681
FXCA62 TJSJ 040858
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
458 AM AST Thu Jul 4 2024

.SYNOPSIS...

The passage of a tropical wave (Invest 96L) will increase the
frequency of passing showers and thunderstorms across the region
through at least Friday. Even with the anticipated increase in
cloud cover and high chances of rain, warm to hot conditions will
continue. A drying trend is expected during the weekend.
Nonetheless, by Tuesday another tropical wave will reach the
islands increasing the potential for shower activity once again.

&&

.SHORT TERM...Today through Saturday...

Calm weather conditions prevailed overnight, with warmer-than-normal
low temperatures across Puerto Rico and the US Virgin Islands around
midnight and before the arrival of a tropical wave by early this
morning. Then, showers and thunderstorms began to affect the Anegada
Passage and Caribbean Water, moving over St Croix early this
morning. The skies were variably cloudy due to a mixture of mid- to
upper-level clouds and the cloudiness from the approaching tropical
wave. The winds were mainly from the east to east-northeast at 10
mph or less.

Today, a tropical wave will bring unsettled weather conditions. The
instability and moisture associated with this wave will likely fuel
the formation of strong thunderstorms, leading to downpours, gusty
winds, and frequent lightning. These conditions will first affect
the local waters, the US Virgin Islands, and eastern PR before
spreading into the interior and north and western sections of PR by
mid-morning into the afternoon. Of particular concern is the
potential for urban and small-stream flooding. It`s crucial that
residents and visitors stay updated on the weather conditions and
take necessary precautions. The wave`s trailing moisture will linger
across the regions through Friday afternoon. During the event, 2 and
4 inches of rain will remain possible, especially across the
southern and eastern third of Puerto Rico, with higher isolated
amounts.

Rain activity will tampered off about Friday afternoon into the
evening as a Saharan Air Layer moves in following the trailing
moisture of today`s wave. Therefore, expect hazy skies due to the
increase in suspended dust particles. However, the interaction of
the available moisture, local effects, sea breeze, excessive
heating, and dust particles could promote the formation of strong
afternoon and evening convection, especially across western PR and
the surrounding waters.

The dry 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.

&&

.LONG TERM...Sunday through Thursday...

The inherited forecast remains unchanged. The latest model guidance
continues to suggest a strong high-pressure system over the
central Atlantic. This weather feature will maintain the wind flow
out of the east-southeast through most of the long-term period.
Precipitable water models show values in the 25th percentile or
below normal climatological levels for this time of the year as a
drier airmass continues to move across the region with low
concentrations of Saharan dust according to the Dust Aerosol
Optical Depth model. Therefore, we foresee dry weather conditions
through Monday. 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.

From late Monday into Tuesday, expect a change in weather
conditions as a vigorous tropical wave approaches the local
islands. The precipitable water content analysis shows values
between 1.90 and 2.30 inches on those days. This wave will move
relatively fast across the region, particularly on Tuesday. With
the passage of this tropical wave, 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. At this time, mid-week is the transition day
to a drier weather pattern.

&&

.AVIATION...

(06Z TAFS)

A tropical wave will bring SHRA/TSRA across the local flying area,
promoting brief MVFR conds. Although VFR conds will prevail,
TSRA/SHRA will spread across ISX through around 4/15z, across
IST/JSJ/JPS btwn 04/10z-15z, and into the rest of PR after 04/14z.
The unsettled wx pattern will extend thru 05/15z, when a drier air
mass with suspended dust particles will move across the islands. The
winds will remain calm-light/var thru 12z, returning from the E to
ENE at 15-20 kt with gusts btwn 20-30 kt near TSRA and sea breeze.

&&

.MARINE...

A tropical wave moving westward will bring more showers and
thunderstorms across the regional waters today and tomorrow. A
surface high pressure over the central Atlantic will strengthen and
extend into the northeast Caribbean on Friday afternoon through the
weekend, leading to moderate and fresh easterly winds across the
local waters. Another tropical wave will be approaching the islands
by late Monday night or Tuesday.

&&

.BEACH FORECAST...

Coastal conditions will continue to improve gradually. There is
moderate risk along all the coastal areas of the islands, with the
exception of the western beaches of PR where the risk is low. In
the island of St. Croix the risk will become high as the
approaching tropical wave get closer. For more information,
consult the Coastal Hazard Message (CFWSJU) issued by the NWS San
Juan office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...

PR...None.

VI...High Rip Current Risk until 6 AM AST early this morning for
     VIZ002.

AM...Small Craft Advisory until 6 PM AST this evening for AMZ723-733-
     735-741.

&&

$$

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