384
FXCA62 TJSJ 260919
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
519 AM AST Sat Apr 26 2025

.SYNOPSIS...

A wet and unstable weather pattern is expected to continue through
at least the end of the workweek, bringing periods of heavy showers
and thunderstorms. Winds are forecast to remain light today,
promoting the development of slow-moving showers and thunderstorms.
Given the expected conditions, the potential for urban and small-
stream flooding remains elevated today. Additionally, a high rip
current risk is likely to return this evening and persist through at
least late Sunday night due to the arrival of a long-period
northerly swell.

&&

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

During the past night and early morning hours, radar data showed
showers across parts of southeastern, eastern, and northeastern
Puerto Rico, as well as surrounding waters and nearby islands. The
heaviest rainfall initially fell in municipalities like Luquillo and
Fajardo, where about 2 to 3 inches were recorded, prompting urban
and small stream flood advisories. Later, showers tracked along the
coast, dropping up to 1 inch of rain, particularly in the Old San
Juan area. Since midnight, the highest radar estimated rainfall
totals have ranged from 0.30 to 1.00 inch. Overnight temperatures
ranged from the lower 60s in the mountains to the mid 70s across
urban and coastal areas of Puerto Rico and the U.S. Virgin Islands,
with light to moderate east northeast winds.

For the short-term period, model guidance continues to suggest a
persistently wet and unstable pattern. A mid to upper level trough
and an associated jet streak will enhance lift and divergence aloft,
creating favorable conditions for the development of showers and
thunderstorms, mainly today. According to the 00Z Tropical Analysis
and Forecast Branch (TAFB) surface analysis, the axis of the surface
trough is located just northwest of the region. Moisture content is
expected to increase from now onward, with model guidance suggesting
precipitable water (PWAT) values rising from around 1.5 inches
(average) to approximately 1.9 to 2.1 inches, which is above
climatological levels for this time of year, and will remain
elevated throughout the forecast period.

Additionally, mid-level relative humidity is expected to increase
significantly, peaking at about two standard deviations above
average. Furthermore, 500 mb temperatures will remain near minus 8
degrees Celsius, maintaining upper level instability and supporting
continued thunderstorm development. The overall pattern suggests
that showers will mainly affect local waters and the U.S. Virgin
Islands during the overnight hours, reaching the northern and
eastern sectors of Puerto Rico in the mornings. As the days
progress, showers and potentially thunderstorms will become more
concentrated across the interior and northwestern sectors of Puerto
Rico.

The primary concern today and likely over the next few days is the
increased risk of flooding, which is expected to remain elevated.
This means flooding in urban areas, roads, small streams, and washes
is likely, along with the potential for landslides in areas of steep
terrain. Isolated flash floods cannot be ruled out. Although the wet
pattern is expected to persist in the coming days, current model
trends indicate that the highest potential for moderate to heavy
rainfall will occur today, particularly during the afternoon hours.
Residents and visitors are encouraged to stay weather aware and
monitor official forecast updates.

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

For the rest of the week, a wet and unstable weather pattern will
persist. A series of shortwave troughs will move across the local
islands, maintaining favorable dynamical conditions aloft for the
development of showers and deep convective activity. As a result,
500 mb temperatures will remain between -8 and -9 degrees Celsius,
with relatively steep 850-700 mb and 700-500 mb lapse rates expected
through most of the long-term forecast period. Meanwhile, elevated
moisture levels will be present at all atmospheric levels, with
columnar moisture remaining well above normal for this time of year.

Most indicators, including precipitable water and the GFS Galvez-
Davidson Index (GDI), suggest that Wednesday will have the highest
chance of thunderstorms and heavier showers. This is when low-level
moisture convergence and steep lapse rates are expected to peak.
Wednesday also presents the greatest potential for widespread urban
and small stream flooding, as well as localized flash flooding,
particularly in areas that may have observed excessive rainfall in
previous days. However, similar risks will also exist on earlier
days, though with less areal coverage. It is important to note that
the timing of the most active day may shift as we approach the week,
depending on the simultaneous combination of favorable dynamics and
above-normal moisture levels. River streamflows will remain
elevated, especially across the interior of Puerto Rico. Therefore,
any additional rainfall could lead to rapid river rises in these
areas. Furthermore, saturated soils will enhance runoff, increasing
the risk of landslides in steep terrain.

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


&&

.AVIATION...
(06z) TAFS

Mainly VFR conds expected at all TAF sites this morning with
better chance of showers resulting in reduced vis across USVI this
morning. After 13Z-15Z VCSH will be likely across most TAF sites
and some may result in SHRA/VCTS reducing CIG/VIS and promoting
MVFR conds btwn 26/17Z-26/23Z. SHRA remains possible for TIST and
TISX after 26/23z. ESE light winds will prevail.

&&

.MARINE...

Light easterly winds will prevail through today increasing to
moderate tonight through early Sunday. Overall conditions should
remain favorable for small craft, but localized hazardous marine
conditions may develop near showers and thunderstorms through at
least midweek. A long-period northeasterly swell is expected to
spread across the Atlantic waters tonight into Sunday, which could
result in hazardous conditions for small craft, mostly across the
Atlantic waters.

&&

.BEACH FORECAST...

A moderate rip current risk will prevail across northern exposed
beaches for most of the day today. However, a long-period northerly
swell is expected to reach the northern coast and persist through
early Monday. Offshore buoy 41044 is already detecting this swell,
and according to these readings, the swell is expected to arrive by
this evening. As a result, a Rip Current Statement has been issued
for the northern beaches of Puerto Rico, from Aguadilla to Fajardo,
as well as the beaches of Culebra.

For more information, please refer to the latest Coastal Hazard
Message (CFWSJU) and Surf Zone Forecast (SRFSJU).

Residents and visitors in affected regions should heed warnings,
avoid swimming in these waters, and stay informed about updates from
local authorities.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk from this evening through late Sunday
     night for PRZ001-002-005-008-012.

VI...None.
AM...None.
&&

$$

SHORT TERM/AVIATION...YZR
LONG TERM/MARINE/BEACH...CVB