364
FXCA62 TJSJ 262109
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
509 PM 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. The likelihood of urban and small
stream flooding, mudslides and rapid river rises will remain high
along with a chance of isolated flash flooding as well. While
there is some uncertainty in the location and timing of this rain,
there is a high confidence of a wet and unsettled period for
Puerto Rico, the Virgin Islands and local waters. Please refer to
the latest Hydrologic Outlook (ESFSJU) for more information. A
long period of northeasterly swell arriving tonight will also
increase the risk of life-threatening rip currents along north-
facing beaches through early next week.

&&

.SHORT TERM...Tonight through Monday...

For tonight, the unstable pattern will continue due to the
proximity of the deep layer trough. Therefore, scattered to
numerous showers and thunderstorms will develop across the
regional waters and along the northern half of PR, gradually
diminishing overnight. However, another round of inclement
weather will spread from the surrounding waters into the local
islands overnight and tomorrow early morning (Sunday), especially
expected mainly across the US Virgin Islands and the eastern
third of Puerto Rico.

The presence of the deep layer trough and near to above-normal
moisture will extend the increasing instability, promoting
scattered to numerous showers and thunderstorms across portions
of Puerto Rico and the U.S. Virgin Islands through Monday into
next midweek. Although mid-level clouds may limit some convective
development on Sunday, available moisture and instability will
still favor afternoon thunderstorms, especially over the interior
and northwest. Dangerous lightning, strong winds, and flooding
rains remain the main threats, as well as landslides and river
flooding due to soil saturation.

By Monday, unstable conditions will persist, with another round of
thunderstorms expected, leading to additional flooding and hazardous
conditions. Stay tuned for further updates, and be prepared for
rapidly changing weather conditions.


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

./FROM PREV DISCUSSION...Issued at 519 AM AST Sat Apr 26 2025/

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...

(18Z TAFS)

Expect SHRA/TSRA across the local flying area throughout the fcst
period. Terminals could be impacted by this activity, producing
periods of MVFR/IFR. We had included TEMPOs for JBQ/JSJ thru 26/23z.
Conds will deteriorate once again after 27/02z due to the arrival of
squally wx from the east. Another round of strong convection is
expected tomorrow. Winds will prevail btwn 5-15 kt, mainly from the
E/ESE under sea breeze variations, becoming calm-light/var after
26/23z, returning from the E at around 10 kt after 27/13z tomorrow.

&&

.MARINE...

Light east to east-southeast winds will prevail through this
evening increasing to moderate tonight through early Sunday. A
deep layer trough will promote an unstable weather pattern across
the local waters. Thus, showers and thunderstorms are expected to
develop from now on, producing localized hazardous marine
conditions. This unstable and wet pattern will persist through at
least next midweek. A long-period northeasterly swell will 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 long period of northeasterly swell arriving tonight will
increase the risk of life-threatening rip currents along north-
facing beaches through early next week. There is a High Risk of
Rip Currents from this evening through 6 AM AST Monday for the
north-facing beaches of Puerto Rico, from Aguadilla to Fajardo,
and Culebra. A High Risk of Rip Currents means that life-
threatening rip currents are likely in the surf zone.

A moderate risk of rip currents is forecast tonight and tomorrow
for the beaches of Aguada, Rincon, northern Vieques, northern St.
Thomas, northern St. John and the eastern half of St. Croix. Low
risk elsewhere, however, life-threatening rip currents often
occur in the vicinity of groins, jetties, reefs, and piers.

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 through late Sunday night for PRZ001-002-
     005-008-012.

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

$$

SHORT TERM...CAM/ICP
LONG TERM....GRS/MRR