Area Forecast Discussion
National Weather Service San Juan PR
428 AM AST Tue Dec 17 2019
A surface high will continue to hold firm across the Atlantic
waters through Thursday. The surface high is creating fresh to
strong winds as well as moderate to rough seas across Atlantic and
Caribbean waters. In fact, this surface high is ushering in
moisture from the Atlantic waters over the area. The high is
forecast to slightly weaken on Thursday as it slides east of
Puerto Rico. This will result in a east to southeast wind flow on
Thursday and Friday. The southerly wind flow will drag moisture
over the U.S. Virgin Island and Puerto Rico, and cause an increase
in showers during the afternoons. Saturday, the surface and mid-
level ridge reestablishes themselves over the area again and
increase the trade wind cap.
.SHORT TERM...Today through Thursday...
A broad ridge tightening the local pressure gradient is promoting
breezy and windy easterly winds across the northeast Caribbean,
including U.S. Virgin Islands and Puerto Rico. Satellite data
indicate a plume of moisture with embedded clouds and showers. As a
result, occasional passing showers producing periods of moderate to
locally heavy rains will affect the Virgin Isles and the eastern
half of PR throughout the day. Periods of sunshine will remain
possible at times. This activity will makes its way into the
interior, north and western section by the afternoon hours. A short
wave upper-level trough will swing by the region inducing a jet
aloft. Therefore the development of an isolated thunderstorm should
not be ruled out late this morning or early in the afternoon. The
strongest activity is expected across the eastern and southern
sections of Puerto Rico.
The pressure gradient is expected to loose by mid-week, promoting a
southeast wind flow. Moisture is also expected to erode across the
islands. Under this weather pattern, maximum temperature will
increase somewhat along the northern sections of the islands.
Although moisture will continue to erode quick passing showers
cannot be ruled out at times.
.LONG TERM...Friday through Wednesday...
The GFS and ECMWF have been persistent in showing the surface and
mid-level ridges holding firm across the U.S. Virgin Island and
Puerto Rico Friday, and through the weekend. A short-wave trough
across the northern Atlantic waters will cause the surface ridge
to weaken. The surface winds are forecast to turn to the east to
southeast through Saturday morning due to the surface high
pressure shifting east.
Friday, lingering moisture from a weak perturbation is progged to
pull across the area, with the bulk of the moisture coming on Friday
afternoon. Guidance continues to show precipitable water values
reaching 1.50 inches during this time. The tropical moisture,
diurnal heating as well as orographic effects will yield isolated to
scattered showers across the U.S. Virgin Islands and eastern Puerto
Rico during the mornings. Followed by scattered showers across
southern, interior, western areas of Puerto Rico during the
afternoons. Also, Daytime high temperatures may exceed the daily
record as a result of the east to southeast wind flow, which
typically causes temperatures to soar.
Saturday and Sunday, guidance shows the surface to mid-level ridge
reestablishing themselves across the area. The strengthening of
the ridges could cause an increase in the trade wind cap through
the Sunday. Both global models depicted this when analyzing the
Skew-T diagrams. The strengthening of the trade wind cap should
inhibit the vertical development of showers into thunderstorms
area-wide, as a result of the mid-level ridge causing sinking air
in the mid- level of the atmosphere.Therefore thunderstorms are
not forecast to develop at this time. Nevertheless, both the GFS
and ECMWF show showers developing during the afternoon across the
U.S. Virgin Islands and Puerto Rico.
The GFS shows precipitable water values falling to the 1.3 inches
early Sunday morning with a continues fall to 1.00 inches by Sunday
afternoon. Sunday afternoon, the precipitable water value is
forecast to be near the 25th percentile for December. Also, several
weak boundaries and their associated moisture are forecast to stall
well north of Puerto Rico through the weekend and early next week.
Next week model guidance continues to diverge on a clear solution
when it comes to the development of the upper-level low or cutoff
low moving across Florida. However, the GFS and ECMWF continue to
show the solution described below.
Monday through Tuesday, the beginning of the week is were the GFS
and ECMWF diverge. The GFS shows an upper-level low deepening near
Nova scotia with the base of the trough reaching the central
Atlantic waters. As the trough deepens well north of the area, its
progged to push a weak front through the region. The GFS shows
abundant moisture ahead of the front, with the moisture filtering
into the U.S. Virgin Islands and Puerto Rico. The GFS has surface
winds out of the west; these winds are forecast to converge with the
frontal boundary just north of Puerto Rico on Tuesday.
On the other hand, the ECMWF shows a low getting cutoff from the
main flow across the southern coastal areas of the U.S. on Monday.
As the low transverse, the coastal locations of the northern Gulf of
Mexico, it should undergo cyclogenesis while it continues to sink
south across the Peninsula of Florida. The ECMWF shows the low east
of the Florida Peninsula by Tuesday afternoon with the bulk of the
remaining moisture positioned northeast of Puerto Rico.
Wednesday, the GFS and ECMWF have the frontal boundary moving
through the area. However, the models don`t agree on the timing of
the frontal passage. What we do know is an increase in showers, and
isolated thunderstorms are possible early next week as a result of a
frontal boundary, which will either stall just north of the area
or a boundary that pushes through Puerto Rico. We should wait to
see how the synoptic pattern evolves across the eastern U.S. and
the Atlantic so that we can determine the effects the frontal
boundary and abundant moisture will have on the forecast.
Winds are forecast to shift more from the E to ESE, with
occasional gust between 20 and 30 kts. Passing SHRA will continue en
route between the Leeward Islands and Puerto Rico. SHRA/-SHRA are
possible across the terminals of USVI, E/S-PR and TNCM/TKPK
throughout the day, spreading into central and western PR between
17/16z-20z. Isold TSRA will be possible mainly across E/S PR during
A firm surface high pressure over the Atlantic waters is creating
fresh to strong winds as well as moderate to rough seas across
Atlantic and Caribbean waters. Sea up to 8 feet are expected this
afternoon, with occasional seas to 10 feet for most local waters.
A high risk of rip currents for the northern, eastern, and
southeastern beaches of Puerto Rico. Also, there is a high risk of
rip currents for the beaches of Saint Croix, the southern beaches
of Vieques, and the northeastern beaches of Culebra.
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 74 89 75 / 60 60 30 30
STT 86 75 85 75 / 30 20 30 30