253
FXCA62 TJSJ 071557 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1157 AM AST Sun Jul 7 2024

.UPDATE...After a brief period where the radar went into clear air
mode, showers have returned to Puerto Rico and Culebra. Moisture
is deeper than yesterday reaching nearly 7 kft, but dry layers are
also present at mid levels with relative humidity values as low
as 3 percent at 550 mb. Nevertheless local shower development is
still possible in western Puerto Rico and cannot rule out the
possibility of a thunderstorm in the enhanced moisture environment
where precipitable water has risen to 1.7 inches.

We are in a moist slot and visibilities have improved tremendously
to more than 15 miles. There will be a brief episode of dust
Monday morning, another quick clearing and then very heavy dust
beginning early Tuesday morning. The tropical wave expected may
arrive a bit earlier than forecast previously with effects
beginning as early as Monday afternoon. Shower activity should
taper off on Tuesday.

Temperatures were good, and PoP grids were adjusted to conform to
current conditions and those expected later on Monday.

&&

.PREV DISCUSSION... /issued 403 AM AST Sun Jul 7 2024/

SYNOPSIS...

The available moisture associated with a trade wind perturbation
and maximum air temperatures in the 90s will promote hazardous
heat index values across the Puerto Rico and US Virgin Islands
coastal areas, where an Excessive Heat Warning and Heat Advisory
are in effect. A relatively stable atmosphere will prevail
throughout the week. However, the chance of rain will increase
due to a couple of tropical waves, the first arriving on Tuesday
and the second by the weekend. Between and around those waves
(especially on Tuesday and Wednesday), air masses with important
suspended dust particles from the African Deserts could inhibit
widespread rain activity and promote hazy skies.

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

Overnight, Puerto Rico and the U.S. Virgin Islands experienced
predominantly calm weather conditions, with hazy skies prevailing
due to the Saharan Air Layer. Like previous days, the prevailing
Saharan Air Layer prevented significant nighttime cooling, resulting
in warmer-than-normal low temperatures. For that reason, official
weather stations reported temperatures in the low 80s along the
coastal/urban areas, while across the higher elevations, they were
in the 70s. Wind-wise, they were mostly out of the east and stayed
below 10 mph.

The latest model guidance suggests a surface high-pressure system
building over the central Atlantic and extending into the northeast
Caribbean, leading to breezy easterly winds and occasional clouds
and showers, especially in the windward sections of the U.S. Virgin
Islands and Puerto Rico. Today, the forecast leans towards hazy
skies with the introduction of a trade wind perturbation that can
bring a slight increase in showery weather. Nonetheless, we
anticipate little cloud growth across the islands, with some
rainfall and isolated thunderstorm activity by late afternoon due to
suspended dust particles. Precipitable water models suggest normal
to below-normal climatological levels for this time of year through
at least the beginning of the workweek. The forecast for Monday
leans towards calm weather throughout the day and a gradual increase
in moisture associated with an approaching tropical wave by late
Monday.

On Tuesday, a weak tropical wave with its associated wind surge will
reach the islands. At this time, we do not anticipate significant
rainfall activity nor cloud growth as an extensive mass of Saharan
Air Layer will move across Puerto Rico and the U.S. Virgin Islands.
The Dust Aerosol Optical Thickness model suggests concentration
values around 0.55, and to be considered a slight to moderate event,
it needs to exceed 0.20. Therefore, it was added to the forecast and
we want to inform citizens and tourists about this upcoming slight
to moderate dust event that could affect people with respiratory
illness.

LONG TERM...WEDNESDAY THROUGH SUNDAY...

Relatively stable atmospheric conditions will prevail throughout
most of the forecast, with above-normal heights at mid- and upper-
level, warmer-than-normal temperatures at 500 MB, and relatively
stable lapse rates at mid- and low levels. Additionally, the 925
MB temperatures suggested warmer-than-normal temperatures, which
may indicate a warm-to-hot spell during those days in the long
term.

The NASA Dust Extinction guidance suggests the presence of
significant suspended dust particles from the African deserts,
especially on Wednesday, with some traces remaining on Thursday.
This dry air mass will surround the trailing moisture from a
tropical wave, which may limit widespread rain activity but create
hazy skies. However, be prepared for the possibility of some
strong thunderstorms, especially on Wednesday afternoon, as steep
lapse rates between 850-700 MB may be possible during those days
due to the passing wave. However, the bulk of moisture and the
African Jet-stream maxima may be located off to the south of the
islands, across the southeastern Caribbean.

Once again, model guidance suggests a surface high pressure
anchored across the North Atlantic Ocean that could promote breezy
easterlies across the islands throughout the second part of the
week. Under this wind flow, we may observe the arrival of dry
slots and pockets of moisture each day, resulting in a mix of
clouds and sunshine or clear skies (during the nighttime) and the
arrival of quick passing showers.

Model guidance suggests a tropical wave arriving by the upcoming
weekend. If the dry air mass surrounding it does not negatively
impact it, we may see a wet and unstable pattern with a better
chance of organizing convection on Saturday or Sunday.

AVIATION...
(06Z TAFS)

VFR conds will prevail across all terminals. However, brief MVFR
conds are possible after 07/17z near TJBQ. Hazy skies will remain
possible as a SAL prevails in the region. The winds will remain from
the E at 5 to 10kt through 07/12 and increase to 15-20 kt with gusts
between 20 and 30 kt near TSRA and sea breeze variations after 14z.

MARINE...

A surface high pressure anchored across the Atlantic Ocean will
tighten the pressure gradient across the Northeast Caribbean,
promoting moderate to fresh easterly winds with choppy seas. A
tropical wave will cross the Lesser Antilles late Monday night and
approach the islands on Tuesday with residual moisture until
Wednesday.

BEACH FORECAST...

The CariCOOS buoy network reported seas around 3 feet (from the
northeast with a 5-second period in the Atlantic and from the
southeast with a 6-second period in the Caribbean) and winds
around 13 knots out from the east. The water temperatures were in
the mid-80s, as were the air temperatures above the water. Winds
will increase at 15 to 20 knots during the day, producing choppy
seas between 4 and 5 feet or higher.

Beachgoers, the risk of rip currents will be moderate in the north
and southwest of PR, Culebra, and St Croix.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     003-005-007-008-010-011.

     Heat Advisory until 5 PM AST this afternoon for PRZ012.

VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.
&&

$$

SHORT TERM...WS
UPPER AIR/PUBLIC....RC