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-- Discussion containing changed information from previous version are highlighted. --
960
FXCA62 TJSJ 231552
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1152 AM AST Fri Apr 23 2021

.UPDATE...
An easterly perturbation will continue to move across the region
today bringing a slight increase in low level moisture. However
recent TJSJ 23/12Z upper air sounding suggests a fairly strong cap
inversion 4-6 degs around 700 millibars with precipitable water
at 1.37 inches and overall dry and stable conditions aloft. Low
level steering winds were from the southeast around 15 knots. That
said, still expect some locally and diurnally induced afternoon
showers over and around the islands, but widespread and significant
accumulations are not anticipated. No change to the inherited short
term forecast package and reasoning at this time.

&&

.AVIATION UPDATE...VFR at all terminals durg prd.SCT-BKN lyrs nr
FL025..FL50...FL090. Quick passing SHRA ovr the regional waters
between PR and the Northern Leeward islands, accompanying the
weak low level easterly disturbance crossing the area. No other
changes to the previous AVN discussion, locally and diurnally
induced afternoon showers are still forecast to develop over parts
of the islands durg the rest of the aftn, followed by clearing
skies and diminishing SHRA ovr lanD areas overnight anD into the
early morning hours. No significant operational wx impacts at this
time.

&&

.MARINE...No change to the previous marine discussion. Overall
fairly tranquil seas are expected through the weekend. However a
light to moderate chop can be expected over the offshore waters
and local passages due to winds between 10 to 15 knots.

&&

.PREV DISCUSSION... /issued 657 AM AST Fri Apr 23 2021/

SYNOPSIS...

An easterly perturbation will move across the islands today,
providing enough low-level moisture to promote passing showers
across the windward portions of PR/USVI in the morning, followed
by afternoon convection across the western sections. A dry air
mass will arrive on Saturday, and a ridge will build aloft,
resulting in fair weather conditions. In the long term, a moist
and unstable weather pattern could be the weather story between
Tuesday and Thursday next week.

SHORT TERM...Today through Sunday...

An easterly perturbation will promote somewhat unstable weather
conditions for today. Passing showers will continue to affect
portions of eastern Puerto Rico and the U.S. Virgin Islands
during the morning hours. This activity will be followed by
afternoon convection over parts of the interior and northwestern
quadrant of Puerto Rico, as well over the San Juan metro area.
This activity could lead to at least ponding of water on roadways
and in poor drainage areas. Showers will diminish by sunset.

For tomorrow, mainly fair weather conditions, with limited
rainfall activity, are expected across the forecast area. Model
guidance now suggest a drier air mass coupled with a surface high
pressure that will yield less available moisture content at the
low levels. Aloft, stable conditions are expected, due to a
building upper-level ridge. Precipitable water values are
forecast to be below normal. Hence, Saturday seems to be the
driest day of the short term period. Incursion of African dust
particles is suggested by observations, thus somewhat hazy skies
could be noticed.

Patches of moisture will reach the islands by Sunday afternoon
due to the influence of a polar trough, located in the western
Atlantic. However, these showers are not expected to be impactful,
as they will not lead to significant rainfall accumulations. The
most active part of the day will be in the afternoon, as local
effects and diurnal heating can spawn showers over parts of
northwestern Puerto Rico.

LONG TERM...Monday through Friday...

A polar trough will move from the western to the eastern Caribbean
Monday through Tuesday. This mid- to upper-level trough will then
amplify over PR/USVI Tuesday through Thursday, to be then
displaced by a ridge building from the west on Friday. Meanwhile,
a surface high pressure will push a disturbance in the easterly
winds over the islands early next week. Then, a cold front will
approach the islands from the northwest Tuesday through Thursday,
enhancing tropical moisture over us. Another surface high will
build over the Western Atlantic Thursday onward.

That said, expect the advective pattern to continue Monday and
early Tuesday, which could bring passing showers across the
windward sections of PR/USVI during the overnight and morning
hours, followed by afternoon convection across the northwest
quadrant and interior portions of Puerto Rico. The weather pattern
should shift to a more unstable one Tuesday through Thursday, as
the polar trough amplifies and tropical moisture increase across
the islands. Therefore, the potential to have inclement weather
due to thunderstorms, and including urban and small stream
flooding, could increase, especially Tuesday through Thursday, as
model guidance continues to suggest. A more stable pattern should
return by Friday.

AVIATION...

Mostly VFR conditions expected to prevail over the next 24 hours
across all terminals. VCSH and brief gusty winds with and around
SHRA expected for TJSJ and TJPS through 23/10. Afternoon convection
will yield SHRA for TJBQ/TJSJ til 23/22. Winds out of east-
southeast at 10 to 15 knots, with sea breeze variations with
occasionally higher gusts during the afternoon.

MARINE...

Mariners can expect tranquil marine conditions during the next
several days. Seas will range between 3 and 5 feet most of the
time. Light to moderate east to southeast winds will prevail
through early next week.

Beachgoers, a moderate risk of rip currents is expected to
continue across some of the east, northern and southern beaches
of the islands.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  75  88  74  89 /  30  10  20  30
STT  75  87  75  86 /  20  20  30  30

&&

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

$$

SHORT TERM...RAM
LONG TERM....GL
PUBLIC DESK...CS