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511
FXCA62 TJSJ 250840
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
440 AM AST Sat May 25 2024

.SYNOPSIS...

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Expect showers and thunderstorms to develop across portions of
Puerto Rico, particularly the central, north, and west sections
each afternoon. Although a ridge aloft is expected to build during
the workweek, plenty of moisture will continue to prevail. As a
result, any prolonged period of showers and thunderstorms will
result in urban and small-stream flooding. The driest day in the
short term, Sunday. The aforementioned available moisture and the
prevailing east to east southeast winds will promote heat index
values that can affect individuals sensitive to heat today and
tomorrow.
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&&

.SHORT TERM...
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Today through Monday...

The broad band of cloudiness associated with the deep-layered trough
west of the area persisted overnight, with showers and isolated
thunderstorms occurring sporadically across the Caribbean waters and
Mona Passage. While some showers did penetrate inland across the
smaller islands and eastern Puerto Rico overnight, they resulted in
minimal rainfall accumulations. During the morning hours, we
anticipate a reduction in convective activity across the regional
waters, although there may still be some showers moving across
windward coastal areas at times. Additionally, there could be patchy
fog in the mountains just before sunrise, so it`s important to be
prepared for sudden changes in visibility and to drive slowly.

Today, low-level moisture is anticipated to persist above 2.0 inches
across most of the forecast area, surpassing typical levels for this
time of year. This elevated low-level moisture will combine with
higher-than-normal temperatures during the day, promoting sweltering
hot conditions, particularly across coastal and urban areas.
Consequently, a limited to elevated heat risk is expected across all
coastal and urban areas as heat index values are forecast to exceed
102 degrees and, in some areas, even reach Heat Advisory criteria.
As a result, a Heat Advisory has been issued for most municipalities
across northern and eastern Puerto Rico and USVI. Aloft, instability
will be marginal as we remain situated between the almost stationary
deep-layered trough to our west and the mid-level ridge to our east.
This atmospheric setup will likely foster another active afternoon,
with shower and thunderstorm activity initially developing across
the Cordillera Central and later clustering over northwestern
portions of the island.

Sunday is expected to be the most stable day, with the deep-layered
trough continuing to weaken while the mid-level ridge remains aloft.
Additionally, a tropical wave with its axis currently positioned at
56W is approaching the area. However, before it moves in, drier air
and subsidence ahead of the wave will settle in during the peak
hours of the day. While moisture content will decrease slightly, it
will remain below normal to near-normal levels. Despite this
decrease in moisture, subsidence aloft will exacerbate warmer
temperatures across the islands, with heat index values reaching
hazardous levels. This is likely to meet Heat Warning criteria,
particularly across northern coastal municipalities. Some convective
activity is expected during the afternoon hours across northwestern
portions of Puerto Rico but is anticipated to be short-lived.

The leading edge of the tropical wave will start to approach by
Sunday evening, with the forecast area experiencing well above-
normal moisture throughout the day on Monday. This will lead to
scattered to numerous showers over eastern Puerto Rico and the
smaller islands by early Monday, as well as deep convective activity
across the region, especially across western Puerto Rico in the
afternoon. Consequently, there could be an elevated risk of flooding
in the area once again.

.LONG TERM...Tuesday through Saturday...

Although the ridge aloft will hold through the forecast cycle, plenty
of moisture will continue to prevail without any erosion expected
in the midterm. In fact, the latest guidance suggested precipitable
water values exceeding 2.0 inches through at least midweek.
Moisture may decrease somewhat by the end of the workweek into the
weekend and remain trapped below 700 MB.

At lower levels, easterly winds will prevail on Tuesday, becoming
more southeast Wednesday and onwards as a surface high relocates
across the central Atlantic. Under this evolving pattern, trade
wind showers will continue to affect windward locations from time
to time, with locally induced afternoon showers expected across
western Puerto Rico each day. With GDI values greater than 25
expected much of the week, diurnal heating and sea breeze
convergence, isolated to scattered thunderstorms, are expected
each afternoon. Therefore, urban and small stream flooding is
expected each afternoon, mainly across central and western areas
of Puerto Rico and the San Juan Metro Area. In addition, available
moisture and southeast winds will promote heat index values that
can affect individuals sensitive to heat.

At this time, the bulk of African Dust will remain south of the
area for much of the week. Nevertheless, some may reach the area
by midweek.
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&&

.AVIATION...

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(06zTAFs)
Areas of SHRA and Isold TSRA over Caribbean waters could move
close to TJPS thru 25/14z. Aft 25/17z, SHRA and VCTS could affect
TJBQ/TJPS/TJSJ. These features will reduce VIS and CIG possibly
producing MVFR/IFR conds and mtn obscurations. Sfc winds ESE 09-13
kts with higher gusts near SHRA till 25/22Z.
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&&

.MARINE...

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An east-to-east southeast wind flow will continue to prevail
across the local waters over the next few days. A trough west of
the area will continue to enhance shower and thunderstorm
activity. Although weather conditions are expected to improve on
Sunday, shower and thunderstorm activity is expected on Monday
with the next tropical wave passage.
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&&

.BEACH FORECAST...

Seas will remain below 5 feet across much of the regional waters.
Therefore, there is a low risk of rip currents across the local
beaches of Puerto Rico and the U.S. Virgin Islands.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
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PR...Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for PRZ001-002-005-008-012-013.

VI...Heat Advisory from 10 AM this morning to 6 PM AST this evening
     for VIZ001-002.

AM...None.
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&&

$$

SHORT TERM...CVB
LONG TERM....OM
AVIATION...CVB