National Weather Service San Juan PR
431 AM AST Tue Dec 21 2021
.SYNOPSIS...
A train of easterly pertubations and accompanying surges of low
to mid level moisture will continue to affect the area for the
next few days, driven by the dominant surface high pressure ridge
and a moderate steering wind flow. Intervals of dry and stable air
will cross the region on occasions resulting in a mixture of sunshine
and clouds. Showers are is expected to be of short duration but some
may cause minor ponding of water on roads and in poor drainage areas.
An upper ridge will build and hold across the area through Thursday.
An induced surface trough is forecast to set up across the area by
Friday, as a cold front stalls across the west Atlantic and north
of the area. Increasing instability aloft and moisture convergence
is forecast by Friday. In the meantime, a seasonal weather pattern
is expected with the prevailing east southeast tradewinds.
&&
.SHORT TERM...Today through Thursday...
Recent infrared satellite imagery, as well as Total Precipitable
Water from GOES-16 show a weak surface trough over the eastern
Caribbean. This feature is expected to cross the islands through the
day, with moisture content increasing all the way to 500 mb, and
thus bringing another round of showers through the day. At sunrise,
it should be mainly tranquil, but by mid-morning into early in the
afternoon, the moisture field will begin to envelope the U.S. Virgin
Islands, then gradually advancing into eastern Puerto Rico. The high
resolution models, as well as the National Blend Model, indicate
moderate rainfall amounts, especially over the eastern half of
Puerto Rico. Additionally, since the trough will be moving over the
area through the day, the combination of low level convergence,
available moisture and diurnal heating should trigger additional
showers over the area, including western Puerto Rico. Ponding of
water on roadways and low-lying areas is anticipated, and isolated
urban and small stream flooding cannot be ruled out.
By Wednesday, as a frontal system exits the eastern coast of the
United States and the surface high pressure migrates toward the
eastern Atlantic, the low to mid level steering flow will shift from
the east-southeast. Then, a similar weather pattern repeats, with a
slot of dry air moving across the area early on the day, followed by
another area of enhanced moisture by the afternoon and evening
hours. This feature, however, looks weaker and with the bulk of the
activity remaining over the Atlantic waters. Then on Thursday, the
front will pull a plume of moisture into the eastern and central
Caribbean. So far, the bulk of the moisture appears to stay west of
the local islands, but close enough to interact with local effects
and result in another round of afternoon showers across western
Puerto Rico.
.LONG TERM...Friday through Wednesday...
An induced surface trough is forecast to develop across the region
through Friday, as a cold front will move and eventually stall across
the west Atlantic and north of the region. An upper level ridge will
erode in response to a polar trough becoming amplified over the west
and southwest Atlantic as it moves eastwards. This in turn is so far
expected to increase instability aloft along with low level moisture
convergence over the region. Model guidance still suggest that the
layered precipitable water values are forecast to increase to around
2.0 inches on Friday and into the weekend.
Latest model guidance continued to be in agreement and maintain the
stalled frontal boundary north and west of the area but with moisture
sinking just north of the islands by Christmas`s Eve and on Christmas
Day. This scenario suggests a continued trend for a wetter pattern
with increasing instability aloft and moisture convergence through
at least Saturday with good potential for enhanced shower development
over the islands as well as the possibility of isolated thunderstorms
over the local Atlantic waters or near the north and east coastal
areas of the islands. This overall scenario will result in increased
potential for minor urban and small stream flooding and or ponding
of water on roads and in poor drainage areas, but mainly in isolated
areas as so far the heavy rainfall and shower activity is not expected
to be widespread.
A gradual improvement in the weather pattern is forecast by Sunday
and through Monday of the following week, as a drier and stable
airmass will enter and spread across the region. Winds will gradually
increase while becoming more northerly as a strong surface high
pressure ridge builds across the west Atlantic and north of the
region. Expect a return of a seasonal weather pattern through Monday
and into Tuesday with a cool advective pattern expected to return.
This will promote the occasional passage of late evening and early
morning tradewind showers along the north and east coastal areas,
followed by limited afternoon convection mainly over parts of the
interior and southwest sections of Puerto Rico. Lesser activity
forecast in and around the U.S. Virgin islands. By Wednesday the
pattern is forecast to change again with approach of a remnant
frontal boundary shearline as the high pressure ridge builds north
of the region and moisture returns to the region from the northeast.
&&
.AVIATION...
Mainly VFR conditions expected through the forecast period. Passing
showers could result in SHRA across most of the terminals with brief
periods of MVFR conditions due to reduction of VIS and low ceilings.
Winds will be out of the east at 10 to 12 knots, with stronger
gusts and sea breeze variations.
&&
.MARINE...Choppy seas up to 6 feet are forecast across the offshore
Atlantic and Caribbean waters as well as the local passages today.
Small craft operators should therefore exercise caution. Seas between
3 to 5 feet can be expected elsewhere. For the rest of today there
is a moderate risk of rip currents for most beaches, however some
beaches of Culebra and east beaches of St Croix may return to a
high risk of rip currents by early Wednesday.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 84 73 87 73 / 60 50 20 40
STT 83 73 85 74 / 60 60 40 60