Area Forecast Discussion
National Weather Service San Juan PR
445 AM AST Sun Dec 4 2022
Patchy moisture is supporting a seasonal weather pattern of
passing showers for the next couple days. Northerly winds will
strengthen overnight, increasing the frequency of showers that
will reach PR and The USVI. Overall, flooding impacts are not
anticipated, but there is still a risk of water ponding on
roadways and poorly drained areas causing hazardous driving
A northeasterly swell has arrived over the last 12 hours and is
expected to peak tonight. As a result, hazardous marine and
coastal conditions will prevail for the next several days,
including seas and breaking wave heights in excess of 10 feet. See
the Marine section below for more details.
.SHORT TERM...Today through Tuesday...
Short-term dominant features are an elongated surface high
pressure extending southwestward from the north-central into the
western Atlantic, a developing mid-to-upper level low and surface-
induced feature with the associated trough's axis extending
southwestward into the northeastern Caribbean region, and a long-
period northeasterly swell spreading across the Atlantic waters
and local Caribbean passages. These features will maintain hazard
risk at limited to significant or high levels for rip currents,
marine conditions, thunderstorms, and winds throughout the cycle.
Today's timeline will begin with a shallow patch of moisture,
identified in radar and satellite imagery by scattered to locally
numerous showers and a few isolated thunderstorms, crossing the
local islands under a northerly wind flow through the early
morning hours. Rainfall accumulations from this activity could
lead to ponding of water on roadways and poorly drained areas,
mainly across coastal municipalities of northeastern Puerto Rico.
Localized urban and small stream flooding is possible, but chances
are limited. After that, a pocket of drier air will reach the
islands late this morning, limiting any opportunity for widespread
showers while supporting more seasonal weather conditions with a
mixture of sunshine and passing showers embedded in the northerly
trade wind flow through this evening. Under the dominant northerly
flow at 10 to 15 mph with higher gust and sea breeze variations,
expect slightly cooler daytime maximum temperatures ranging from
the mid 80s across coastal areas to lower 70s across higher
For tonight and continuing through Tuesday, as the surface-
induced low gradually builds northeast of the region and the
pressure gradient tightens, expect an increase in wind speeds of
around 15-25 mph, causing more frequent patches of low-level
moisture to reach the local islands and, thus, an increase in
shower activity. This activity will favor the northern half and
portions of the interior of Puerto Rico, Vieques, Culebra, and the
U.S. Virgin Islands. Showers may even move further inland as wind
speeds increase. Showers are expected around the clock, but more
frequent showers are likely at nighttime when the influence of
advective cooling processes is more significant, and precipitable
water vapor values are at their highest concentrations of around
1.5 inches. Uncertainties on isolated thunderstorm development are
present. While the exclusion of thunderstorms on the official
forecast is explicit, one or two could develop due to some
instability generated by the upper-level trough. Overall, flooding
impacts are not anticipated, but there is still a risk of water
ponding on roadways and poorly drained areas causing hazardous
driving conditions. Slightly cooler daytime maximum and minimum
temperatures are expected each day, falling as low as the mid to
upper 50s across higher elevations during the clearest nights.
.LONG TERM...Wednesday through Monday...
A dry, breezy northeasterly flow will prevail on Wednesday and
Thursday due to cool air advection as a deep trough in the Central
Atlantic remains the predominant feature. Rainfall should be
limited during this time due to patchy moisture and at times low
precipitable water values. By Friday, local winds begin to lighten
into the weekend as the trough moves further east into the
Atlantic Basin. Weak concentrations of moisture continue to
dominate during this period, therefore no significant rainfall is
forecast. During these days, the most shower activity is expected
across northern and eastern sections due to passing windward
showers. There is a chance each afternoon for convection to form
over land, potentially providing rain to the interior and southern
sections of Puerto Rico.
By Sunday, another trough will begin crossing the Western
Atlantic and there will be an area of high pressure at the
surface in the Gulf of Mexico, together providing an irregular
windflow, light and variable from the west. The end of the period
remains quite dry with a continuation of patchy to drier moisture
values. Confidence on the exact location of rainfall is low at
the moment, however no significant amounts are expected. Marine
models suggest that these low pressure systems will be significant
swell producers, therefore the greatest hazards will be found
across the coastal and marine zones during this time.
.AVIATION...Mainly VFR conditions will prevail across all local
terminals through the cycle. Increased trade wind SHRA may result
in brief MVFR conditions at TJSJ, TJBQ, and TIST throughout the
day, with a few TSRA bringing VCTS through 04/14Z. Light and
variable winds across all PR terminals and light northeasterly
winds at USVI terminals will prevail through 04/14Z. Winds will
remain from the north and increase to 10-15 knots with higher
gusts and sea breeze variations after 04/14Z.
.MARINE... Currently observed across the regional waters is a
northeasterly swell between 6 and 8 feet with a period ranging
between 10 and 11 seconds. Seas will increase even more today and
over the next few days, therefore Small Craft Advisories are in
effect for most of the Atlantic Waters and Caribbean Passages. The
long- period northeasterly swell will peak late tonight, lasting
into Tuesday with heights around 10 feet, with occasional seas up
to 13 feet, and periods around 11 seconds. Another long-period
northeasterly swell will reach the local Atlantic waters from
PR's beachgoers, A High Surf Advisory and a high risk of rip
currents also remain in effect due to expected breaking waves
around 6 to 12 feet, mainly across north-facing beaches of Puerto
Rico and Culebra.
USVI's beachgoers, a high risk of life-threatening rip currents
remains is in effect for most local waters.
PR...High Rip Current Risk through Tuesday afternoon for PRZ001-002-
High Surf Advisory until 6 AM AST Tuesday for PRZ001-005-008.
High Surf Advisory until 6 AM AST Tuesday for PRZ002-010.
High Surf Advisory from 6 PM this evening to 6 AM AST Tuesday
High Rip Current Risk from this evening through Tuesday
afternoon for PRZ013.
VI...High Rip Current Risk from this evening through Tuesday
afternoon for VIZ002.
High Rip Current Risk through Tuesday afternoon for VIZ001.
.AM...Small Craft Advisory until 8 PM AST Tuesday for Atlantic
Waters of Puerto Rico and USVI from 10 NM to 19.5N-Coastal
Waters of Northern Puerto Rico out 10 NM-Mona Passage
Southward to 17N-Coastal Waters of Northwestern Puerto Rico
out 10 NM.
Small Craft Advisory from 8 AM this morning to 8 PM AST
Tuesday for Coastal Waters of Northern USVI and Culebra
out 10 NM.
Small Craft Advisory from 8 PM this evening to 8 AM AST
Tuesday for Anegada Passage Southward to 17N-Coastal Waters
of Southern USVI Vieques and Eastern Puerto Rico out 10 NM.