086
FXCA62 TJSJ 272137
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
537 PM AST Sat Apr 27 2024


SYNOPSIS...

Unsettled weather conditions are expected to prevail the rest of
the afternoon hours, with an elevated risk of flooding, especially
across southwestern Puerto Rico with improving conditions
tonight. Additional shower development should continue tomorrow.
Some improvement is anticipated for the first couple of days of
the workweek, but the pattern will turn wet again around mid-week,
with additional periods of showers and thunderstorms. Marine and
coastal conditions are expected to deteriorate Monday onwards as a
northerly swell arrives.

&&

SHORT TERM...Tonight through Monday...

Another day with cloudy skies across the forecast area. During
the morning hours, isolated showers moved across northern,
eastern, and central sections of Puerto Rico, as well as the U.S.
Virgin Islands. Easterly winds between 5 to 10 mph turned more
from the northeast through the day under the influence of a
building surface high pressure across the western Atlantic.
Daytime highs peaked mainly in the mid to upper 80s along coastal
areas and 70s to lower 80s across the Cordillera Central. During
the early afternoon hours, showers and thunderstorms began to
develop across the southwestern quadrant of Puerto Rico, where a
flood advisory was issued. At this time, the highest rainfall
accumulations are around 2.00 to 4.00 inches of rain along that
region based on radar estimates.

Tonight, expect variable conditions to continue with isolated to
scattered showers. The associated moisture and rainfall activity
will filter across the northern and eastern sectors of Puerto Rico
and the surrounding waters. Precipitable water levels should
remain above normal values most of the day on Sunday, resulting in
additional showers and isolated thunderstorms possible. High
resolution models are showing best rain chances across the
northeastern and the interior of PR during the morning, then
moving to the southwest by the afternoon.

Based on the latest model guidance, at the beginning of the workweek,
the broad high pressure is expected to help push the moisture field
southward allowing a slightly drier airmass to filter into the local
islands. However, Precipitable Water models suggest that moisture
levels will not drop that much and remain around seasonal levels.
That should result in at least some convection across the interior
and southwestern Puerto Rico due to the prevailing northeasterly
winds and local effects, but accumulations should be much less in
comparison with the last couple of days.

&&

LONG TERM...Tuesday through Saturday...
.PREV DISCUSSION... /issued 400 AM AST Sat Apr 27 2024/

A high pressure system at the lower levels of the atmosphere will
drive the winds from the east at the surface, and from the
northeast a little bit aloft. This feature will maintain the
moisture trapped in the lower levels of the atmosphere. In
general, Tuesday looks like the driest day of the forecast period,
but moisture will still remain at seasonal levels, triggering
convection in the interior and southwestern Puerto Rico.

The pattern will turn wet again for the rest of the workweek as yet
another mid and upper level trough digs into the western Caribbean.
This trough will pull plenty of moisture from the Caribbean Sea into
the local islands, and precipitable water values will be above
normal. On Wednesday, the steering flow will be from the southeast,
so local effects should trigger for the interior, north and
northwest of Puerto Rico, as well as around the Virgin Islands and
local waters. Then, on Thursday, as the trough further approaches,
the steering winds will collapse, with very light winds anticipated.
Additionally, the mean flow will be from the southwest, so activity
could switch focus for the eastern interior, the San Juan metro
area, and then the Virgin Islands.

One thing that could delay the development of rain, however, is that
if a thick cloud layer get established, then local effects will be
milder. Also, temperatures could be below normal for a change,
except for the southern coast of Puerto Rico, where values are still
expected to reach the low 90s. On the other hand, if there are
breaks of sunshine, then the risk for showers and thunderstorms will
be elevated, likely producing urban and small stream flooding, water
surges along rivers, as well as mudslides.

By Friday, the influence of the mid and upper level trough
diminishes, but a surface reflection remains, maintaining enough
moisture, and a light wind flow from the northeast. More moisture
will also be added into the mixture from another trough approaching
the eastern Caribbean from the east.  Thus, another active afternoon
is anticipated for the interior of Puerto Rico, with activity then
spreading toward the coastal areas. Since some drier air will filter
at the mid and upper levels, temperatures will warm up again to more
seasonal values, reaching the mid and upper 80s, and even the low
90s.

On Saturday, the pressure gradient begins to recover as a surface
high pressure exits the eastern coast of the United States. The
winds will be around 10 knots, and out of the southeast. Lingering
moisture around the northeastern Caribbean will maintain a showery
pattern, with another active afternoon and an elevated risk for
flooding and mudslides.

&&

AVIATION...
(18z TAFs)

SHRA will end at TJPS before 28/00z, but passing SHRA are
anticipated to continue across the local waters, reaching the USVI
and TJSJ terminals after 06Z. Periods of reduced VIS and low
ceilings are anticipated. Additional round of SHRA and VCTS is
anticipated for TJPS after 28/17Z. Mountain obscuration is expected
for the Cordillera Central from 14-21Z. Winds will be on the
increase, out of the NE at 11-16 kts, and stronger gusts after 28/13-
14Z.

&&

MARINE...

Showers and thunderstorms will continue to affect the local waters
for the next several days as lingering moisture from a surface
trough/old frontal boundary remains over the local area. Gentle to
moderate northeast winds will continue tonight and Sunday morning.
Winds will increase from moderate to fresh Sunday night through at
least Tuesday night under the influence of a surface high pressure
moving off the eastern coast of the United States. A northerly
swell is anticipated to spread across the local waters Monday
deteriorating coastal and marine conditions through at least mid-
week.

&&

BEACH FORECAST...

Breaking waves around 5 feet will maintain the risk of rip
currents moderate through tomorrow along the north-facing beaches
of Puerto Rico and the U.S. Virgin Islands. A northerly swell will
be arriving on Monday, increasing the risk for these beaches to
high through at least late Wednesday. For more information and
details about the latest forecast please refer to the Surf Zone
Forecast (SRFSJU), and the Coastal Hazard Message product
(CFWSJU).

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory from 6 PM Sunday to 6 AM AST Wednesday for
     AMZ711.

&&

$$

SHORT TERM/MARINE/BEACH FORECAST...YZR
LONG TERM/AVIATION....ERG/RC
PUBLIC...ERG