National Weather Service San Juan PR
517 AM AST Tue Dec 20 2016
.SYNOPSIS...Moist flow in fresh to strong trade winds will produce
showers each day. A pocket of cold air in a mid and upper level
trough will enhance showers and thunderstorms today and tomorrow.
At upper levels...The trough over Puerto Rico and the
U.S. Virgin Islands shifts north by Wednesday night. Beginning
Thursday southwest flow develops over the area as a ridge develops
over the southeast Caribbean and the equatorial Atlantic.
At mid levels...High Pressure continues from the Gulf of Mexico
east northeast into the Atlantic. Weak cold-core low pressure is
found over Puerto Rico today and Wednesday that will gradually
disappear over the weekend when high pressure ridges east from
Florida. Thereafter high pressure will dominate the Bahama islands
and Cuba and overflow into the local area. Mid level moisture will
increase today through Friday, then drop off next week when flow
switches to northeast.
At lower levels...Strong high pressure over the northeast central
Atlantic will shift eastward toward the Azore islands. High
pressure will move from the mid-Atlantic seaboard into the western
Atlantic ocean over the weekend maintaining fresh to strong trade
winds over much of the sub-tropical and tropical Atlantic.
&&
.DISCUSSION...Rapidly moving showers continued across the local
waters and onshore over eastern and northern Puerto Rico. Cold air
aloft is enhancing the instability and has generated some
thunderstorms over the Atlantic waters and around the U.S. Virgin
Islands. The center of this cold pocket will be 2 standard
deviations below normal and will settle over Puerto Rico today
and slowly warm. Active showers and isolated thunderstorms may
continue after Wednesday, but the upper level trough will give way
to a ridge building from the southeast afterwards. Pockets of
divergence aloft will likely enhance showers activity later in the
week and over the weekend as upper level flow turns southwest, but
convergence will generally prevail at upper levels. Still too
early to get a handle on Christmas Eve and Christmas Day, but
shower are likely--mainly on the windward coasts.
&&
.AVIATION...Brief MVFR conds will remain possible at Leeward and
USVI TAF sites as well as JBQ/JSJ in passing SHRA with isolated TSRA
through the forecast period. Mountain obscurations expected. ENE
surface winds of 15-20kts to continue with gusts to 30 kt at
times.
&&
.MARINE...Marine conditions remain hazardous during the next
several days. Waves remained above 10 feet in easterly seas at
buoy 41043 north of the forecast area and between 7 and 8 feet
overnight at the local inner buoy near San Juan. A pocket of cold
air has increased instability considerably and thunderstorms have
been observed over Atlantic waters and around the U.S. Virgin
Islands all night--spreading west into the Mona passage. This has
created very gusty conditions. Small craft advisory conditions
will continue over outer waters and passages for much of the week.
Inner waters will see some improvement after mid-week. Rip
currents will begin to subside after today, but moderate to high
risk applies to most of the coasts of Puerto Rico and the U.S.
Virgin Islands.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 84 75 83 76 / 60 50 40 40
STT 84 74 85 74 / 50 40 40 40