National Weather Service Today's Weather Impact Levels

358
FXCA62 TJSJ 200729
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
329 AM AST Wed May 20 2026

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

.KEY MESSAGES...
Issued at 324 AM AST Wed May 20 2026

* Breezy, to locally windy, east to east-southeast winds will
  continue through much of the week, steering periods of fast moving
  showers towards windward sectors. This can cause unsecured items
  to blow around.

* Afternoon showers and isolated thunderstorms are expected each
  day across interior and west to northwest Puerto Rico limited to
  elevated flooding risk) due to daytime heating, local effects,
  and nearby troughing. Lines of showers can also develop
  downwind of the local islands and El Yunque.

* Breezy conditions will also sustain choppy seas, hazardous marine
  conditions and up to a moderate risk of rip currents. A Small
  Craft Advisory remains in effect for the offshore Atlantic through
  at least Thursday afternoon.

* A limited heat risk will persist this week, with heat indices
  ranging from the mid 90s to the low 100s across urban and coastal
  areas each afternoon.

* Instability should gradually increase Saturday onwards, as an
  upper-level trough extending into the tropics may shift eastward
  and linger north of the CWA while moisture content increases.
  &&

.Short Term(Today through Friday)...
Issued at 324 AM AST Wed May 20 2026

Current satellite derived precipitable water (PWAT) values indicate
below normal to normal columnar moisture for this time of the year.
PWAT values are currently at around 1.40 inches over the USVI,
Vieques and Culebra, and 1.45 to 1.66 inches over PR. Shallow
moisture and quick moving showers have continued to be steered
towards the region by breezy east to east-southeast winds. At least
minimal accumulations since midnight have been detected over the
eastern half of PR (including north-central and south-central PR),
Vieques, Culebra and the USVI. The highest accumulations from
midnight to 3 AM AST have been over Yabucoa with up to a radar
estimated around 0.6 inches of rain. Patchy fog was also detected
over sectors of interior PR. Similar to the last few days, lows have
been in the low 70s to locally near 80 over urban and coastal areas
of the islands, and in the upper 50s to the 60s at higher elevations
of PR.

A surface high pressure over the western to central Atlantic will
tighten the local pressure gradient and promote breezy, to locally
windy, east to east-southeast winds, with sea and land breeze
variations. This flow will sustain an advective pattern, with
moisture and fast-moving showers, moving over the region. PWAT
values will be at normal values for most of the period (1.50 to 1.90
inches) with model guidance suggesting that a patch of drier air can
reach the islands overnight to Thursday afternoon, prompting a dip
in PWAT values to below normal values. PWAT values can also reach
above normal values (above 2 inches) during afternoon convection,
each day. A mid- to upper- level trough NW of the islands will
remain during the period, adding some instability and allowing the
available moisture to reach the mid-levels. However, patches of
drier air in the mid- levels will also be present. The breezy, to
locally windy, east to east-southeast steering flow will continue to
result in quick moving showers advecting towards windward sectors
through the workweek and in periods of variable to showery weather
(with locally higher gusts). This can also result in ponding of
water over roads and poorly drained areas, with a chance of urban
and small stream flooding. An afternoon convective pattern will also
develop each day as diurnal heating, local orographic effects and
sea breeze convergence fuel this activity. Convective showers and
isolated t-storms are forecast to develop over interior towards W to
NW Puerto Rico, resulting in a limited to elevated flooding risk.
Flood advisories might need to be issued each afternoon. Lines of
showers can also develop downwind of El Yunque and the USVI, towards
eastern PR. The breezy, to locally windy, steering flow can also
help limit rainfall accumulations as showers and t-storms move
faster than usual. However, this flow can result in unsecured items
blowing around. Current model guidance suggests that 925 mb wind
speeds will continue at high-end normal to above normal values
through the rest of the period. Current model guidance suggests that
925 mb temperatures will remain at seasonal levels today and Friday
and at high-end normal values to slightly above normal on Thursday.
Maximum temperatures will reach the upper 80s to lower 90s at urban
and coastal areas, while heat indices can exceed the upper 90s and
100 degrees, during the afternoon hours. This will result in a
limited heat risk. The east to east-southeast flow will also steer
low concentrations of saharan dust towards the islands during the
period.

&&

.Long Term(Saturday through Wednesday)...
Issued at 324 AM AST Wed May 20 2026

No major changes were introduced to the long-term forecast. As
mentioned in the previous discussion, a broad surface high pressure
will linger over the Western Atlantic through most of the period,
promoting E-ESE winds and resulting in breezy to locally windy
conditions across the CWA. Instability should gradually increase
Saturday onwards, as an upper-level trough extending into the
tropics may shift eastward and linger north of the CWA. According to
the latest model guidance, moisture content will increase as well,
with PWAT values approaching near-normal levels (up to 2.0 inches).
Additionally, the latest Grand Ensemble shows members tending to
wetter conditions for the first part of next workweek (difference
around 0.2 - 0.3 inches), increasing confidence. Under the influence
of the trough, mid-level temperatures may become slightly cooler
than normal (near -9 degrees Celsius), while winds aloft may become
stronger and allow ventilation. Additionally, the latest Galvez-
Davison Index (GDI) solutions dont rule out the potential for
isolated to scattered thunderstorms across the islands each day,
becoming frequent Monday onward during afternoon into night hours.
Hence, windward sections of the islands can expect passing showers
moving each day, while afternoon showers and thunderstorms will
likely concentrate over western/northwestern Puerto Rico. Due to
strengthening winds, showers should remain progressive, with
rainfall accumulations likely to result in ponding of water over
roadways and poorly drained areas, including minor flooding.
Nevertheless, urban and small streams flooding cannot be ruled out
over the aforementioned areas.

As mentioned in the previous discussion, warmer than normal
temperatures and abundant moisture content will increase the
likelihood of heat indices exceeding 100 degrees Fahrenheit.
Although a limited risk is expected across low elevations and urban
areas of the islands, individuals sensitive to heat must take
necessary precautions, such as remaining hydrated and avoiding long
sun exposure.

&&

.AVIATION...
(06Z TAFS)
Issued at 324 AM AST Wed May 20 2026

Mainly VFR conditions across the terminals. Breezy, to locally
windy, E to ESE winds up to 15-22 kts after 20/13Z, with higher
gusts and sea breeze variations, decreasing after 20/23Z. these
winds will steer rounds of -SHRA/VCSH towards windward terminals
throughout the period. Btwn 20/16-23Z, SHRA/+SHRA, with isolated TS,
will develop across interior to W-NW PR, possibly resulting in brief
MVFR conditions for TJBQ. Lines of -SHRA/VCSH can also develop from
the local islands and El Yunque, affecting TJSJ.

&&

.MARINE...
Issued at 324 AM AST Wed May 20 2026

Marine conditions will remain somewhat hazardous through the next
several days as strong high pressure over the Atlantic continues to
support moderate to fresh easterly winds across the regional waters.
Winds will occasionally reach locally strong levels, resulting in
choppy to rough seas at times. Small Craft Advisories remain in
effect through at least Thursday afternoon for the offshore
Atlantic waters. In addition, isolated afternoon and early evening
thunderstorms may develop each day across the western coastal
waters of Puerto Rico, producing locally higher winds and seas
near storms.

&&

.BEACH FORECAST...
Issued at 324 AM AST Wed May 20 2026

Moderate rip current risk conditions are expected to persist across
most local beaches through much of the week and into the upcoming
weekend due to persistent breezy conditions and ongoing wind-driven
seas. The highest risk will continue along north and east-facing
beaches. Beachgoers should exercise caution, avoid swimming alone,
and remain near lifeguard-protected beaches whenever possible, as
life-threatening rip currents can develop even under moderate risk
conditions.

&&

.FIRE WEATHER...
Issued at 324 AM AST Wed May 20 2026

Increasing moisture today could prevent RH values from decreasing
below threshold values for prolonged periods; however, rainfall
should again remain limited across the southern plains, allowing
relative humidity to drop to critical fire weather thresholds in the
region. Additionally, sustained winds of 15 to around 20 mph, with
higher gusts, combined with elevated KBDI values, will favor the
rapid spread of wildfires across the southern coastal plains. The
low 7-day percent of normal rainfall will also persist across the
southeastern coastal plains. Fire management partners are once
again urged to remain vigilant.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 PM AST Thursday for AMZ711.

&&

$$

SHORT TERM/AVIATION/FIRE WEATHER/KEY MESSAGES...MRR
LONG TERM....MNG
MARINE/BEACH FORECAST...ICP

NWS SJU Office Area Forecast Discussion