754
FXCA62 TJSJ 221429
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1029 AM AST Tue Apr 22 2025

.UPDATE...

The Rincon and San Juan buoys indicated seas between 6 and 7 feet
this morning. Therefore, the Atlantic coastal waters were added
to the Small Craft Advisory and extended through midnight tonight.
Also, the beaches of Rincon and Aguada were added to the High Rip
Current Risk through at least 6 PM this evening. Otherwise, the
weather forecast remains on track.


&&

.AVIATION...

(12Z TAFs)

Mainly VFR conds expected across all terminals thru the period.
However, BKN cigs btw FL030-FL050 are possible thru 22z at TJPS/TISX
due to diurnally induced -SHRA. The 22/12z TJSJ upper air sounding
indicated NNE winds btw 10-19 kt blo FL100.


&&

.PREV DISCUSSION... /issued 444 AM AST Tue Apr 22 2025/

SYNOPSIS...

Hazardous marine conditions and life-threatening rip currents
remain the main weather threats today, particularly across the
Atlantic waters and Mona Passage, as well as north-facing
beaches. Fair and stable weather will persist through midweek, but
a shift toward a wetter pattern is expected by the end of the
workweek into the weekend, bringing increased chances of showers
and thunderstorms. Warmer-than-normal temperatures, combined with
rising moisture and atmospheric instability, will elevate the
risk of flooding, especially from Friday through the weekend.

SHORT TERM...Today through Thursday...

Current satellite derived precipitable water (PWAT) values indicate
around 1.2 inches over the islands, below normal values for this
time of the year. Since midnight, radar estimated rainfall
accumulations show around 0.2 inches over some isolated sectors of
northern PR but overall show none to minimal accumulations over most
of the islands. Lows ranged from the upper 50s at higher elevations
of Puerto Rico and in the the upper 60s to low 70s over the rest of
the interior to southern half of PR. Lows were in the low to mid 70s
across the northern, NW and NE coastal areas of PR, as well as
across Vieques, Culebra and the U.S. Virgin Islands.

A drier air mass, currently northeast of the area, is forecast to
filter in later today, prompting PWAT values at or below 1 inch over
mainly the eastern region. Below normal PWAT values will persist
over most of the short term period. Steering flow will be north-
northeast to start the period but will gradually veer during the
short term period, eventually becoming east-southeasterly on
Thursday as a weak surface trough moves over the region. Breezy wind
risks will also decrease today as winds veer. It is not until late
Wednesday night and Thursday morning that a surge in moisture
(moisture from a past frontal boundary) from the east will increase
PWAT values over St. Croix to 1.5 inches, at normal values for this
time of the year. By Thursday night, this surge in moisture will
increase PWAT values over easternmost PR and the USVI to between 1.5
to 1.7 inches. In the upper levels the mid to upper trough that
affected us during the weekend is currently northeast of the region,
this feature will continue to move away from the region. Overall
generally stable conditions with a trade wind inversion and 500 mb
temperatures at -5 to -6 degrees Celsius will inhibit any deep
convection today and through most of the period. Most available
moisture will be confined to below 800 mb, with drier air above.

Local effects will still result in limited afternoon convective
showers, mainly over southern PR today, southwestern PR tomorrow and
northwestern PR on Thursday. Flooding risks are none to minimal.
Advective showers will continue to affect windward sectors at times
as patches of low-level moisture reach the islands in the form of
isolated to scattered showers. 925 mb temperatures are forecast to
be below normal today, gradually climbing to normal values during
the rest of the period.

LONG TERM...Friday through Monday...

The long-term forecast indicates a warm, wet, and marginally
unstable weather pattern. Easterly winds will continue to
transport tropical moisture into the region, while a mid-to-upper-
level trough and associated jet streak will enhance conditions
favorable for thunderstorm development. A weakened trade wind
inversion and typical temperatures at 500 mb will further support
instability. Additionally, temperatures are expected to exceed normal
thresholds, particularly in coastal and urban areas, leading to
slightly elevated perceived temperatures. These warmer-than-normal
conditions will further contribute to the overall instability and
potential for heavy rainfall.

Precipitable water values are expected to rise from around 1.5
inches on Friday to over 1.7 inches by Saturday evening,
indicating well-above-normal moisture levels. This, combined with
marginal instability, daytime heating, and above-normal
temperatures, will support the development of afternoon and early
evening showers and thunderstorms. The latest guidance suggests
that Monday will likely be the wettest day of the forecast period, with
moderate confidence in this trend, though the exact timing and
intensity of rainfall and convection remain uncertain and may
shift with future updates. As moisture increases, so does the risk
for moderate to heavy rainfall, which could lead to urban and
small stream flooding, especially in areas prone to frequent
afternoon convection. Additionally, localized flash flooding,
mudslides, and water surges along rivers and tributaries remain a
concern.

With forecast uncertainty still present, its important to
monitor future updates closely. Conditions are favorable for
another active weather weekend, with potential impacts that may
evolve as new guidance becomes available.

AVIATION...
(06Z TAFs)

VFR conditions will prevail throughout the forecast period. Winds
are will start from the NNE this morning, gradually veering to
become ENE. Speeds will be at 10-15 knots, with higher gusts and sea
breeze variation after 22/14Z, decreasing after 22/23Z. VCSH
possible for TJBQ/TJSJ/TISX/TIST during the period and for TJPS
during the early morning and afternoon.

MARINE...

A surface high over the western Atlantic and a low northeast of
the region will bring moderate northeast winds today, combined
with a weak northeasterly swell, maintaining hazardous marine
conditions through this evening. Small Craft Advisories are in
effect for the offshore and coastal Atlantic waters of Puerto Rico
and the U.S. Virgin Islands, as well as the Mona Passage. Winds
will become light and variable on Wednesday and Thursday as the
low weakens, with easterly winds returning by the end of the
workweek.

BEACH FORECAST...

Rough to hazardous seas will continue to pose a high risk of life-
threatening rip currents along the northern coasts of Puerto Rico
through late tonight. While the rip current risk will decrease
starting Wednesday, life-threatening rip currents remain possible
at most north-facing beaches. These currents are particularly
dangerous near groins, jetties, reefs, and piers. For more
information, refer to the Coastal Hazard Message (CWFSJU) and the
Surf Zone Forecast (SRFSJU).

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late tonight for PRZ001-002-005-
     008.

     High Rip Current Risk until 6 PM AST this evening for PRZ010.

VI...None.
AM...Small Craft Advisory until midnight AST tonight for AMZ711-712-
     741.

&&

$$

UPDATE...DSR