018
FXCA62 TJSJ 160834
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
434 AM AST Wed Apr 16 2025

.SYNOPSIS...

Increase instability should remain through the forecast period,
meaning that there is a risk of flooding and lightning over the
next few days. The risk of life- threatening rip currents will
continue moderate for most north- and east- facing beaches.
Increasing winds and a northerly swell will likely deteriorate
marine and surf zone conditions over the weekend.

&&

.SHORT TERM...Today through Friday....

Variable weather conditions prevailed along the local islands with
periods of isolated light to moderate showers across eastern
sections of Puerto Rico. Shower activity remained relatively
localized with minimal rainfall accumulations. Overnight minimum
temperatures were in the low 70s across the mountain areas and in
the upper to lower 80s across the coastal areas.

A subtropical jet will continue to extend eastward into the central
Atlantic, placing Puerto Rico and the U.S. Virgin Islands under an
increasingly unstable atmospheric pattern through Friday, with peak
influence expected on Thursday. This upper-level feature will
enhance divergence aloft, supporting convective development.
Although precipitable water (PWAT) values will generally follow a
seasonal pattern, a slight decrease to around 1.4 inches is expected
later today, followed by an increase to approximately 1.8 inches by
Thursday. Surface winds will remain from the east today, becoming
more southeasterly on Thursday due to an induced surface trough
developing north of Hispaniola. This will promote additional
moisture advection into the region, favoring passing showers,
particularly during the overnight and early morning hours.

GFS model guidance suggests that the induced surface trough will
move into the local area from late Thursday into Friday, weakening
the surface pressure gradient. As a result, lighter winds and
enhanced low-level convergence will increase the potential for
widespread and persistent shower activity. Isolated to scattered
thunderstorms are likely each afternoon over interior and western
Puerto Rico, and across the surrounding waters during the night,
supported by cold 500 mb temperatures ranging between -8C and -9C.
Meanwhile, 925 mb temperatures are expected to peak on Friday,
contributing to warmer conditions across the northern coastal areas
of Puerto Rico. Given the anticipated rainfall and slower storm
motion, there is an elevated risk of urban and small stream
flooding, particularly from Thursday afternoon through Friday.


.LONG TERM...Saturday through Wednesday...

The latest model guidance continues to indicate the approach of a
trough accompanied by a cold front, which is expected to move into
the region on Saturday. These features will enhance the potential
for shower and thunderstorm development, particularly across the
interior and southwestern portions of Puerto Rico during the
afternoon and early evening hours. Winds are also forecast to
increase and become breezy through at least Monday as a surface high-
pressure system strengthens over the western Atlantic.

Precipitable water models suggest values between 1.8 and 1.9 inches,
500 mb temperatures ranging from -8 to -9 degrees Celsius, and deep-
layer moisture extending from the surface up to around 500 mb
through the weekend. These conditions will support the potential for
urban and small stream flooding, rapid river rises, mudslides, and
frequent lightning associated with thunderstorm activity. As of now,
the weekend is the wettest part of the long-term period.

A gradual improvement in weather conditions is expected from late
Sunday through Tuesday as a drier and cooler air mass moves into the
region behind the front. This transition will bring lower humidity,
reduced rain chances, and a return to more stable and seasonable
conditions.

&&

.AVIATION...
(06Z TAFs)

VFR conditions will prevail for most of the TAF sites
during the period. Some TSRA and SHRA at the afternoon hours will
result in MVFR conditions across TJBQ and TJSJ at around 16/18Z with
a reduction in VIS and lower ceilings. Winds will pear at 16/15Z
from the east at 15 knots with gusty winds near the strongest
showers.

&&

.MARINE...

A high pressure system building over the western Atlantic will
promote moderate to locally fresh easterly winds, gradually veering
to become east-southeasterly today. This high will continue to push
the remnants of a frontal boundary across the region, leading to
increased showers and thunderstorms across local waters. Any
thunderstorm activity will likely generate locally hazardous marine
conditions, small craft should exercise caution. Increasing winds and
a weak northerly swell will likely deteriorate marine conditions
over the weekend.

&&

.BEACH FORECAST...

Increasing winds will promote a moderate risk for most north- and
east- facing beaches of Puerto Rico and St. Croix in the U.S.
Virgin Islands. These conditions are expected through Thursday,
improving by Friday. However, deteriorating marine conditions
will bring the risk of rip currents to high during the weekend. Be
aware that afternoon thunderstorms pose a threat of lightning,
mostly for the beaches in the west and southwestern coast of
Puerto Rico.

&&

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

$$

SHORT TERM/AVIATION...LIS
LONG TERM/MARINE/BEACH FORECAST...MMC