437
FXCA62 TJSJ 182020
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
420 PM AST Fri Apr 18 2025

.SYNOPSIS...
An induced surface trough and plenty of moisture will continue to
generate a wet and unstable weather pattern through this weekend.
Therefore, periods of strong showers with thunderstorms will
increase the potential for urban flooding and minor small- stream
flooding. The heaviest activity will be lightning, with the threat
of lightning present as well. Tranquil marine conditions will
persist and deteriorate for the upcoming workweek.

&&

.SHORT TERM...Tonight through Sunday...

During the morning hours, showers continued to move across the U.S.
Virgin Islands, the eastern half of Puerto Rico, and the municipal
island of Culebra. Radar estimates indicated rainfall amounts
between 1.0 and 1.5 inches in those areas. Later, a line of showers
accompanied by isolated thunderstorms developed over southern Puerto
Rico, prompting flood advisories for Mayagez, Hormigueros, and Cabo
Rojo. As of 3 PM AST, those areas received between 1 to 2.5 inches
of rainfall. Winds remained light, generally from the east to
northeast. Daytime temperatures stayed in the low to mid-80s across
coastal areas and ranged from the upper 70s to low 70s in the
mountainous regions.

The short-term forecast remains on track. Weather conditions will
continue to be influenced by a mid-to-upper-level trough approaching
from the west, which is expected to linger through the weekend. At
the surface, an induced trough is moving across the area, enhancing
low-level convergence across the northeastern Caribbean. According
to the latest model guidance, precipitable water values are forecast
to range from 1.8 to 2.10 inches through early Sundayabove the
climatological average for this time of year. This afternoon,
additional showers and thunderstorms are expected to develop across
Puerto Ricos interior and spread outward, fueled by daytime heating
and sea breeze convergence. The risk of flooding remains elevated
today and into Saturday. Expect ponding of water on roadways and in
poorly drained areas, with the potential for more significant urban
and small stream flooding. Saturday is expected to be another
variable day, with cloudy skies and periods of scattered to numerous
showers. Isolated to scattered thunderstorms are also likely to
continue, particularly over the interior and western half of Puerto
Rico, and across surrounding waters overnight.

By Sunday, expect showers mainly over the surrounding waters, the
U.S. Virgin Islands, and eastern Puerto Rico, with a limited flood
risk early in the day. Afternoon convection may develop, supported
by available moisture and local effects. A gradual improvement in
rainfall conditions is expected later in the day.

.LONG TERM...Monday through Friday...

An unsettled and moist weather pattern will prevail at the
beginning of the workweek. On Monday, upper-air analysis indicates
a pronounced trough evolving into a cut-off low just north of the
forecast area. This synoptic feature will induce significant
instability, as reflected in model-derived 500 MB temperatures
dropping between -8C and -10C, which is supported by the latest
TJSJ RAOB and GFS analysis. This colder air aloft will promote
robust convective development and increase the potential for
thunderstorm activity across the region, particularly over interior
and western Puerto Rico during the afternoon hours. At the
surface, a strengthening Atlantic high will enhance the local
pressure gradient, with 925 MB wind speeds peaking between 15 to
25 knots and occasional gusts above that range, according to GFS
guidance. Winds will shift from the northeast, resulting in breezy
to windy conditions and advecting fast-moving showers across the
northern and eastern municipalities of Puerto Rico, as well as the
U.S. Virgin Islands. However, given the enhanced steering flow,
rainfall totals are expected to be limited from late Sunday into
Monday despite periods of active weather. Additionally, both
700500 MB and 850700 MB relative humidity values exceed the 75th
percentile, suggesting a deeply saturated column and increased
potential for vertically developed convection during this period.

From Tuesday through Friday, weather conditions are expected to
gradually improve. The upper-level divergence zone tied to the
cut-off low will migrate eastward, allowing for the development of
a mid-level ridge that will introduce drier and more stable air
masses over the region. The global model shows a notable drop in
PWAT values, reaching below 1.0 inch by Sunday, indicative of a
more suppressed convective environment. Despite this drying trend,
residual cloudiness and embedded moisture from the proximity of
the lingering surface low may still bring passing showers,
especially across eastern Puerto Rico and the U.S. Virgin Islands
under continued northeasterly trade winds.


&&

.AVIATION...
(18Z TAFs)

Mostly VFR conds expected. However, SHRA/TSRA could result in
reduced VIS and CIGs at TJPS through 18/23z. Mountain obscuration is
expected for the western Cordillera Central. Additional activity is
expected aft 18/23z mostly across the waters occasionally reaching
TJSJ/TIST/TISX TAF sites. This will promote short periods of MVFR,
or even brief IFR, conds. Winds will be from the NE at less than 10
knots.


&&

.MARINE...
An induced surface trough and a broad surface high pressure over
the central Atlantic will promote light southeasterly winds across
the region through tomorrow. By Sunday, increasing surface winds
due to another surface high pressure over the western Atlantic
will result in wind-driven seas between 6 to 7 feet, enhancing
choppy marine conditions along the area. Thunderstorm activity
will be present across the local waters today into Sunday due to
strong showers and thunderstorms, resulting in localized hazardous
marine conditions.

&&

.BEACH FORECAST...
A low risk of rip currents is expected across all coastal areas
through Saturday. However, the risk will increase to moderate on
Sunday and become high by Sunday afternoon and into the upcoming
workweek due to strengthening winds. Moderate easterly winds will
raise the rip current threat along most north- and east-facing
beaches of Puerto Rico and St. Croix. Additionally, deteriorating
marine conditions over the weekend will further elevate the
hazard. Beachgoers should also remain vigilant for afternoon
thunderstorms, especially along the western and southwestern
coasts of Puerto Rico, where dangerous lightning may occur.

&&

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

$$

SHORT TERM...EM
LONG TERM....GRS
AVIATION...EM