Heat Advisory 21, NWS JSJ

860 
WWCA72 TJSJ 271426
NPWSJU

URGENT - WEATHER MESSAGE
National Weather Service San Juan PR
1026 AM AST Mon Jul 27 2026

PRZ001>005-007-008-010>013-VIZ001-002-272100-
/O.CON.TJSJ.HT.Y.0021.000000T0000Z-260727T2100Z/
San Juan and Vicinity-Northeast-Southeast-Eastern Interior-North
Central-Ponce and Vicinity-Northwest-Mayaguez and Vicinity-
Southwest-Culebra-Vieques-St.Thomas...St. John...and Adjacent
Islands-St Croix-
Including the cities of Naguabo, Penuelas, San Juan, Barceloneta,
Toa Alta, Culebra, Canovanas, Anna's Retreat, Charlotte Amalie,
Bayamon, Ponce, Ceiba, Cidra, Fajardo, Dorado, Gurabo, Frederiksted
Southeast, Anasco, Guaynabo, Luquillo, Maunabo, Cayey, Aguas Buenas,
Patillas, Comerio, Christiansted, Lajas, Guayama, Juncos,
Hormigueros, Yauco, Loiza, San Lorenzo, Arecibo, Humacao, Trujillo
Alto, Arroyo, Hatillo, Juana Diaz, Vieques, Quebradillas, Carolina,
Florida, Frederiksted, Caguas, Catano, Aguada, Rio Grande, Salinas,
Santa Isabel, Yabucoa, Rincon, Cruz Bay, Grove Place, San German,
Mayaguez, Guayanilla, Moca, Vega Baja, Vega Alta, Isabela, Charlotte
Amalie West, Charlotte Amalie East, Las Piedras, Cabo Rojo, Camuy,
Aguadilla, Toa Baja, Manati, and Guanica
1026 AM AST Mon Jul 27 2026

...HEAT ADVISORY REMAINS IN EFFECT UNTIL 5 PM AST THIS AFTERNOON...

* WHAT...This level of heat affects most individuals sensitive to
  heat, especially those without effective cooling and/or adequate
  hydration.

* WHERE...Urban and coastal areas of Puerto Rico and U.S. Virgin
  Islands.

* WHEN...Until 5 PM AST this afternoon.

* IMPACTS...Hot temperatures and high humidity may cause heat
  illnesses.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Drink plenty of fluids, stay in an air-conditioned room, stay out of
the sun, and check up on relatives and neighbors.

Take extra precautions when outside. Wear lightweight and loose
fitting clothing. Try to limit strenuous activities to early morning
or evening. Take action when you see symptoms of heat exhaustion and
heat stroke.

&&

$$

YZR

778 
WWCA72 TJSJ 272041
NPWSJU

URGENT - WEATHER MESSAGE
National Weather Service San Juan PR
441 PM AST Mon Jul 27 2026

PRZ001>005-007-008-010>013-VIZ001-002-272145-
/O.EXP.TJSJ.HT.Y.0021.000000T0000Z-260727T2100Z/
San Juan and Vicinity-Northeast-Southeast-Eastern Interior-North
Central-Ponce and Vicinity-Northwest-Mayaguez and Vicinity-
Southwest-Culebra-Vieques-St.Thomas...St. John...and Adjacent
Islands-St Croix-
Including the cities of Charlotte Amalie East, Canovanas, Loiza,
Barceloneta, Lajas, Carolina, Patillas, Comerio, Gurabo, Aguadilla,
Santa Isabel, Mayaguez, Ceiba, Moca, Manati, Cayey, Luquillo, Arroyo,
Vega Baja, Hormigueros, Juncos, Trujillo Alto, San Juan, Florida,
Juana Diaz, Vieques, San German, Charlotte Amalie West, Guayanilla,
Naguabo, Aguas Buenas, Christiansted, Rio Grande, Grove Place, Cruz
Bay, Culebra, Fajardo, Charlotte Amalie, Toa Baja, Las Piedras,
Ponce, Yauco, Aguada, Isabela, Guayama, Catano, Penuelas,
Quebradillas, Frederiksted, Salinas,
Humacao, Hatillo, Anna's Retreat, Camuy, Vega Alta, Maunabo,
Rincon, Cabo Rojo, Guanica, San Lorenzo, Anasco, Caguas, Yabucoa,
Arecibo, Dorado, Cidra, Toa Alta, Frederiksted Southeast, Guaynabo,
and Bayamon
441 PM AST Mon Jul 27 2026

...HEAT ADVISORY WILL EXPIRE AT 5 PM AST THIS AFTERNOON...

The threat has ended.

$$

GRS

Area Forecast Discussion
National Weather Service San Juan PR
308 PM AST Sun Jul 26 2026

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

.KEY MESSAGES...
Updated at 308 PM AST Sun Jul 26 2026

* A more unsettled weather pattern is forecast for tonight into
  Monday as a tropical wave and an upper instability move across
  the area. Therefore, there is a limited to elevated flooding
  threat along the northwestern side for tomorrow.

* Warm conditions will persist today under an east-southeasterly wind
  flow. Therefore, peak heat indices could approach 100°F TO
  110°F across coastal and urban areas, particularly north-
  central Puerto Rico.

* A moderate risk of rip currents remains in effect for the northern
  and eastern exposed beaches of Puerto Rico, Culebra, Vieques,
  and St. Croix.

* For the U.S. Virgin Islands, the potential for showers starts
  late tonight into early on Monday with periods of passing
  showers and some localized thunderstorms in the waters.

&&

.Short Term(This evening through Tuesday)...
Issued at 308 PM AST Sun Jul 26 2026

Current Satellite Derived Precipitable Water (PWAT) values range
between 1.34 and 1.65 inches across most of the region, with the
exception being St. Croix with PWAT values around 1.70 inches and
gradually increasing as the moisture field from a tropical wave
continues to approach the islands. Lingering Saharan Dust and
subsidence ahead of this tropical wave promoted a tranquil, albeit
warm to hot and hazy over some sectors, morning and early
afternoon. Diurnal heating, local orographic effects and sea
breeze convergence have quickly triggered convective shower and
t-storm activity within the past hour over interior to western
Puerto Rico, a limited flooding risk continues over those areas
this afternoon and evening although available moisture should stay
below 800 mb. Ponding of water in roads and poorly drained areas
with a chance of urban and small streams flooding and locally
strong t-storms continue to be forecast. Shower 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 reached the
100s to even the low 110s.

The leading edge of deeper tropical moisture (interacting with a
mid- to upper-level trough that is over the islands and will
persist through the period) will support an increase in scattered
showers and isolated t-storms over the region. The moisture field
from the wave and related showers and t-storms will arrive from
the southeast and reach St. Croix first tonight, increasing PWAT
values to 1.90 in. As the tropical wave advances west-
northwestward across the northeastern Caribbean, moisture levels
and atmospheric instability will continuously rise through Monday
and Tuesday. Deep tropical moisture, above normal PWAT that can
reach above 2 inches and increasingly favorable conditions for
shower and t-storm development will continue overspreading the
region tonight through Tuesday evening PWAT values will continue
above normal and reach above 2 inches. This evolving
meteorological pattern will support periods of scattered to
numerous showers along with isolated to scattered thunderstorms;
this convective activity is expected to be most pronounced during
the overnight and morning hours on Monday, and during the
afternoon hours on both Monday and Tuesday under an east-southeast
steering flow. Therefore, a heightened risk of flooding is
expected for both days. The probability of flooding will increase
in urban zones and small streams, while a restricted possibility
for localized flash flooding remains in sectors experiencing the
most severe and enduring convection.

Forecast 925 mb temperatures remain anomalously warm alongside
the anticipated rainfall threat. Consequently, sectors with sparse
rainfall are anticipated to experience above-normal daytime
maximums and unseasonably warm overnight minimums. These factors,
in tandem with surging tropical moisture, will encourage elevated
to potentially significant heat impacts across parts of the
islands.

&&

.Long Term(Wednesday through next Saturday)...
Issued at 308 PM AST Sun Jul 26 2026

Recent model guidance continues to suggest abundant moisture across
Puerto Rico and the U.S. Virgin Islands as a tropical wave moves
through the region early in the period. The latest precipitable
water (PWAT) values are near 2.0 inches, reflecting normal to above-
normal moisture for this time of year. Under this scenario,
scattered trade wind showers remain possible during the morning,
followed by afternoon showers and isolated thunderstorms across
central and northwestern Puerto Rico driven by daytime heating and
local effects.

However, these conditions will be short-lived as a drier air mass
quickly moves into the area by Wednesday night. At the surface, a
strengthening high-pressure system will tighten the pressure
gradient, promoting breezy conditions through the rest of the
period. Stability aloft will be further supported by a building mid-
to-upper level ridge, limiting overall shower activity across the
islands through the second half of the week as PWAT values drop
below normal to 1.25-1.30 inches. Nonetheless, quick patches of
moisture embedded in the trade winds will bring occasional fast-
moving showers with minimal to no impact. Moisture levels should see
a slight increase by late Saturday into Sunday as another tropical
wave approaches the region. Near to above-normal temperatures will
persist daily, with daytime highs reaching the upper 80s to low 90s
across coastal and urban areas.

&&

.AVIATION...
(18Z TAFS)
Issued at 308 PM AST Sun Jul 26 2026

VFR conditions prevail through this evening with easterly winds
at 15 to 20 kts with gusts up to 30 kts will diminish after
26/23Z, then pick up again from the east-southeast around 15 mph,
with higher gusts and stronger near SHRA/TSRA at around 27/12Z.
Afternoon SHRA/TSRA across interior to western PR will continue to
affect the vicinity of TJPS and TJBQ through 26/23Z, bringing
brief MVFR/IFR conditions and mountain obscurations. Overnight
increasing SHRA/TSRA will affect TIST/TISX/TJSJ/TJPS as a tropical
wave approaches and continues moving eastward, creating an
unstable and wet pattern along with possible brief MVFR/IFR
conditions.

&&

.MARINE...
Issued at 308 PM AST Sun Jul 26 2026

Surface high pressure across the Central Atlantic will maintain
moderate easterly winds across the regional waters through Sunday,
ranging between 10 and 15 knots with gusts up to 20 knots at least
until tonight before the arrival of the axis of a tropical wave.
Therefore, seas will generally remain between 2 and 4 feet across
protected waters and up to 5 feet across open Atlantic and
Caribbean waters. Marine conditions will begin to deteriorate late
tonight into Monday as an approaching tropical wave interacts with
an upper-level cut-off low. This system will bring a significant
increase in passing showers and thunderstorm activity across the
regional waters, starting in the Anegada Passage and USVI waters
before spreading west. Mariners should expect sudden localized
gusts in excess of 25 knots, frequent lightning, and locally
elevated seas near the strongest thunderstorms.

&&

.BEACH FORECAST...
Issued at 308 PM AST Sun Jul 26 2026

A moderate risk of rip currents will continue through the
remainder of the day for the northern, eastern, and southeastern
beaches of Puerto Rico, as well as Culebra, Vieques, and St.
Croix. While breaking wave heights will remain relatively low at 2
to 4 feet. For tomorrow, a decrease in winds will lead to a low
risk of rip currents; however, other threats, such as thunderstorm
activity, might result in hazardous beach conditions.

The moderate risk of rip currents returns on Tuesday.

&&

.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...LIS