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120
FXCA62 TJSJ 262131
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
531 PM AST Sun May 26 2024

.SYNOPSIS...
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The passage of a tropical wave, the associated
trailing moisture, and the proximity of a deep-layered upper-level
trough will yield wet and unstable conditions and, thus, an
increased flood risk during the next few days. A more seasonal
shower pattern will return by midweek, with possible localized
flooding impacts. Cooler temperatures are likely on Monday, but
elevated to critical excessive heat risks will return by Tuesday
onward. As of 2 PM AST today, tropical cyclone activity is not
anticipated during the next seven days.
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&&

.SHORT TERM...
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Tonight through Tuesday...

Daytime observations from satellite and radar have indicated
mainly sunny to partly cloudy skies, with localized afternoon
shower development across northwestern Puerto Rico and downwind
from Saint Croix. So far, the highest rainfall totals between one
and two inches were observed between Lares, San Sebastian, and
Moca, where a Flood Advisory was in effect. Daytime highs ranged
from around 80 degrees across higher elevations to around lower
90s across lower elevations and urban areas. Winds were generally
easterly, peaking at 10-15 mph.

A tropical wave`s arrival tonight into Monday is forecasted to
elevate precipitable water (PWAT) levels, potentially surpassing
two standard deviations above the typical thresholds for this time
of year, reaching as high as 2.5 inches. As the wave progresses
westward, moisture levels will likely decrease, but lingering
moisture will keep PWAT values elevated, ranging from 2.2 to 2.4
inches for the remainder of the period. Additionally, the
proximity of a deep-layered upper-level trough will maintain
marginally unstable conditions, conducive to deep-convective
development, aided by jet dynamics aloft and cool 500 mb
temperatures hovering around -6 to -7 degrees Celsius. East to
east-southeast winds at 5-15 mph will persist, resulting in a weak
steering flow.

Weather conditions will likely deteriorate tonight, particularly
impacting the U.S. Virgin Islands and eastern Puerto Rico,
including Vieques and Culebra. On Monday, the activity will likely
expand across most of the forecast area, with the highest activity
concentrated over the interior and west to northwestern Puerto
Rico during the afternoon. Based on recent GOES-16 Geostationary
Lightning Mapper (GLM) data and according to the Galvez-Davidson
Index and latest model guidance, there`s a potential for scattered
to widespread heavy rain-producing thunderstorms, raising
concerns for elevated flooding risks, including urban areas,
roads, small streams, and washes. Isolated flash floods and
mudslides in steep terrain areas are also possible. While
convective development may decrease on Tuesday, the flood risk
will continue, following a similar shower pattern and rain
distribution.

Increased cloud cover and potential rainfall could yield warmer
nighttime and lower daytime temperatures tonight into Monday
night. Still, warmer-than-normal conditions will persist. Lows in
coastal and urban areas will likely remain as high as the lower
80s, while higher elevations may see lows as low as the mid 60s.
In contrast, highs will range from the lower 90s in lower
elevations to the upper 70s across higher elevations. With ample
moisture present, heat indices may exceed 102 degrees Fahrenheit
in most coastal and urban areas, with some locations likely
experiencing periods surpassing 108 degrees or higher, especially
from Tuesday onward.

.LONG TERM...Wednesday through Sunday...

From previous discussion issued at 530 AM AST Sun May 26 2024

The long-term forecast remains on track. A mid-to upper-level
ridge will hold for most of the period over the eastern Caribbean,
resulting in drier air aloft and warmer 500 mb temperatures.
However, enough low-level moisture will persist through the
forecast period as the precipitable water content (PWAT) remains
around 2.00 inches or higher through Sunday. Given the expected
conditions, the combination of low-level moisture and local
diurnal effects will enhance shower and thunderstorm development,
especially in the afternoon hours over the interior and western PR.
As the surface high pressure builds over the central Atlantic,
the pressure gradient will decrease, and veering winds will focus
the shower activity over the northwestern quadrant and the San
Juan Metro area. Given the expected conditions, there is a limited
to moderate flood threat for each afternoon, and mainly over
the interior and northern half of Puerto Rico.

From Friday to Sunday, a similar weather pattern will persist as
the surface high pressure over the central Atlantic interacts
with another high building over the western Atlantic, and in
between a weak surface trough is expected to develop north of the
islands. In response, increasing moisture and southeasterly winds
will prevail across the region. Under this influence, warmer
conditions with heat indices between 102-110 degrees Fahrenheit
or higher are expected, resulting in excessive heat conditions for
the northern coastal and urban areas.
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&&

.AVIATION...
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(18z TAFs)

SHRA/VCTS possible at TJBQ thru 26/22z. There is some light HZ due
to Saharan dust, but VSBY will remain P6SM. Meanwhile, SHRA/iso TSRA
en route from the Leeward Islands due to an approaching tropical
wave may cause tempo MVFR conds at TISX/TIST/TJSJ from this evening
thru Monday morning. ESE winds will continue at 7-18 knots blo
FL050.
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&&

.MARINE...

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A moderate east to southeast wind flow will prevail across the region
over the next few days. A tropical wave will move across the Caribbean
waters, increasing shower and thunderstorm activity through at least
Monday night across the local waters.
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&&

.BEACH FORECAST...

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There is a low risk of rip currents today across all beaches of Puerto
Rico and the U.S. Virgin Islands. Isolated thunderstorms are
possible with the arrival of tropical wave during the afternoon.
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&&

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

$$

SHORT TERM...DSR/LIS
PACKAGE...ICP
AVIATION...DSR/LIS
PUBLIC...MRR/MMC