands.

&&

.Short Term(This evening through Monday)...
Issued at 209 PM AST Sat Mar 21 2026

Fairly calm weather conditions were observed during the morning
hours. A few isolated showers developed over the surrounding
waters and moved into the southern coastal areas of Puerto Rico
under a southerly wind flow; however, rainfall accumulations
remained minimal.

Currently, the southern extent of a frontal boundary associated with
a polar trough is affecting the Dominican Republic. Mid-level
conditions remain relatively dry, as indicated by the TJSJ 21/12Z
upper-air sounding, which shows mid-level relative humidity near 28
percent. This dryness may initially limit convective development.
However, the 700500 mb advected layered precipitable water product
indicates that moisture will gradually increase through the
afternoon and evening as the frontal boundary approaches the region,
allowing for a gradual increase in shower activity. For the
remainder of the afternoon, isolated to scattered showers may
develop over interior Puerto Rico and drift northward by evening.
Despite the increasing moisture, the overall flooding risk is
expected to remain limited.

As the polar trough and associated frontal boundary continue to
approach, the frequency of showers and thunderstorms is expected to
increase, particularly late tonight into Sunday. Activity will
initially affect northwestern Puerto Rico late tonight into early
Sunday morning, followed by additional shower and thunderstorm
development over interior areas by midday and into the afternoon.

Winds will veer from southerly tonight to southwesterly or westerly
Sunday morning, becoming northwesterly to northerly by Sunday
afternoon while decreasing in speed. This evolving wind pattern may
result in erratic storm motion, especially on Sunday. The reduction
in steering winds could also enhance rainfall accumulation over
localized areas. Cloud cover will be an important limiting factor
for convective development.

Although upper-level conditions will become more unstable as the
polar trough approaches and the frontal boundary moves across the
area, the bulk of the deeper convection is expected to remain north
of the islands over the Atlantic waters. Therefore, local weather
conditions will depend largely on advective shower activity as well
as localized convection driven by diurnal heating and sea breeze
convergence, both of which will be influenced by cloud coverage.

Nonetheless, conditions should continue to be monitored, as
persistent heavy rainfall could result in excessive runoff and rapid
river rises. Although relatively dry antecedent conditions may
initially mitigate flooding impacts, prolonged rainfall could still
lead to flooding in vulnerable areas, particularly in flood-prone
locations and along steep terrain where landslides cannot be ruled
out. Additionally, any thunderstorm activity may produce gusty winds
and frequent lightning.

Residents and visitors are advised to monitor the forecast as
conditions evolve and to avoid rivers and flood-prone areas,
particularly on Sunday.

By Monday, conditions will depend on how quickly the frontal
boundary moves east of the region. This will likely be a transition
day, with lingering showers and thunderstorms possible, mainly
across northeastern Puerto Rico, Vieques, Culebra, and the northern
U.S. Virgin Islands.

&&

.LONG TERM(Tuesday through Saturday)...
Issued at 304 AM AST Sat Mar 21 2026

A broad mid- to upper-level trough will persist across the western
Atlantic and northeastern Caribbean through the period, maintaining
unsettled conditions across Puerto Rico and the U.S. Virgin Islands.
Moisture will remain near to above normal throughout the period.

On Wednesday, lighter steering flow and sufficient moisture will
favor slow-moving showers, particularly across windward areas during
the morning and over the interior and western portions of Puerto
Rico during the afternoon. Thunderstorms are possible, with locally
heavy rainfall that may lead to urban and small stream flooding as
well as frequent lightning.

From Thursday onward, moisture will deepen across the region while
500 mb temperatures remain below -8 degrees Celsius, resulting in a
more unstable atmosphere. This combination of cold air aloft and
abundant moisture through a deep layer will promote more efficient
rainfall processes and an increased coverage of showers and
thunderstorms each day. Periods of heavy rain will maintain an
elevated risk of flooding, especially in areas that receive repeated
rounds of precipitation. Gusty winds will be possible in and near
heavier showers, while saturated soils will increase the likelihood
of flooding, ponding of water on roadways, and minor landslides in
steep terrain.

Overall, an extended period of unsettled weather is expected from
midweek through Saturday, driven by persistent troughing aloft, cold
mid-level temperatures, abundant moisture at all levels, and
periodic disturbances moving through the region.

&&

.AVIATION...
(18Z TAFS)
Issued at 209 PM AST Sat Mar 21 2026

Mainly VFR conds expected at the terminals early in the period
with - SHRA/VCSH developing aft 21/23Z, becoming more frequent ovr
PR terminals, particularly TJBQ, overnight into Sunday. THe
passage of a shearline will support SHRA/TSRA development,
especially aft 22/12Z across interior and NWRN PR. Periods of
MVFR/brief IFR conds possible in SHRA/TSRA, with reduced VIS and
lower CIGs (BKN/OVC010- 020). Mtn obsc likely, particularly across
interior PR. Winds will veer during the period, from S-SW at
1020 kt with higher gusts thru tonight, becoming W by Sunday
morning, then NW-N aft 22/15Z.

&&

.MARINE...
Issued at 209 PM AST Sat Mar 21 2026

A pre-frontal trough and associated boundary approaching from the
northwest will continue to weaken the pressure gradient, promoting
light to moderate south to southwest winds tonight. Periods of
unsettled weather will increase shower activity and possible
thunderstorm development across the local waters, especially
during the overnight hours into Sunday. Localized hazardous marine
conditions near storms and gusty winds, particularly across the
Atlantic waters and the Mona Passage. Late Sunday into early next
week, high pressure building over the western Atlantic will
promote the return northeasterly winds and increasing seas,
leading to deteriorating marine conditions.

&&

.BEACH FORECAST...
Issued at 255 PM AST Sat Mar 21 2026

Mainly, a moderate risk of rip currents will prevail over the next
few days along the northern and eastern beaches of Puerto Rico,
including Vieques, Culebra, as well as the U.S. Virgin Islands. A
moderate risk means that life-threatening rip currents remain
possible in the surf zones, and beachgoers should remain cautious.
A small pulse of northerly swell may locally increase the risk in
exposed beaches. A low risk are expected along the more protected
southern beaches.

Additionally, increasing moisture and instability will bring
periods of showers and isolated thunderstorms posing a danger to
those in or near the water, mainly tomorrow, Sunday. Stay weather-
aware! For additional information and location specific rip current
details, please visit: weather.gov/beach/sju.

&&

.HYDROLOGY...
Issued at 255 PM AST Sat Mar 21 2026

The expected increase in moisture associated with the approaching
frontal boundary and upper-level trough will result in periods of
showers and isolated thunderstorms through the next 24 hours.
While relatively dry antecedent conditions may initially limit
excessive runoff and delay river responses, the potential for
persistent or slow-moving showers, enhanced by weakening winds,
could lead to localized ponding of water, urban and small stream
flooding, and rapid river rises. The risk of flooding will remain
generally limited but could increase in areas experiencing
repeated rainfall from late tonight into Sunday. Additionally,
isolated landslides cannot be ruled out in steeper terrain where
soils may become saturated.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION/HYDRO...CVB
LONG TERM...MMC
MARINE/BEACH...YZR

NWS SJU Office Area Forecast Discussion