issued 557 PM AST Sun Jan 14 2018/
SYNOPSIS...Brief passing trade wind showers will prevail through
most of the week with increasing moisture and a relatively deep
moist layer below the weak mid level inversion. After Wednesday,
moisture will decline into the next weekend. Northwest flow
continues aloft until Friday when the ridge of high pressure
crosses the area from the west.
SHORT TERM...Tonight through Wednesday...
There will be a slight increase in moisture behind a trough now over
Saint Martin that is oriented northeast/southwest. This trough will
arrive Monday morning in the U.S. Virgin Islands and Puerto Rico.
The increase in moisture is expected to continue through Tuesday
16/18Z. In response surface winds will veer a little arnd 15/18Z to
become east to east southeast, but will return to a more east
northeast orientation afterward. Hence the pattern of scattered
brief trade wind showers we have had over the weekend will increase
somewhat through the first several days of the week. At 700 mb a
ridge will form northeast of the area and generate a weak high
almost 400 miles northeast of the area Wednesday. At first it will
lessen moisture overnight Tuesday but then as it approaches with
more northeasterly flow, moisture out of the cooler Atlantic will
arrive to continue the onshore pattern of up slope trade wind
showers Wednesday and beyond. Temperatures will remain seasonable,
but just above normal.
LONG TERM...Although the surface high pressure decays in the
east central Atlantic Ocean, a second weaker high moves out into
the western Atlantic at mid latitudes, causing the cold front to
become more east west at around 25 north. This will allow the
easterly trade wind flow to continue virtually uninterrupted and
cool mid-Atlantic air to carry brief trade winds showers across
the area. Moisture does decline Thursday through Saturday, but the
diurnal fluctuations will affect the area more than the weak
trend. Hence shower activity will depend more on the bands of
showers that form in the low level unstable air. Capping of these
showers around 16-18 kft is more due to the extremely dry air over
the area rather than any weak inversions that are present from
the 500 mb high that remains close to the local area through the
period. Although cooler air approaches the local area from the
northeast at the end of the 10 day period, it is not clear that
any frontal passage can occur.
AVIATION...Mainly VFR conditions will prevail across all terminals
through the forecast period. However, SHRA over SW PR will cont til
14/21Z with lcl MVFR and mtn obscurations and isold trade wind
showers will move across the Leeward/USVI/eastern PR terminals
through 15/14Z . Surface winds will be east at 12-18 kt with higher
gusts and sea breeze variations til 14/22Z Maximum winds 30 to 35 kt
btwn FL370-470 from WNW.
MARINE...Seas have begun to subside again at the outer buoy
after reaching 10.8 feet earlier this morning. They are still
over 9 feet. This surge is still working its way through the local
waters so some small craft advisories had to be added as seas in
the eastern end of the waters surrounding the northern U.S. Virgin
Islands rise above 7 feet. Seas in the inner Atlantic waters will
subside overnight below 6 feet and remain 6 feet or less until
Monday night when 7 foot seas return. It was decided to let the
SCA expire for Monday in those zones before raising them again
Monday night. Most SCAs will have to be raised again after Monday,
but should all be gone by Wednesday night.
.PRELIMINARY POINT TEMPS/POPS...
SJU 83 75 84 75 / 30 20 20 40
STT 84 75 84 75 / 20 30 30 50
Visit the Caribbean-Central America Weather Thread where you can find at first post web cams,radars
and observations from Caribbean basin members Click Here