National Weather Service San Juan PR
539 AM AST Tue Dec 5 2017
.SYNOPSIS...A shear line with good moisture along the boundary is
generating showers and occasional thunderstorms in the area. The
boundary will drift slowly southeast until cooler and somewhat
drier air moves into the area Friday and Saturday. Winds will
increase considerably tonight and Wednesday.
&&
.SHORT TERM...Today through Thursday...
At lower levels, a shearline and a frontal boundary to
the north of the islands will continue to sink across the local
Atlantic waters. At upper levels, a trough moving southeast out
of Florida will provide good divergence aloft through at least
Wednesday. These features will continue to result in better than
normal precipitable water content and good instability to aid in
the development of showers and thunderstorms each day across the
forecast area. For today, under an east to east-northeasterly
steering wind flow, showers and thunderstorms are expected mainly
over the mountain ranges of PR and into the interior and west-
southwest portions of the islands. Across the USVI, under the
evolving trough pattern aloft, isolated thunderstorms and
scattered showers can`t be ruled out.
Later on Wed-Thu, a more easterly wind flow will dominate the local
area and showers should focus more across the western interior
portions of PR with streamers are expected to develop each day
across the USVI. A mid to upper level ridge building over the
forecast area late in the forecast period will bring drier air and
fair weather conditions across the region.
.LONG TERM...Friday through Monday...
Precipitable water begins to fall late Thursday afternoon and
continues through Friday evening when the shear line backs into
the area from the northeast bringing cooler and drier air behind
it. A ridge aloft will build and sharpen over the Mona Channel
Friday and Saturday to enhance this drying effect. The lowest
levels will continue to see bands of moisture riding through on
the northeast flow, however and this will bring instability and
showers to the area. The showers will mainly affect the U.S.
Virgin Islands and the windward side of Puerto Rico, however.
Another front will approach the area from the northwest with
moisture ahead of it beginning to infringe on the northwest corner
of the forecast area Sunday and Monday. The ridge aloft will
weaken and round out early next week, but remain in the area.
&&
.AVIATION...Mainly VFR conditions are expected to prevail through at
least 05/15z. Between 05/16z-22z SHRA/TSRA are expected to affect
the eastern interior and western portions of PR, MVFR and mtn
obscd are expected with this activity. Across the USVI terminals,
SHRA/TSRA mainly expected in and around the northern USVI`s
terminals. Winds E- ENE at 8-18 kts blo FL100. Sea breeze
variations aft 05/13z. Maximum winds WSW 75-85 kt around FL410.
&&
.MARINE...Seas will increase with winds and swell entering the
area from a system to our northeast. Small craft advisories will
begin to go up this evening and seas will spread out of the outer
Atlantic waters and through the local passages and channels
tonight and Wednesday. Highest seas may reach 10 to 11 feet by
Wednesday night and Thursday. Small craft advisories should all be
down by Sunday night. High surf advisories may be necessary on the
northeast coast of Puerto Rico by Thursday.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 85 76 85 75 / 60 40 40 40
STT 85 76 85 75 / 60 50 20 50