125
FXCA62 TJSJ 232036
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
436 PM AST Sun Jun 23 2024

.SYNOPSIS...

Fast moving showers will continue to move over the local islands
during the overnight hours. An approaching tropical wave will
increase shower and thunderstorm activity starting early on
Monday through the evening hours. A mass of drier air accompanied
by a a dense plume of Saharan dust is expected to move over the
local area from Tuesday onward.

&&

.SHORT TERM...Tonight through Tuesday...

Variable weather was observed over the local islands during the
morning hours. By mid-morning the San Juan streamer formed, leaving
moderate precipitation over the San Juan metro area. As the day
progressed, strong showers and thunderstorms began to develop over
the interior towards western and northwestern Puerto Rico. Doppler
radar estimated rainfall accumulations between 2.5 to 3.0 inches. A
Special Weather Statement (SPS) and Flood Advisory (FLS) was issued
for the mentioned sectors. The rainfall activity started to
dissipate by mid-afternoon, leaving only few showers over the
eastern municipalities of Puerto Rico. The U.S. Virgin Islands
were under a fair weather pattern for most of the day. Today,
daytime temperatures were seen in the upper 80s to mid 90s along
most of the coastal and urban areas. Heat indices were over the
107 degrees Fahrenheit across the northern, western, and southern
portions of Puerto Rico, Culebra, Vieques and St. Croix. A Heat
Advisory was in effect for these areas.

Surface winds will veer from the southeast today as the surface high
pressure located over the central moves toward the eastern Atlantic.
A tropical wave is forecast to move over the local islands by
Monday, increasing moisture and the potential for stronger showers
and thunderstorms over the area. By Tuesday, a drier airmass will
filter into the region as the tropical wave exits the islands.
Moderate to high concentrations of Saharan dust are expected to move
into the area as well, promoting hazy skies through at least
Thursday.

A warm trend is anticipated by the middle of the week with daytime
temperatures reaching the low to mid 90s along the coastal and urban
portions of Puerto Rico and the U.S. Virgin Islands. Therefore, heat
advisories or excessive heat warning are possible during this period.

.LONG TERM...Wednesday through Sunday...

./FROM PREV DISCUSSION...Issued at 425 AM AST Sun Jun 23 2024/

The NASA Dust Aerosol Optical Thickness model shows a dense Saharan
Air Layer (SAL) lingering over the local islands through Thursday.
In addition, there will be a mid-to upper-level ridge across the
northeastern Caribbean, resulting in drier conditions aloft and
inducing a very stable weather pattern with hazy skies, sunny
mornings, and some afternoon showers across the western sections of
Puerto Rico. Instability will slightly increase from late Thursday
and Friday as an induced low-level trade wind perturbation streams
across the local area. Moisture content will increase once again and
isolated thunderstorms are expected to develop over portions of the
waters and across the islands.

Later in the period, as the SAL exits the region, the islands will
be influenced by the easterly winds resulting from the interaction
between a surface high pressure over the central Atlantic and an
induced surface trough just northwest of the region over the Western
Atlantic. This wind pattern will drag tropical moisture into the
islands. According to global model guidance, PWAT values will
fluctuate from 1.70 to 2.10 inches, just above climatology for the
region. That moisture, along with diurnal heating, will promote the
development of afternoon showers and isolated thunderstorms each day
across the western interior sections of Puerto Rico. Winds will
return from the southeast by Saturday into Sunday, increasing the
potential to observe heat indices near 108 to 112F across the
coastal and urban areas of the islands.

&&

.AVIATION...

(18Z TAFs)

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. SHRA/TSRA en route from the SE
Caribbean should increase across the USVI terminals aft 24/12z due
to an approaching tropical wave. Similar conditions should spread
across the eastern PR terminals by 24/15z. Sfc winds from the ESE up
to 15 kts, bcmg light and variable overnight.

&&

.MARINE...

A surface high pressure over the central Atlantic will promote
light to moderate easterly winds through tonight. Winds are expected
to veer from the east southeast by Monday morning. Trade wind
showers will move across the regional waters from time to time. The
next tropical wave will approach the local islands from late tonight
through Monday. Increasing winds and showers with thunderstorms are
expected through Monday night across the local waters. Hazy skies are
expected from Tuesday through Thursday due to Saharan dust.

&&

.BEACH FORECAST...

There is a low to moderate risk of rip currents across PR and the
USVI. Showers and isolated thunderstorms are possible across the
coastal areas of the USVI and eastern PR during the morning hours,
and across the west/northwest coast of PR during the afternoon.
The afternoon thunderstorms could produce frequent lightning and
gusty winds. This weather pattern is expected to continue for the
next few days with the arrival of a series of tropical waves.

&&

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

$$

SHORT TERM...LIS
LONG TERM....GRS
AVIATION...DSR
PUBLIC DESK...MRR