414
FXCA62 TJSJ 092035
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
435 PM AST Sun Mar 9 2025

.SYNOPSIS...

Current afternoon showers over the interior, west, northwest and
south and southwest Puerto Rico will gradually dissipate during
the evening. Generally stable weather will persist through at
least midweek with the normal diurnal pattern under E to ESE
winds. However, a wet and unstable weather pattern is forecast by
the end of the workweek into the weekend. A long- period northerly
swell will spread across the Atlantic waters and local passages
next weekend, resulting in hazardous marine and coastal
conditions.

&&

.SHORT TERM...Tonight through Tuesday...

Stable weather conditions with little or no rain prevailed across
PR and the USVI during the morning hours. However, local effects
with the arrival of a low-level moisture patch resulted in some
showers across the windward locations, followed by afternoon
convection across the interior and western portions of PR and
downwind from the Virgin Islands. Maximum temperatures were in the
upper 80s or even low 90s at coastal and urban areas, with heat
index values in the upper 90s.

The afternoon convection affecting interior, west, northwest and
south and southwest Puerto Rico will gradually dissipate and/or
move offshore during the evening, leaving mostly clear skies this
evening and overnight. Although we expect calm and stable
weather, the easterlies will bring a pocket of moisture with
clouds and a few showers across the windward locations overnight.

A stable weather pattern will persist through Tuesday evening,
driven by a mid to upper-level ridge. Even with a dry and stable
air mass, the prevailing winds will continue bring pockets of
moisture, leading to clouds and occasional showers in the
windward locations until Tuesday. Expect afternoon convection to
develop, mainly due to sea breeze and local effects, over the
mountains and western regions of Puerto Rico, as well as downwind
from the Virgin Islands. The ridge aloft will weaken early Tuesday
night as an upper-level trough approaches the Northeast Caribbean
from the west.

.LONG TERM...Wednesday through Sunday...

./FROM PREV DISCUSSION...Issued at 510 AM AST Sun Mar 9 2025/

The long-term forecast remains on track as model guidance continues
to suggest an unstable weather pattern developing by the end of the
workweek into the weekend. Wednesday will start fairly calm, with
well-below-normal moisture, as ridging aloft dominates the forecast
area. However, by Thursday, the weather pattern will begin to change
as the mid-level ridge erodes and is pushed eastward by a series of
back-to-back mid-to-upper-level troughs crossing the northern
Caribbean.

This pattern will promote cold air advection at 500 mb, dropping
temperatures to between -7C and -9C. Additionally, 250 mb heights
and 1000-500 mb thickness values will drop to near-normal levels,
accompanied by strong cyclonic vorticity advection in the vicinity.
This instability will combine with above-normal moisture content as
a frontal boundary, associated with upper-level dynamics, stalls
over the islands, leading to low-level convergence. Considering the
09/00Z model run cycle, the median quartile of precipitable water
values in the region, based on ensemble data from the ECMWF, GFS,
and CMC global models, ranges between 1.5 and 1.9 inches by
Friday. This supports high forecast confidence in near to well-
above-normal moisture content across the region.

Therefore, expect a gradual increase in shower and thunderstorm
activity across the northeastern Caribbean. On Thursday, the
weather pattern will follow a more seasonal trend, with showers
developing over windward coastal areas of the islands in the
morning and later across interior and southwestern portions of
Puerto Rico in the afternoon, as diurnal heating and sea breeze
convergence combine with instability aloft.

The areal coverage of showers and thunderstorms will begin to
increase from Thursday night through the weekend as instability and
moisture content continue to rise. A limited to elevated flooding
threat will prevail, leading to urban and small stream flooding, as
well as potential landslides and rockfalls in vulnerable and steep
terrain.

&&

.AVIATION...

(18Z TAFs)

VFR conditions are expected at all TAF sites. However, SHRA/-SHRA
will develop in western and central Puerto Rico through 09/23Z,
mainly in the VCTY of JPS/JBQ. Mountain obscuration expected along
the Cordillera Central. VCSH will occasionally affect ISX, IST, and
JSJ. Winds will be mainly from the E at 5-15 knots with higher gusts
and sea breezes, although expect calm to light and variable winds
overnight (between 09/23-1013z). SHRA/+SHRA will likely form in the
interior and western Puerto Rico tomorrow afternoon.

&&

.MARINE...

A surface high pressure will build and migrate toward the central
Atlantic early this week, while a surface low and associated
frontal boundary moves across the western Atlantic. This will
shift the winds from the east-southeast through Thursday. By the
end of the week, the front shall increase the potential for
showers across the local waters by Thursday and Friday. Another
long-period northerly swell will spread across the Atlantic waters
and local passages next weekend, resulting in hazardous seas once
again.

&&

.BEACH FORECAST...

A High Risk of Rip Currents (life-threatening rip currents are
likely in the surf zone) was extended through 6 PM AST this
evening for the north- oriented beaches of Puerto Rico, from
Rincon to Fajardo, Culebra and the northern USVI. This is due to a
fading long-period north- northwesterly swell. For tonight and
tomorrow, Monday, the risk of rip currents is forecast to be
moderate (life-threatening rip currents are possible in the surf
zone) for the aforementioned beaches and for St. Croix as the
swell continues to dissipate. For more information, please refer
to the latest Coastal Hazard Message (CFWSJU) and Surf Zone
Forecast (SRFSJU).

If you become caught in a rip current, yell for help. Remain
calm, do not exhaust yourself and stay afloat while waiting for
help. If you have to swim out of a rip current, swim parallel to
shore and back toward the beach when possible. Do not attempt to
swim directly against a rip current as you will tire quickly.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 PM AST this evening for PRZ001-002-
     005-008-010-012.

VI...High Rip Current Risk until 6 PM AST this evening for VIZ001.

AM...None.
&&

$$

SHORT TERM...CAM/MRR
LONG TERM....CVB
AVIATION.....CAM/MNG
MARINE.......ERG
BEACH........MRR