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000
FXCA62 TJSJ 200851
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
451 AM AST Sat Apr 20 2024

.SYNOPSIS...

Some improvement may be observed today. However, a wet and unstable
weather pattern is expected to return by the end of the weekend and
persist through the workweek.

&&

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

Runoffs from previous rainfall caused Rio Grande de Manati in
Manati, and Rio de la Plata in Dorado to reach flood stage once
again. However, waters receded around 4 AM AST. Under a northeast
flow, showers moved across north-central Puerto Rico and over Saint
Croix, with rainfall accumulations up to 1 inch in Vega Baja. For
western Puerto Rico, skies cleared out as a drier air mass moved
over that region.

The most recent satellite-derived precipitable water value product
shows a moisture gradient over the region. First, below normal
moisture values are observed along western Puerto Rico and the
Atlantic waters, but plenty of moisture lingers in the Caribbean in
the vicinity of Saint Croix. In fact, the high resolution models
show that showers will continue to stream across this region, so
ponding of water on roadways is possible today. Shower activity will
be possible, but not as much across Saint Thomas, Vieques and
Culebra.

For Puerto Rico, the wind flow at the mid and upper levels is now
zonal, as a jet stream passes just north of the region, and the
upper level trough departs toward the east. Most of the moisture
should remain east of the region today, but there is enough to
trigger showers in the afternoon along the interior and southwestern
Puerto Rico. The showers are not expected to be as widespread as the
previous days, but since the soils are so saturated and rivers are
running high, flooding and mudslides will remain possible in these
areas.

The slight improvement in the weather conditions will not last long.
An area of moisture associated with the trough that affected the
area earlier this week and another one approaching from the Lesser
Antilles will combine to increase moisture above climatological
levels on Sunday and Monday. First, showers will move from the east,
affecting the Virgin Islands and eastern Puerto Rico during the day,
and then additional convection will kick in in the afternoon across
the interior and western Puerto Rico. Again, the likelihood of
flooding, rapid river rises, and mudslides will be elevated.

Also on Monday, temperatures at 925 mb will be two standard deviation
above normal, meaning that it should be hot too. It is possible that
the cloud coverage may prevent temperatures for rising too much, but
coastal areas experiencing enough sunshine could see heat indices
above the 100 degree mark.

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

A wet and unstable weather pattern is expected to return in the long-
term forecast. A surface high pressure across the central Atlantic
and a surface low over the western Atlantic will induce east-
southeast winds across the CWA. Consequently, this wind pattern will
promote warmer temperatures over the islands and pull tropical
moisture into the area. Precipitable water content is expected to
remain within the range of 1.9 to 2.2 inches. These values exceed
the 90th and 99th percentiles of the climatological normal,
indicating well above-normal moisture levels for this time of year.

In terms of instability, an upper-level trough is expected to deepen
throughout the week, inducing an inverted surface trough west of
Puerto Rico. These features will gradually progress eastward during
the week, sustaining unstable weather conditions capable of
generating deep convective activity. The likelihood of convective
activity will be highest during afternoon hours as high moisture
content and instability combine with surface heating and local
effects.

Soils are expected to remain somewhat saturated by Tuesday due to
rains from previous days, with rivers still experiencing elevated
streamflows. Consequently, the flood risk will remain elevated to
significant through at least Friday, particularly during afternoon
hours. Quick river rises, river flooding, and landslides are also
possible, primarily across Puerto Rico. In the US Virgin Islands,
minor flooding with brief periods of excessive runoff is possible
each day of the week.


&&

.AVIATION...
(06z) TAFS

Mainly VFR conditions expected. SHRA will stream across the Atlantic
waters and passages, occasionally reaching TSJU and the USVI
terminals. After 17Z, additional activity is expected in the
vicinity of TJPS. All this activity may result in brief periods of
reduced VIS and low ceilings. Mountain obscuration is expected for
the western Cordillera Central. Winds are from the NE at 5-12 kts.

&&

.MARINE...

Gentle to moderate northeasterly wind flow will continue to prevail
across the local waters through at least early the next workweek,
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...

Although there is no high rip current risk statement, life-
threatening rip currents remain possible 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