446
FXCA62 TJSJ 232030
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
430 PM AST Thu May 23 2024

.SYNOPSIS...

Fair weather conditions will continue to prevail across the
region tonight. However, there is the potential for an increase in
shower activity mainly across the eastern half of Puerto Rico due
to an approaching perturbation located over the Leeward islands.
Wet weather conditions are expected to persist throughout next
week as a tropical wave approaches from the southeast by Monday.
In addition, warm temperatures are expected, once again, by this
weekend with heat indices in the upper 100s.

&&

.SHORT TERM...Tonight through Saturday...

After a very wet weather period this morning, drier air filtered
into the area limiting the shower formation across the region.
Therefore, mainly fair weather conditions were observed during the
most part of the day. Warm conditions were also observed due to the
combination of good moisture and southeasterly winds. Overall,
daytime temperatures were observed in the upper 80s to low 90s along
the coastal and urban areas of Puerto Rico and the U.S. Virgin
Islands. In addition, heat indices were seen between 110 to 115
degrees Fahrenheit during the early afternoon hours mainly across
the northwestern quadrant of Puerto Rico.

The latest model guidance continue to suggest an increase in shower
activity across the region due to the arrival of a perturbation
located over the Leeward Islands. The eastern portions of Puerto
Rico and the U.S. Virgin Islands are expected to receive the most
activity. For the rest of the area, passing showers will continue
through the night into Friday. Variable weather conditions are
expected by Friday into Saturday as moisture continues to move from
the Caribbean into the local area. Expect active afternoons with the
potential for moderate to locally heavy showers and isolated
thunderstorms across the northwestern quadrant of Puerto Rico.
Therefore, the potential for urban and small stream flooding, rapid
river rises, and mudslides remains elevated. This wet and unstable
weather pattern is forecast to persist until the upcoming week.

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

PREV DISCUSSION.../issued 330 AM AST Thu May 23 2024/

Model guidance indicates that the wet period will persist through
the long-term forecast. Sunday remains very moist, with precipitable
water values above 2.0 inches and ample instability aloft. A mid-
level ridge just east of the forecast area is expected to move over
us by late Sunday into early Monday, coinciding with a slight
reduction in low-level moisture, which will cause precipitable water
values to drop to almost 1.8 inches. However, this "reprieve" won`t
last long as a tropical wave moves from the southeast over the
islands by late Monday, causing moisture content to surge above 2.0
inches once again, persisting through the long-term forecast.
Instability aloft will be marginal, as weak ridging will alternate
with a series of weak short-wave troughs throughout the week.
Therefore, the combination of above-normal moisture and marginal
instability will maintain the flood risk across the area.

The National Blend of Models indicates approximately a 3-in-5 (60%)
chance for thunderstorm activity and a 1-in-5 (20%) chance for 24-
hour rainfall accumulations to exceed an inch each day of the long-
term forecast, particularly across western/northwestern Puerto Rico
under an east-southeast steering flow. The Galvez-Davison Index also
suggests a similar scenario, with a high potential for observing
isolated to scattered thunderstorms capable of producing heavy
rainfall. With saturated soils and elevated streamflows from
previous rainfall, the flooding risk will remain elevated.
Therefore, we could expect urban and small stream flooding, as well
as flash flooding and landslides in areas of steep terrain.

Another hot episode is expected on Sunday and Monday, accompanied by
suspended Saharan dust particulates by early next week. Therefore,
in areas with limited showers, hot and hazy conditions will prevail.
The combination of high moisture content and high temperatures will
produce hazardous heat index values, particularly in coastal and
urban areas, possibly reaching Heat Advisory criteria once again.

&&

.AVIATION...

(18z TAFS)

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. However, SHRA en route from the
Windward Islands could reach the USVI terminals during the overnight
hours. Late afternoon convection is still possible in and around
TJBQ, which may cause tempo MVFR conds. Low-level winds will
continue from the ESE at 10-20 kt.

&&

.MARINE...

The interaction of an induced surface trough and a surface high
pressure to the northeast will promote moderate to fresh east-
southeast winds. In addition, the proximity of a deep layer trough
close to the Dominican Republic will continue to enhance thunderstorm
development with the potential for squally conditions across the
regional waters and local passages through at least Friday. Winds are
forecast to diminish once again by Friday night.

&&

.BEACH FORECAST...

Low to moderate rip current risk will persist throughout the period.
In the presence of moderate rip current risk, there is still the
possibility of life-threatening rip currents in the surf zone.
Please remember to heed the advice of the local beach patrol and
flag warning systems.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...RAM
LONG TERM....GRS
AVIATION...DSR
PUBLIC DESK...LIS