National Weather Service Today's Weather Impact Levels

244
FXCA62 TJSJ 112048
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
448 PM AST Tue Mar 11 2025

.SYNOPSIS...

Fair weather conditions and warmer temperatures will persist
through tomorrow under southeasterly winds. A wet and unstable
weather pattern is forecast by the end of the workweek into the
weekend due to the arrival of upper-level troughs and a frontal
boundary. A long-period northwesterly swell will spread across
the Atlantic waters and local passages around Thursday, resulting
in hazardous seas and life-threatening rip currents. Another swell
is anticipated during the weekend into early next week maintaining
marine and coastal conditions deteriorated.

&&

.SHORT TERM...Tonight through Thursday...

Stable weather conditions under clear to partly cloudy skies were
seen across Puerto Rico (PR) and the US Virgin Islands (USVI)
today. Some showers moved across the windward areas due to a
moisture patch during the morning hours. Maximum temperatures
reached the upper 80s to low 90s, with heat index values in the
upper 90s. Occasional life-threatening rip currents developed
along the north-facing beaches of northern USVI and Puerto Rico.

Calm weather conditions will dominate overnight and Wednesday
morning due to a mid-to-upper-level ridge and a drier air mass.
However, an approaching frontal boundary will increase moisture as
early as Wednesday afternoon into Thursday. Meanwhile, a mid to
upper-level trough aloft will also increase atmospheric instability.
Based on the expected weather pattern, we can expect the rain
activity to slowly increase by passing showers across the windward
sections during the early morning and afternoon, with afternoon
convection developing across the interior and northwest quadrant
due to the east-southeast wind flow.

For Thursday, winds will become variable, gradually shifting from
the south-southwest before transitioning to a north-northeasterly
flow as the pre-frontal trough approaches the islands, pooling
additional moisture. The potential for thunderstorms and flooding
rains will increase due to these factors and the arrival of the
trough aloft, too. Therefore, an unstable and wet pattern will
likely evolve by the second part of this workweek due to the factors
mentioned above, among others.

Although minimum temperatures will drop into the low 70s along the
coast and in the low 60s or even lower in the mountains overnight,
maximum temperatures will again climb into the upper 80s or low 90s
on Wednesday due to the weak east-southeast wind flow.

The arrival of a long-period northwesterly swell will result in life-
threatening rip currents from the beaches near Rincon northward to
Fajardo and the northern beaches in Culebra and the US Virgin
Islands from late Wednesday night into Thursday.

.LONG TERM...Friday through Tuesday...
.PREV DISCUSSION... /issued 402 AM AST Tue Mar 11 2025/

A short-wave trough will move quickly just north of the area on
Friday, and the 500 MB temperatures are now expected to warm by a
few degrees to around minus 6/5 degrees Celsius. However, a
surface front, pushed by a building surface high pressure over the
western Atlantic, is expected to move over or close to the islands
and linger across the region over the weekend. The relatively
colder mid-level temperatures, combined with above-normal
precipitable water (PWAT) content of 1.50 to 1.70 inches, will
promote a wetter pattern across Puerto Rico and the U.S. Virgin
Islands in general during the first part of the long-term period,
with increasing chances of isolated thunderstorm development.

Consequently, there will be a limited to elevated risk of
excessive rainfall, leading to urban and small stream flooding and
potential landslides and rockfalls in vulnerable and steep
terrain. In the U.S. Virgin Islands, the highest probability of
precipitation (40% to 60%) will occur from late Friday through the
weekend as shower activity increases due to the expected frontal
passage.

An upper-level ridge building from the western Caribbean and a
developing TUTT-low northeast of the Leeward islands will promote
a drying trend across the local area for Monday and Tuesday, with
PWAT values dropping between 1.00-1.20 inches. Meanwhile, the
surface high pressure over the western Atlantic will strengthen,
and the low-level winds will increase between 20 and 25 knots
across the Atlantic waters and the Anegada Passage, particularly
on Monday. Therefore, breezy conditions and an advective weather
pattern are expected, with passing showers moving at times across
the windward areas of the islands and limited afternoon shower
development over the SW quadrant of PR.

&&

.AVIATION...
(18Z TAFs)

Expect VFR conditions today. The potential to observe SHRA/+SHRA
will increase after 12/12z, spreading into the interior and
northern PR after 12/15z. This activity will impact the terminals,
possibly creating MVFR conditions. Winds will prevail from the
E-ESE at 10-15kt with sea breeze variations until 11/23z, when we
expect they to become calm to light and variable, returning from
the E-ESE after 12/13z.

&&

.MARINE...

A surface high pressure over the eastern Atlantic and a surface low
and associated front over the western Atlantic will maintain winds
from the east-southeast. These winds will become lighter and from
the south on Wednesday. By the end of the week, the potential for
showers over the local waters will increase as the front moves closer
to the area, and winds turn from the east to northeast. A long-
period northwesterly swell will spread across the Atlantic waters and
local passages around late Wednesday night or early Thursday, with
another swell from the north-northeast by the end of the weekend,
resulting in hazardous seas.

&&

.BEACH FORECAST...

There is a low risk of rip currents across the islands tonight and
Wednesday. A 4 to 5 feet northwesterly swell will spread across
the Atlantic waters on Thursday and Friday, increasing the
potential for life-threatening rip currents. A larger swell (6 to
8 ft) is expected later during the weekend into early next week,
potentially resulting in large breaking wave action along the
north-facing beaches of the islands. High Surf Advisory conditions
and life-threatening rip currents are likely, particularly on
Sunday.

&&

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

$$

SHORT TERM/AVIATION...CAM/YZR
LONG TERM...DS
MARINE/BEACH FORECAST...ERG/YZR

NWS SJU Office Area Forecast Discussion