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000
FXCA62 TJSJ 210850
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
450 AM AST Sun Apr 21 2024

.SYNOPSIS...

Suspended Saharan dust will be present early this week, promoting
hazy skies. However, this will be short-lived as a moist and
unsettled weather pattern will prevail throughout the week.

&&

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

Persistent shower activity has been observed across Saint Croix and
the northern coast of Puerto Rico all night long. However, rainfall
rates were not that strong, and accumulations generally remained at
around 1.5 inches for Vega Baja and below one inch elsewhere. The
most recent satellite-derived precipitable water imagery show plenty
of moisture in the vicinity of Saint Croix, but drier air to the
northwest of Puerto Rico. This moisture field is associated with a
surface trough that will gradually move over the region, once again
increasing the frequency of showers. The activity should increase in
the afternoon or evening hours, first spreading into St. Thomas and
St. John, then Vieques, Culebra and eastern Puerto Rico. Also,
showers are expected to develop due to local effects over the
eastern interior and another cluster in the southwest.

The unsettled weather pattern persists on Monday as the moisture
field advances into the region. At the same time, a surface high
pressure migrating toward the northeast just north of the region
will make the winds shift from the southeast at the surface, and
from the south from 900 to 600 mb. With all that moisture and under
a southern wind flow, heat indices will increase in areas that are
not under the cloud shield of the trough (likely coastal areas of
Puerto Rico and Saint Croix). Aside from the heat, the main focus
will be the rain again, since additional showers and thunderstorms
are anticipated. The light wind flow will make the activity to
develop along the Cordillera Central, then spreading mostly toward
the north, and possibly moving into the San Juan metro area. Soils
in the interior municipalities are saturated and rivers are running
high , so another round of at least urban flooding is likely.

Saharan dust will follow this rain event on Tuesday. So far, the
aerosol models show that the denser cloud of dust will remain over
the Caribbean waters, and reaching St. Croix, where hazy skies and
periods of reduced visibilities are possible. Elsewhere, grayish
skies will be possible at times, but with less concentration of dust
in general.

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

Traces of suspended Saharan dust particulates will still be
present on Wednesday, leading to some hazy skies, especially
during the morning hours. For the rest of the week, a wet and
unstable weather pattern will persist. A series of short-wave
troughs will move over the local islands throughout the week,
creating favorable dynamical conditions aloft for shower and deep
convective development. This will maintain 500 mb temperatures
between -6 and -7 degrees Celsius and steep lapse rates for most
of the long-term forecast. Meanwhile, elevated moisture levels
will be present at all levels, with columnar moisture well above
normal for this time of year.

Most indicators, including precipitable water and the GDI (Galvez-
Davidson Index), suggest that Friday will have the highest chance
of thunderstorms and heavier showers. This is when low-level
moisture convergence and steep lapse rates will be at their
maximum. Friday also poses the greatest potential for widespread
urban and small stream flooding, as well as localized flash
flooding, although similar risks will also exist on previous days
with less areal coverage. As mentioned earlier, river streamflows
will remain elevated, especially those across interior Puerto
Rico. Therefore, any additional rainfall could lead to quick river
rises in these areas. Additionally, saturated soils will enhance
runoff, increasing the risk of landslides in steep terrain.

Weather conditions will gradually improve by the weekend. However,
lingering moisture, coupled with diurnal heating and local effects,
will continue to foster afternoon convection across interior
portions of Puerto Rico and downwind of the smaller islands.


&&

.AVIATION...
(06z) TAFS

SHRA will continue in the vicinity of TISX, with period of reduced
VIS and low ceilings. Additional SHRA will develop after 17Z for the
Cordillera Central, resulting in mountain obscuration. After 21Z,
the frequency of SHRA will also increase around TJSJ and TIST. Winds
will be from the ENE at 9-14 kts.


&&

.MARINE...

Gentle to moderate northeasterly wind flow will continue to prevail
across the local waters through at least early this week, becoming
light to gentle from the east southeast on Tuesday. A small
northeasterly swell will move across the Atlantic waters through the
weekend. However, seas will remain less than 5 feet for the most
part.

&&

.BEACH FORECAST...

Moderate rip current risk will persist today, indicating the
possibility of life-threatening rip currents in the surf zone,
especially along north-facing beaches of the local islands. 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/AVIATION...ERG
LONG TERM/MARINE/BEACH FORECAST....CVB