821
FXCA62 TJSJ 100900
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
500 AM AST Fri Jan 10 2025

.SYNOPSIS...

Today`s main hazards are pulses of northerly swells, strengthening
tonight into Saturday, creating hazardous seas for small craft,
dangerous swimming conditions, and life-threatening rip currents
along north-facing beaches. Light to moderate rains from a passing
frontal boundary will affect the region today. However, stable
conditions with mostly clear skies are expected for the weekend
and likely into early next week, with a wetter pattern starting
Tuesday and a cooling trend into next week. Stay informed on
marine conditions and hazards through our social media channels
and the Experimental Graphical Hazard Weather Outlook:
https://www.weather.gov/erh/ghwo?wfo=sju.


&&

.SHORT TERM...Today through Sunday...

During the night, mostly cloudy skies with variable conditions were
observed due to a front currently moving through our region. Light
to moderate showers persisted across Puerto Rico, moving quickly
without leaving significant accumulations. Based on radar estimates,
precipitation totals are close to a quarter of an inch since
midnight. The prevailing winds came from the north, as anticipated,
at speeds of 8 to 15 mph. Temperatures remained in the 70s in
coastal and urban areas, while the mountainous and rural regions
experienced cooler conditions in the 60s.

As the front finishes crossing this morning and a surface high
pressure continues to establish itself over the Bahamas, we will
experience breezy northerly winds and some additional passing
showers. No significant accumulations are expected. Tonight into
Saturday, the winds will shift from the northeast as the high
pressure moves eastward, getting closer to the northern part of our
region. Great news, the weekend will be characterized by mostly
stable weather conditions and pleasant temperatures, as moisture
levels decrease significantly after the frontal passage, and a cool
advective pattern sets in. Precipitable water values are expected to
drop and remain around 0.85 to 1.0 inch, which is well below normal
for this time of year. Additionally, a mid-level ridge will dominate
and enhance atmospheric stability, with a strong trade wind cap
inversion and a deep layer of dry air persisting over our region. As
a result, expect mostly sunny skies, which will provide ideal
conditions for outdoor activities.

Temperatures will remain pleasant across the islands throughout the
forecast period. The 925 mb temperatures expected to dip to below-
normal values. The mountainous areas of Puerto Rico are forecast to
see minimum temperatures ranging from the mid to upper 50s. Coastal
areas in Puerto Rico and nearby islands could experience
temperatures in the low 70s or upper 60s tonight and over the coming
nights. Daytime highs are expected to reach the 80s in coastal and
lower elevations, while the mountains will experience highs in the
70s.

.LONG TERM...Monday through Thursday...

Model guidance now suggests transitioning to a wetter pattern on
Tuesday as moisture from a frontal boundary arrives earlier than
anticipated, increasing PWAT values to near-normal levels (1.21.5
inches). A brief weakening of the mid-level ridge will allow for
an increase in showers on Tuesday. Even with hostile conditions
aloft, moderate to fresh northeasterly winds will maintain
typical moisture levels across the region on Wednesday and
Thursday, with trade wind showers producing localized rainfall
accumulations, especially across windward areas.

By Thursday night, the GFS model suggests the proximity of a
second frontal boundary, driven by stronger northeasterly winds
associated with a broader and more robust surface high pressure
system. In contrast, the ECMWF does not support this scenario. If
the second boundary materializes, PWAT values could reach well-
above-normal thresholds, nearing 1.7 inches. A significant
weakening of the trade wind cap inversion would accompany this
scenario, leading to the potential for heavier rainfall and a
heightened risk of flooding by the end of the forecast period.

A cooling trend is still expected across the region, as model-
estimated 925 mb temperatures suggested, with the most noticeable
cooling occurring early Monday and Wednesday.

Confidence in this weather scenario is high early in the forecast
period. Still, it decreases later due to discrepancies in model
guidance, especially regarding the timing and impact of the
second frontal boundary. Stay tuned for updates as the forecast
continues to evolve.


&&

.AVIATION...
(06Z TAFs)

VFR conditions are expected to prevail across all terminals through
the forecast period with just brief periods of -RA, particularly
across PR northern TAF sites during the morning and late tonight,
dry conditions are expected to prevail afterwards. Breezy northerly
winds are expected today at around 10 to 15 knots with higher
periods of gusts. Winds veering NE tonight into Saturday and
becoming somewhat lighter.


&&

.MARINE...

Following the passage of the frontal boundary, winds will
strengthen and turn from the north today. By Saturday, winds will
veer to the northeast before gradually weakening and turning more
easterly on Sunday. Moderate to locally fresh winds will return
early next week. A long-period northerly swell continues to spread
across the Atlantic waters, with a stronger pulse expected to
arrive tonight into Saturday. Seas building up to 10 feet will
maintain hazardous conditions for small craft.


&&

.BEACH FORECAST...

Hazardous and rough seas fueled by northerly swell pulses will
create life-threatening rip current conditions along the northern
beaches of the northern U.S. Virgin Islands, Culebra, and Puerto
Rico today. Large breaking waves, reaching up to 11 feet, will
result in dangerous swimming conditions and may cause localized
beach erosion. Consequently, High Rip Current Risk and High Surf
Advisories are in effect. These hazardous conditions are expected
to persist and expand to additional surf zone areas through the
weekend. For further details, consult the Coastal Hazard Message
(CWFSJU) and the Surf Zone Forecast (SRFSJU).


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Sunday afternoon for PRZ001-002-
     005-008-010-012.

     High Surf Advisory until 6 AM AST Sunday for PRZ001-002-005-008-
     010.

     High Surf Advisory from 6 AM Saturday to 6 AM AST Sunday for
     PRZ012.

VI...High Rip Current Risk through Sunday afternoon for VIZ001.

     High Surf Advisory from 6 AM Saturday to 6 AM AST Sunday for
     VIZ001.

AM...Small Craft Advisory until noon AST Sunday for AMZ711.

     Small Craft Advisory until noon AST Sunday for AMZ712-716-741-
     742.

&&

$$

SHORT TERM...YZR
LONG TERM....ICP
AVIATION...YZR
PUBLIC/IDSS...ICP/YZR