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346
FXCA62 TJSJ 271920
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
320 PM AST Mon Jul 27 2026

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

.KEY MESSAGES...
Updated at 315 PM AST Mon Jul 27 2026

* Moisture associated with a tropical wave will increase the potential
  for showers and thunderstorms through Tuesday, elevating the
  risk of urban and small-stream flooding, particularly across the
  interior and western Puerto Rico during the afternoon hours.

* Moisture will decrease by Wednesday, reducing the flooding threat
  while winds continue to increase.

* A Heat Advisory remains in effect through this afternoon. Warm to
  hot conditions will persist across Puerto Rico and the U.S.
  Virgin Islands through the rest of the week. Additional Heat
  Advisories or even Extreme Heat Warnings may be needed in the
  coming days, especially in areas where rainfall remains limited.

* Moderate concentrations of Saharan dust will gradually return
  tomorrow, Tuesday, and persist through at least early Friday.

&&

.Short Term(This evening through Wednesday)...
Issued at 315 PM AST Mon Jul 27 2026

The moisture field, related to a tropical wave moving over the
region, with precipitable water (PWAT) values around 2 inches has
remained mostly over the southeastern and eastern waters and over
the USVI, Vieques and Culebra. PWAT values over northwestern Puerto
Rico have reached similar values due to recently formed t-storms.
Subsidence ahead of this tropical wave promoted a generally fair but
warm to hot morning and early afternoon, with the exception of early
morning showers over the eastern region. These showers over the
eastern region were passing as the bulk of showers and t-storms
related to the wave stayed over the Caribbean waters during the
early morning. The tropical wave interacting with an upper level
trough, diurnal heating (with highs in the low 90s), local
orographic effects and sea breeze convergence have resulted in
convective shower and t-storm activity within the past hours over
interior, western and northwestern Puerto Rico, sustaining a
flooding risk. These t-storms have resulted in frequent lightning
and heavy rain with rainfall totals between 1.5 to around 2 inches
being observed. While these t-storms can persist and spread to
surrounding areas during the afternoon, convective activity can
also still develop downwind of the local islands and El Yunque. A
Heat Advisory also remains in effect for coastal and lower
elevation areas of the islands through 5 PM as heat indices have
once again reached the 100s to even the low 110s.

Through at least tomorrow, Tuesday, the interaction between this
lingering tropical wave, and its moisture field (PWAT values around
1.90 to 2 inches), and the upper-level trough will continue to
affect the region. As the wave advances west-northwestward across
the northeastern Caribbean, tropical moisture and atmospheric
instability will stay above normal through tomorrow, maintaining the
possibility of  bands of showers and t-storms reaching the region
and convective showers and t-storms developing during the afternoon
at interior to W-NW Puerto Rico and downwind of El Yunque and the
local islands, sustaining a flooding risk. Additionally, stronger t-
storms may produce frequent lightning and locally gusty winds. As
the surface high over the Atlantic strengthens, ESE flow will become
breezy tomorrow to locally windy on Wednesday. A drier air mass is
forecast to spread across the region by Wednesday afternoon, leading
to a gradual reduction in moisture levels, with precipitable water
values dropping to below normal values. Furthermore, the return of
pulses of Saharan dust is anticipated to bring hazier skies and
diminish the likelihood of t-storm and shower development. 925 mb
temperatures are forecast to remain above normal tomorrow and
Thursday. Due to this, areas with little rainfall are anticipated to
experience above-normal highs and warm overnight lows under
persistent east-southeast flow. These factors, along with increased
moisture tomorrow, will continue to result in elevated to
potentially significant heat risks across parts of the islands.

&&

.Long Term(Thursday through Monday)...
Issued at 305 AM AST Mon Jul 27 2026

The latest model guidance continues to suggest the arrival of a
drier air mass across the region, dropping precipitable water values
below normal to around 1.30 to 1.40 inches on Thursday. Along with
this drier air, NASA GEOS-5 model guidance indicates low-to-moderate
concentrations of Saharan Dust arriving through the first half of
the period.

Strengthening surface high pressure over the central Atlantic will
create a tight pressure gradient, bringing breezy to locally windy
east-southeast winds through Saturday. Aloft, a mid-to-upper-level
ridge will promote stable conditions, limiting widespread shower
activity. However, the forecast remains on track for patches of low-
level moisture embedded in the trade winds to move across the area.
This will bring passing showers, particularly over the windward
portions of the islands during the morning hours. In the afternoons,
this moisture combined with daytime heating and local effects could
trigger shower activity along the interior and northwestern portions
of Puerto Rico.

By late Sunday into Monday, an approaching tropical wave will
increase moisture alongside the potential for widespread showers and
thunderstorms. Elevated temperatures will persist through the
period, driven by normal to above-normal temperatures at the 925 mb
level, with daytime highs reaching the upper 80s to low 90s across
coastal and urban areas. Residents and visitors are encouraged to
monitor forecasts and stay updated on changing conditions as the
week progresses.

&&

.AVIATION...
(18Z TAFS)
Issued at 315 PM AST Mon Jul 27 2026

Mainly VFR conditions will prevail through this evening with ESE
winds at 15 to 20 kts, and gusts up to 25 kts (higher near TSRA),
these winds will diminish after 27/23Z before picking up again
from the ESE at 15 to 20 kts at around 28/12Z. As a tropical wave
moves over the region, afternoon SHRA/TSRA across interior to
western PR will continue to affect mainly the vicinity of TJPS and
TJBQ through 27/23Z, bringing brief MVFR/IFR conditions and
mountain obscurations. Bands of SHRA/ and possible TSRA can also
reach eastern terminals through the period.

&&

.MARINE...
Issued at 315 PM AST Mon Jul 27 2026

Moisture associated with a tropical wave will increase the potential
for showers and thunderstorms through Tuesday. Gentle to moderate
east to east-southeast winds will continue through the rest of
today, becoming moderate to fresh Tuesday afternoon into Wednesday
as high pressure strengthens over the Atlantic, tightening the
pressure gradient. These winds will maintain choppy seas across
portions of the Atlantic waters, offshore Caribbean waters, and the
local passages, with seas up to 6 feet expected in localized areas
from midweek through at least the end of the week.

&&

.BEACH FORECAST...
Issued at 315 PM AST Mon Jul 27 2026

Under light winds, an overall low risk of rip currents is expected
tonight. Caution is always recommended, as even when the risk is
low, life-threatening rip currents can occur near reefs, jetties,
groins, and piers. As winds begin an increasing trend on Tuesday, a
moderate risk of rip currents is forecast across northern Puerto
Rico. Then, easterly winds will continue to increase as high
pressure over the Atlantic strengthens from Wednesday onwards,
becoming breezy to locally windy. As a result, the moderate risk of
rip currents will expand to most beaches of Puerto Rico and the U.S.
Virgin Islands. Life-threatening rip currents will remain possible,
exercise caution. Additionally, warm to hot conditions will prevail,
mainly across coastal and low-elevation areas. Stay hydrated and
take frequent breaks in the shade.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM/AVIATION...MRR
LONG TERM....GRS
MARINE/BEACH FORECAST/KEY MESSAGES...YZR

NWS SJU Office Area Forecast Discussion