National Weather Service San Juan PR
432 AM AST Wed Jan 20 2021
.SYNOPSIS...
A moist air mass will promote passing showers across the windward
sections through the morning hours. Shower activity is then
forecast to diminish by this afternoon into the evening hours when
a drier air mass is forecast to arrive. Although stable weather
conditions will continue through the forecast period, pockets of
moisture embedded in the trades will reach the islands from time
to time.
&&
.SHORT TERM...Today through Friday...
Latest GOES-16 Total Precipitable Water imagery shows an area of
enhanced moisture moving across Puerto Rico. This has resulted in
passing shower activity affecting mainly the northern half of Puerto
Rico during the overnight hours. This pocket of moisture is expected
to quickly move out of the area during the early morning hours, with
an area of drier air quickly following behind and remaining over the
area through tonight. Therefore, from the mid-morning hours onwards,
mainly fair weather conditions with very little shower activity are
expected. A few showers cannot be ruled out across western Puerto
Rico this afternoon.
At mid to upper-levels, a polar trough is forecast to move across
the western Atlantic later today into tomorrow, weakening the mid-
level ridge somewhat. The polar trough will drive a surface low-
pressure and an associated front across the western Atlantic,
weakening the pressure gradient and thus the low-level steering
flow. Another patch of low-level moisture is expected to move across
the area on Thursday and combined with a lighter steering flow,
there is a better potential to observe afternoon convection across
interior, western and southwestern portions of Puerto Rico as the
available moisture will combine with diurnal heating and local
effects. Below normal moisture then moves across the area for Friday
and thus limited shower activity can be expected once again.
However, some afternoon convection cannot be ruled out across
portions of western Puerto Rico.
.LONG TERM...Saturday through Wednesday...
The mid-level high will remain in place, promoting subsidence and
dry air aloft. At the surface, pockets of moisture embedded in the
trades will bring clouds and showers from time to time. However,
due to the mid-level high, do not anticipate organized convection
across the region.
A surface-high pressure, moving from the Eastern Seaboard into the
Atlantic, will sink southward the remnants of an old boundary,
keeping it to the north of the islands Sunday through mid-week.
Model guidance has been very inconsistent with the moisture field
through the long-term period.
&&
.AVIATION...
Mainly VFR conditions expected across the local terminals. Passing
SHRA will continue to affect portions of the northern half of PR
through 20/12z, resulting in VCSH across TJSJ and TJBQ. Brief MVFR
conditions cannot be ruled with these passing SHRA. Thereafter,
mainly fair wx conditions expected through the rest of the period.
Winds will continue from the E-ENE below 10 kts through 20/12z,
increasing to up to 15 kts with sea breeze variations after
20/14z.
&&
.MARINE...
The northerly swell is subsiding across the local waters. However,
moderate to locally fresh easterly winds will create choppy seas
of up to 6 feet today. Therefore, small craft operators should
exercise caution across most of the local waters.
A surface-low moving across the western Atlantic will loosen the
local pressure gradient, promoting light to moderate trades across
the local islands. As a result, the seas will also improve.
Beachgoers, there is a high risk of rip currents along the north-
facing beaches of Puerto Rico and Culebra. It`s moderate for most
of the north and east-facing coasts.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 73 84 74 / 30 30 30 30
STT 85 73 84 72 / 10 30 30 10