499
FXCA62 TJSJ 050835
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
435 AM AST Fri Jul 5 2024

.SYNOPSIS...

Residual moisture from the tropical wave (Invest 96L) will lead
to occasional passing showers across the region through at least
this afternoon. A drying trend is expected during the weekend
along with a Saharan Air Layer. Warm to hot conditions will
continue. The next tropical wave will reach the islands Monday
night into Tuesday increasing the potential for shower activity
once again.

&&

.SHORT TERM...Today through Sunday...

Last night, a tropical wave caused showers and thunderstorms in the
local waters. These showers and thunderstorms then moved inland
across the US Virgin Islands, as well as the south and east of
Puerto Rico. Additionally, the arrival of a Saharan Air Layer led to
hazy skies, and the GOES-E infrared channel detected cloudy skies.
These two factors - the suspended dust particles and cloud cover -
blocked outgoing longwave radiation, preventing nighttime cooling
and resulting in warmer-than-normal low temperatures. The winds were
mainly from the east-southeast at 10 mph or less.

From this morning through this afternoon, the trailing moisture
associated with the tropical wave will maintain high low-level
moisture values, which, combined with the maximum air temperatures,
will result in hazardous heat index values along PR and the USVI`s
coastal and urban sections. Therefore, a Heat Advisory is in effect
for most of these locations. The skies will also be hazy due to
moderate to high suspended Saharan dust particles. The proximity of
a mid to upper-level trough north of the islands will promote enough
instability, resulting in periods of showers and thunderstorms
across the local waters, eastern Puerto Rico, and the Virgin
Islands. The combination of the lingering tropical moisture,
excessive heating, suspended dust particles, topography, and sea
breeze variations will promote some isolated strong thunderstorms,
especially during the afternoon.

Rain activity will tampered off late this afternoon into the evening
as the concentrations of the suspended African dust particles
increase, making the haziness more evident and promoting stable
conditions. The dry, stable, and hazy weather pattern will extend
into the weekend. However, a surface high-pressure building across
the central Atlantic and extending into the northeast Caribbean will
promote breezy easterly winds, advecting occasional patches of
clouds and showers, especially across the windward sections of the
US Virgin Islands and Puerto Rico. A trade wind perturbation may
increase the potential for showery weather on Sunday, as a sharp
increase in moisture would be possible.

&&

.LONG TERM...Monday through Friday...

On Monday, residual Saharan dust particles will remain in low
concentrations across Puerto Rico and the U.S. Virgin Islands.
Nonetheless, by late Monday, expect an increase in moisture and
shower activity as a tropical wave, along with a wind surge,
reaches the islands on Tuesday. Precipitable water models show
values in the 50th percentile or around normal climatological
levels for this time of the year with the passage of this tropical
wave. This wave will move relatively fast across the region,
particularly on Tuesday. With it, expect the passing showers to
increase across the islands during the morning hours, with
moderate to heavy showers and isolated thunderstorms expected
across the western and northwestern portions of Puerto Rico during
the afternoon hours. Trailing moisture associated with the
tropical wave will prevail over the long-term period.

The latest model guidance continues to suggest a strong surface to
mid-level high-pressure system over the central Atlantic. This
weather feature will maintain the wind flow out of the east-
southeast through at least Friday. With these weather conditions,
we anticipate warm weather conditions with temperatures in the 90s
along the coastal and urban areas of the islands and in the upper
70s to low 80s in the mountainous areas. As mentioned in previous
discussions, the mixture of warm temperatures and the available
moisture will result in higher heat indices across the region,
surpassing the mid-100s.

&&

.AVIATION...

(06Z TAFS)

Although VFR conds will prevail for most of the period. SHRA/TSRA
will affect the surrounding waters of USVI and may move inland,
affecting ISX or IST through at least 05/18z; then this activity may
spread into PR, affecting JSJ/JPS or JBQ between 05/16-05/22z,
promoting brief MVFR conds. Hazy skies will remain possible as a SAL
enters the region. The winds will remain calm to light and variable
through 12z, returning from the E to ESE at 15-20 kt with gusts
between 20 and 30 kt near TSRA and sea breeze variations after 12z.

&&

.MARINE...

A departing tropical wave will continue to bring showers and
thunderstorms across the regional waters through this morning. A
strengthening surface high pressure over the central Atlantic will
extend into the northeast Caribbean by this afternoon through the
weekend, leading to moderate to fresh easterly winds. The next
tropical wave will be approaching the islands by late Monday night
into Tuesday, increasing the chance of showers across the local
waters.

&&

.BEACH FORECAST...

Today, the rip current risk should remain moderate along the
southern beaches of Puerto Rico, Vieques, Culebra, Saint Thomas,
Saint John, and Saint Croix. Elsewhere should remain low. For
additional information, consult the Surf Zone Forecast (SRFSJU)
issued by the NWS San Juan Office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...

PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001-003-005-007-008-010-011.

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

AM...None.
&&

$$

SHORT TERM/AVIATION/MARINE...CAM
LONG TERM/BEACH FORECAST...MMC