660
FXCA62 TJSJ 060909
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
509 AM AST Sat Jul 6 2024

.SYNOPSIS...

A Saharan Air Layer (SAL) will promote hazy skies and limit
longwave radiational cooling, promoting warmer-than-normal low
temperatures today and tomorrow. Combining the available moisture
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. The next tropical wave will arrive between Monday
afternoon and Tuesday. Another pulse of SAL may move in by the
middle of next week. Another tropical wave will swing by the
islands around the following weekend.

&&

.SHORT TERM...Today through Monday...

During the overnight hours, Puerto Rico and the U.S. Virgin Islands
experienced mostly calm weather conditions, with some tradewind
showers in the southern and eastern municipalities of Puerto Rico.
The showers were not significant, and accumulations were below 0.30
inches. Hazy skies were observed as a Saharan Air Layer prevailed.
Satellite observations also suggested partly cloudy skies across
the islands. The combination of cloudiness and hazy skies
prevented significant nighttime cooling, resulting in warmer-than-
normal low temperatures. Temperature-wise, they remain in the low
80s along the coastal/urban areas, while across the higher
elevations, they remain in the 70s. Official weather stations
reported southeasterly winds of less than 10 mph throughout the
night.

Todays forecast leans towards hazy skies and dry air dominating the
area, with the possibility of some rainfall and isolated
thunderstorm activity by late afternoon and evening due to suspended
dust particles. These weather conditions should prevail through at
least Sunday. Model guidance suggests a surface high-pressure system
building across 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.

An increase in moisture is expected on Sunday as a trade wind
perturbation moves through the region, potentially bringing showery
weather. Precipitable water models suggest normal to below-normal
climatological levels for this time of year through the beginning of
the workweek. The forecast for Monday leans towards calm weather
throughout the day, with Saharan dust particles reaching the area by
late Monday and a gradual increase in moisture associated with an
approaching tropical wave.

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

Model guidance suggests the lingering moisture of a tropical wave
on Tuesday, pooling total precipitable water (TPW) between 2.0
and 2.30 inches across PR and the USVI`s region. Additionally, we
observed steep lapse rates at the 850-700 MB layer, and the
African Easterly Jet maxima are moving south of the islands on
Tuesday and Wednesday. Under this weather pattern, scattered to
numerous showers and thunderstorms may develop across the region
each day. We noted that model guidance suggests surges of SAL
between the available moisture, which may also cause hazy skies.
If thunderstorms develop, the interaction with the suspended dust
particles could enhance them.

A surface high pressure anchored across the North Atlantic Ocean
will promote breezy easterlies across the islands throughout the
week. This wind flow will promote a mixture between the arrival of
dry slots and pockets of moisture each day. This means that we
may observe a mix of clouds and sunshine or clear skies (during
the nighttime) with the arrival of quick passing showers advect by
the winds. Although we may observe periods of rainy conditions,
the interaction of plenty of low-level moisture, above-normal sea
surface temperatures, and maximum summer heat temperatures could
result in an extended warm-to-hot spell during the upcoming week
if we do not observe prolonged periods of rain activity.

GFS suggests another tropical wave moving across the northeast
Caribbean around next weekend, which could promote unsettled
weather conditions.

&&

.AVIATION...

VFR conds will prevail across all terminals. However, brief MVFR
conds are possible after 06/17z in TJBQ and TJSJ. Hazy skies will
remain possible as a SAL prevails in the region. The winds will
remain from the ESE at 5 to 10kt through 06/12 and increase to 15-20
kt with gusts between 20 and 30 kt near TSRA and sea breeze
variations after 12z.

&&

.MARINE...
A surface high pressure across the Atlantic Ocean will tighten the
local pressure gradient, promoting choppy seas across the islands
through the next few days. Seas will range between 3 and 5 feet
across most of the coastal waters. Winds will be mainly from the
east to southeast at 10 to 15 knots, increasing between 15 and 20
knots later in the day.

&&

.BEACH FORECAST...
There is a moderate risk of rip currents for southwest PR,
Vieques, Culebra and St Croix today, becoming moderate for most
of the beaches tomorrow due to the increase in the easterly winds.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning from 10 AM this morning to 5 PM AST this
     afternoon for PRZ001-003-005-007-008-010-011.

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ002-012-013.

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

     Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ001.

AM...None.
&&

$$

SHORT TERM...MMC
LONG TERM....CAM