081
FXCA62 TJSJ 060902
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
502 AM AST Sun Oct 6 2024

.SYNOPSIS...

Variable weather conditions will persist across the islands today
due to the divergent side of an upper-level trough and sufficient
tropical moisture in the region. Hazardous marine conditions will
continue due to northeasterly swells generated by the distant
Hurricane Kirk in the Central Atlantic, impacting most northern
Atlantic waters and the northern coastal areas of Puerto Rico. As
a result, a High Surf Advisory and a high risk of rip currents are
in effect for the exposed northern coastal areas today. A Small
Craft Advisory also remains in effect until late today, extending
into Sunday for the offshore Atlantic waters. For more detailed
information, visit www.weather.sju/marine.

&&

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

Since 8 PM AST, radar observations have detected a few showers
affecting land areas, especially the far eastern sections of Puerto
Rico and parts of the local islands, leading to minimal rainfall
accumulations. Satellite imagery and reports from trained observers
indicated that skies remained mostly clear, with only brief
interruptions from the shower activity. Most surface observing
stations reported overnight lows in the 70s, with some exceptions in
northeastern and southern Puerto Rico, as well as the US Virgin
Islands. Winds were generally light and mostly variable, as
evidenced by the nearly stationary showers developing over local
waters.

In the next 12 to 24 hours, an upper-level trough to the west,
combined with abundant moisturewith precipitable water values
peaking around 2.2 to 2.3 incheswill maintain favorable conditions
for showers and thunderstorms across the region. Initially,
scattered showers and a few isolated thunderstorms will impact local
waters and some coastal areas this morning. As the day continues and
diurnal heating, along with sea-breeze convergence, intensifies,
anticipate an increase in showers and potentially severe
thunderstorms across the islands, including the US Virgin Islands.
After sundown, the focus of the activity will shift back to the
local waters and parts of the coastal areas. Similar to previous
days, winds are expected to remain light and variable, driven by an
area of weak pressure gradient over the northeastern Caribbean,
indicating that showers will not move much, if at all, during their
duration. Additionally, in this scenario, any developing activity is
likely to favor the interior and slowly extend to nearby areas,
following the dominant steering flow. Given the anticipated nature
of these storms, expect not only excessive rainfall (flooding)
hazard risks but also risks of strong gusty winds, lightning
strikes, and possibly small hail if conditions are optimal. While
these rains may provide brief relief from the heat, overall, warm
conditions are expected to dominate todayconditions that could
particularly affect individuals who are extremely sensitive to heat,
especially when outdoors without effective cooling or adequate
hydration.

The upper-level trough is expected to weaken while drifting
southwestward by Monday, gradually making way for a mid-to-upper-
level high. However, conditions will not become completely
unfavorable for thunderstorm development. The forecast still
indicates that showers with isolated thunderstorms will persist over
the coming daysgoing from the local waters and coastal areas during
the night and early morning into central and downwind areas in the
afternoon, fueled by intense diurnal heating and driven by sea-
breeze convergence. Light and variable winds on Monday are expected
to produce a similar shower distribution and associated risks.
However, reduced available moisture under more southeasterly winds
on Tuesday will likely lead to fewer and more localized showers.
Warm conditions will persist, so those planning outdoor activities
or working outside should take appropriate precautions.

.LONG TERM...Wednesday through Sunday

A variable weather pattern will dominate the long-term forecast,
with a persistent mid to upper-level ridge providing stable
conditions that inhibit convective showers. Although stability
will prevail, intermittent tropical moisture will affect the
region, mainly brought by a broad surface high pressure over the
Central Atlantic. According to Global Forecast System (GFS) model
guidance, this system will push an extensive area of tropical
moisture with precipitable water values (PWAT) exceeding 2 inches
under a southeasterly wind flow. Combined with local effects, this
moisture could result in active afternoons, particularly over the
northwestern sections of Puerto Rico. The strongest showers will
likely be accompanied by isolated to scattered thunderstorms, as
suggested by the Galvez-Davison Index (GDI), especially on
Wednesday and Thursday. From Friday through Sunday, interactions
between the surface high pressure and Tropical Storm Leslie will
cause winds to shift across the forecast area. This wind shift
will lead to a more east-to-southeast wind flow, increasing
surface pressure. With higher relative humidity between 850-500 MB
and cooler temperatures at 500 MB (around -6C), this pattern is
expected to bring showers and thunderstorms during the afternoons.

Global model guidance for 925 MB temperatures indicates a warm
period for much of the region due to the southeasterly wind
component and the presence of tropical moisture, which will
increase relative humidity at 850-700 MB. Consequently, warm
temperatures and high moisture levels could result in heat indices
near the threshold for heat advisories, posing health risks for
vulnerable communities, particularly in coastal and urban areas.
Please follow the health departments recommendations to prevent
heat-related illnesses.

&&

.AVIATION...
( 06Z TAF )
Favorable environmental conditions will support SHRA/TSRA
development across the region. The strongest activity, resulting
in SCT-BKN around FL020-030 and reduced visibilities (MVFR
conditions) and possible impacts on operations across most
terminals, is likely between 06/14-20Z. For TJSJ and USVI
terminals, VCSH/VCTS are expected throughout the forecast cycle.
Light to calm and variable winds will prevail during the next 24
hours, peaking at 5-10 knots around 06/16-20Z.

&&

.MARINE...
Hazardous marine conditions will persist through Monday due to
northeast swells affecting regional waters, generated by Hurricane
Kirk. A weak high-pressure system over the western Atlantic and
Hurricane Kirk in the Central Atlantic will support light to
gentle north-to-northeast winds across local waters. Localized
hazardous marine conditions are expected near thunderstorms,
especially in the afternoon near coastal areas of Puerto Rico.

&&

.BEACH FORECAST...
Today, another pulse of northeasterly swell will reach the
Atlantic coastal waters this morning, resulting in breaking waves
up to 12 feet. These conditions will generate very dangerous
marine conditions across the northern and northeastern exposed
beaches, where a High Surf Advisory remains in effect until 6 PM
tonight. Additionally, the high risk of rip currents continues
throughout the weekend for most northern beaches of Puerto Rico,
Culebra, and St. Croix. Beachgoers are urged to exercise extreme
caution, as the breaking waves pose significant dangers along the
northern and eastern coasts of Puerto Rico and nearby islands.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Monday night for PRZ001-002-
     005-008.

     High Surf Advisory until 6 PM AST this evening for PRZ001-002-
     005-008.

     High Rip Current Risk through this afternoon for PRZ012.

VI...High Rip Current Risk through late tonight for VIZ002.

AM...Small Craft Advisory until 6 AM AST Monday for AMZ711-712.

     Small Craft Advisory until 6 PM AST this evening for AMZ723.

&&

$$

SHORT TERM & AVIATION...ICP
LONG TERM & MARINE....LIS