639
FXCA62 TJSJ 222056
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
456 PM AST Sat Jun 22 2024

.SYNOPSIS...

Showery weather conditions will prevail through the rest of the
afternoon and early evening. Weather conditions will start to
deteriorate by late Sunday with the arrival of a tropical wave.
Expect an increase in showers and thunderstorms until at least
Tuesday.

&&

.SHORT TERM...Tonight through Monday...

Relatively calm conditions prevailed during the morning hours across
the local islands. By mid-morning the cloud coverage began to
increase across the Cordillera Central, resulting eventually in the
development of strong showers and thunderstorms. The strongest
activity was seen from Adjuntas to Cabo Rojo, where Doppler radar
estimated rainfall accumulations between 1.0 to 3.0 inches. The San
Juan streamer also formed, leaving rainfall accumulations of 2.0
inches in the Guaynabo area. Moving into eastern Puerto Rico, late
afternoon convective activity was observed over Humacao and
Yabucoa. Flood Advisories were issued for the mentioned sectors.
For the rest of the afternoon, the shower activity should move
over the Mona Passage and dissipating before sunset. However,
trailing showers over the Anegada Passage could reach the eastern
portions of Puerto Rico and the U.S. Virgin Islands leaving minor
rainfall accumulations.

As the surface high pressure moves towards the central Atlantic,
winds will start to veer from the east to southeast by Sunday
morning. A tropical wave will approach the local area from the east
on Sunday afternoon, increasing instability and the potential for
convective activity through Monday. Squally weather conditions are
anticipated, along with moderate winds with higher gusts embedded
with the heaviest showers. By Monday, the stronger activity is
expected across the western and northwestern portions of Puerto Rico
during the afternoon hours. Despite the expected weather, daytime
temperatures will remain warm to hot across the region, with highs
in the range between the upper 80s to lower 90s along the coastal
and urban sectors, and in the mid 80s in the mountains. Heat indices
are expected to be elevated as well, with a limited heat risk
anticipated.

.LONG TERM...Tuesday through Saturday...

./FROM PREV DISCUSSION...Issued at 356 AM AST Sat Jun 22 2024/

After the tropical wave departs, a surface high pressure will take
control of the atmospheric conditions. First, this feature will
drive the trade winds from the southeast, and will tighten the
pressure gradient, with surface speeds around 20 knots. Also, a
trade wind cap inversion will form at around 800 mb, trapping all
the available moisture below this level. Moderate to strong
concentrations of Saharan dust may arrive too, with hazy skies and
little precipitation expected through at least early Thursday. This
conditions, however, will make temperature soar, with very hot
conditions expected to return these days.

By Thursday, as an induced surface trough (more evident in the 700
mb pressure field) approaches, the winds are expected to shift from
the northeast at these heights, and the trade wind cap will weaken.
Instability should be enough to fire up showers and thunderstorms
for the interior and western portions of Puerto Rico, with an
elevated risk for urban and small stream flooding, and lightning.
Also, by the end of the week, an upper level trough will approach
the Leeward Islands. This trough is not expected to affect the local
islands directly, but it will allow for better shower and
thunderstorm production each afternoon across the usual interior and
western Puerto Rico. Shower activity will become more frequent too
for eastern Puerto Rico and for the Virgin Islands. Saharan dust
will not completely go away, but concentrations are expected to be
lower. This low concentrations of dust could contribute to enhance
the frequency of the lightning activity within the thunderstorms.

&&

.AVIATION...

(18Z TAFs)

SHRA/TSRA over mainland PR should end around sunset. Thereafter,
mainly VFR conditions should prevail across all terminals through
Sunday morning. Afternoon SHRA/TSRA expected to develop once again
across the area terminals after 23/16z. HZ due to Saharan dust
expected later this evening through Sunday, but VSBY should remain
P6SM. Low-level winds from the east at 10-15 kt, bcmg light and
variable overnight.

&&

.MARINE...

A surface high pressure over the central Atlantic will promote
light to moderate easterly winds through the rest of the weekend.
Trade wind showers will move across the regional waters from time to
time. The next tropical wave will approach the local islands by the
beginning of the next workweek. Increasing winds and thunderstorm
activity is expected from late Sunday through Monday.

&&

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

$$

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