890
FXCA62 TJSJ 141850
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
250 PM AST Mon Jul 14 2025

.KEY MESSAGES...

* Hot temperatures and high humidity content this week will
  promote an elevated heat risk across lower elevations and urban
  areas of Puerto Rico.

* A couple of tropical waves will increase shower and thunderstorm
  activity tomorrow and Wednesday.

* A Saharan Air Layer with moderate to high concentrations of
  Saharan dust will promote hazy skies and poor air quality
  conditions from Thursday into the weekend.

* A moderate risk of rip currents returns tomorrow night across
  the east and northern beaches of the islands.

&&

.SHORT TERM...Tonight through Wednesday...

By 130 PM AST, the highest radar estimated accumulations were
over several southeastern and northwestern municipalities as well
as over northern San Juan (including Old San Juan), Guaynabo, and
Catano. Values ranging from 0.6 to 1.3 in were observed over these
areas. By 230 PM AST 1.2 to 1.7 inches of radar estimated rain were
observed over northern San Juan due to El Yunque streamer. A
Flood Advisory is in effect through 515 PM for northern San Juan.
Heat indices ranged in the upper 90s to low 100s over coastal
areas of the islands. Current satellite- derived precipitable
water (PWAT) values indicate up to around 1.6 to 1.9 inches over
the area. PWAT values should briefly decrease tonight to below 1.5
inches, before increasing again tomorrow as moisture from a weak
tropical wave reaches the area. A TUTT-low well west of our area
will continue to move away from the region as a mid to upper level
high remains over the region and promotes more stable conditions
aloft. PWAT values tomorrow can reach 1.9 to 2.30 inches tomorrow
and Wednesday, as moisture from another tropical wave moves over
the area. ESE steering flow will persist, possibly becoming more E
on Wednesday, with surges in wind speeds expected during the
period. This wind direction, local orographic effects and
available moisture will promote afternoon showers and
thunderstorms, mainly over interior to W-NW PR with streamers also
forming downwind of the USVI, Vieques, Culebra and El Yunque
(towards the metro area). Under this diurnal pattern, showers will
continue to filter over windward sectors of the islands. With the
tropical waves moving through on Tuesday and Wednesday, the
potential for showers and thunderstorms over the islands
increases. There is a limited to elevated risk to observe ponding
of water in roads and poorly drainage areas, as well as localized
flooding in urban areas, roads and small streams. 925 mb
temperatures will remain above normal for the period, with highs
in the upper 80s to low 90s along the coastal and urban areas each
day. The increase in moisture and these temperatures will also
lead to heat indices above 100 degrees each day, with an elevated
heat risk persisting.

.LONG TERM...Thursday through Monday...

./FROM PREV DISCUSSION...Issued at 501 AM AST Mon Jul 14 2025/

On Thursday, southeasterly winds and an area of high moisture
content trailing a departing tropical wave will aid in the
development of early morning showers between the USVI and eastern
PR, followed by afternoon thunderstorms over west/northwest PR.
At the same time, a Saharan Air Layer (SAL) with moderate to high
concentrations of Saharan dust is expected to gradually fill
across the region. This will promote hazy skies, reduced
visibility, and poor air quality conditions through at least
Friday. The higher humidity content at the surface and the
southeast component in the winds will cause higher than normal
heat indices across most coastal and urban areas of the islands,
but particularly across the north and west coast of PR. Latest
guidance suggest Heat Advisory to Extreme Heat Warning conditions
before the onset of afternoon convection, and in cloud/rain-free
areas.

Also, warmer minimum temperatures at night due to the SAL preventing
efficient cooling at the surface will cause that days begin
warmer than normal. Therefore, hot temperatures are expected to
continue through at least Saturday and mainly across most coastal
and urban areas of PR. Heat advisory conditions will likely
continue each day. Residents and visitors, are advised to take
appropriate precautions during this period, particularly those who
are sensitive to heat and poor air quality conditions.

A TUTT-low and an induced low-level trough are expected to move
across the northeastern Caribbean early in the weekend. This will
bring an increase in shower activity again across the islands.
Another tropical wave is expected to move on Monday.

&&

.AVIATION...
(18Z TAFS)

Mainly VFR conditions at the terminals during the period. SHRA/-SHRA
will continue to filter in under ESE flow, these could briefly
affect the eastern terminals. Afternoon SHRA/TSRA will develop
across the interior and west PR, affecting the VCTY of TJBQ and TJSJ
thru 14/23z. ESE winds are at at around 15 kt with higher gusts and
sea breeze variations and will become light/variable after 14/23Z.

&&

.MARINE...

A surface high pressure will strengthen over the western Atlantic
early this week. Two tropical waves will move across the eastern
Caribbean on Tuesday and Wednesday. Therefore, moderate to locally
fresh trades and choppy seas are expected across the regional waters
by midweek. Increasing shower and thunderstorm activity is expected
from Tuesday through Thursday. Hazy conditions due to Saharan dust
will return from Thursday onward.

&&

.BEACH FORECAST...

Although there is currently a low risk of rip currents. By late
tomorrow, a moderate risk of rip currents will return for the
north and east facing beaches of Puerto Rico, Culebra, Vieques,
and St. Croix, driven by increasing winds. Life-threatening rip
currents often occur in the vicinity of groins, jetties, reefs,
and piers. Hot and humid conditions will continue throughout the
week. Stay hydrated, take breaks in the shade, and limit outdoor
activity during peak heat hours.

&&

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

$$

MRR/MNG