Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 091733

Area Forecast Discussion
National Weather Service San Juan PR
133 PM AST Mon Dec 9 2019

Brisk east-northeast winds will continue across the islands due
to a strong high pressure system over the Atlantic Ocean. Under
this flow, passing showers are favored across eastern and northern
Puerto Rico and over the U.S. Virgin Islands. Marine conditions
will deteriorate beginning tonight and will continue hazardous for
much of the workweek.


.SHORT TERM...Tonight through Wednesday...
A high pressure system located over the Atlantic Ocean will
continue to strengthen, tightening the pressure gradient across
the area. This will result in brisk east-northeast winds for the
next couple of days. As a result, passing showers are favored
across portions of northern and eastern Puerto Rico as well as
over and around the U.S. Virgin Islands. For tonight, this
activity could be a bit enhanced by the presence of a trough at
the mid and upper levels located just to the north of the islands.
On Tuesday, although this trough will linger north of the area,
drier air is anticipated to move in, limiting the amount of
moisture. Nonetheless, a few showers could still be advected over
the islands. Finally, on Wednesday, the same pattern holds, but
with a bit more moisture available, the frequency of showers
should increase once again.

.LONG TERM...Thursday through Tuesday...

Uneventful weather conditions are expected to prevail in the
long-term forecast. Near average moisture will be with us, with
precipitable water values mostly between 1.5 and 1.7 inches. Also,
trade winds stay fairly brisk as well, helping to keep showers
moving along and reducing the amount of rainfall accumulations. No
major fronts or tropical waves are forecasted for the next week.
Furthermore, the upper levels of the atmosphere remain less
conducive to thunderstorm activity with plenty of ridging aloft.

Despite the generally average conditions in the
long-term, there will still be plenty of opportunity for passing
showers. Advective showers will develop in the nighttime and
morning hours, threatening the east coast of Puerto Rico and the
USVI, particularly Thursday and Friday when the highest moisture
content will occur. Furthermore, the usual afternoon shower
activity is possible, particularly for western Puerto Rico.
However, for reasons mentioned in the last paragraph, these
showers will likely have minimal impact. Long range models suggest
drier conditions heading into the next week, with the chance for
some Saharan dust working into the region. However, there is
minimal confidence in that forecast this far out.


.Aviation...Mostly VFR conds are expected through the forecast
period. VCSH and/or -SHRA may affect the TAF sites at times, but
overall VFR conds will prevail. ENE winds of 15-25 kts likely below
FL250. Brief winds gusts are possible with SHRA as they pass by.


Hazardous marine conditions are expected to begin tonight, with
seas increasing between 5 to 7 feet due to an increase in winds.
Winds will be out of the east-northeast at 15 to 20 knots. Small
craft advisories will be in effect beginning tonight through much
of the workweek. For the beachgoers, there is a moderate risk of
rip currents for most of the local beaches.


SJU  75  84  76  84 /  50  70  70  40
STT  75  86  75  85 /  40  40  40  30


AM...Small Craft Advisory from 8 PM this evening to 8 AM AST Friday
     for Anegada Passage Southward to 17N-Atlantic Waters of
     Puerto Rico and USVI from 10 NM to 19.5N-Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to



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