889
FXCA62 TJSJ 230825
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
425 AM AST Sun Jun 23 2024

.SYNOPSIS...

There is a Heat Advisory in effect for most urban areas and lower
elevations of the islands. A weak upper-level trough and the
available low-level moisture will promote shower and thunderstorm
development across the area today. A tropical wave will increase
shower activity on Monday. A strong Saharan Air Layer with a
dense area of dust is expected to move over the islands by
midweek.


&&

.SHORT TERM...Today through Tuesday...

Advective showers and isolated thunderstorms will continue to effect
windward sections throughout the morning hours as relatively drier
air filters into the region under a surface easterly to
southeasterly flow. High pressure continues to dominate the Atlantic
Basin and as it shifts towards the eastern Atlantic and a tropical
wave moves into the eastern Caribbean, winds will begin to veer
slightly remaining from a southeasterly direction through the short
term. Afternoon convection later on today is expected to bring heavy
thunderstorms across the western section of Puerto Rico, mainly in
the center of the western part of the island as sea breezes keep
most of the rain inland. Streamer formation downwind of El Yunque
and the smaller islands may bring some good showers are well.
Flooding is possible with the heaviest activity and thunderstorms
capable of producing frequent lightning are possible.

The tropical wave is forecast to move across the region on Monday
afternoon, bringing a moist airmass that will elevate the chance of
passing showers and overall cloudy, muggy conditions. Winds may
increase to strong values at times, which may result in squally
weather conditions especially across the southeastern sections of
Puerto Rico and the USVI. A drier airmass with high concentrations
of Saharan Dust is forecast to replace this airmass as the tropical
wave exits to the northwest of the region on Tuesday morning. Hazy,
dry conditions are forecast on Tuesday as a result of this. Monday
night may see a good amount of thunderstorms as the interaction of
the saharan dust causes somewhat of dryline effect on the moisture
in front of it. Yet as mentioned, Tuesday appears to be quite dry
with high maximum temperatures which may yield for a heat advisory
or warning, stay tuned.

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

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...

(06Z TAFs)

Mainly VFR conditions expected to prevail through the forecast
period with periods of reduced VIS and low ceilings possible during
the afternoon. SHRA will continue across the region, impacting the
USVI terminals and TJSJ through the day. Widespread SHRA and
isolated TSRA expected after 15Z, mainly effecting TJPS and TJBQ.
Winds will be out of the E at 14-18 kts, with gusts around 25 kts
from 13-22z.


&&

.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 from
late tonight through Monday. Increasing winds and showers with
thunderstorms are expected through Monday 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. The weather conditions should improve during the
night.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001-003-005-007-008-010>013.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ002.

AM...None.
&&

$$

SHORT TERM...RC
LONG TERM....LIS/DSR
AVIATION...RC