Area Forecast Discussion
National Weather Service San Juan PR
510 AM AST Wed Sep 5 2018
.SYNOPSIS...An upper level low north of Hispaniola will provide
enough instability to result in another round of afternoon
convection across the interior and western sections of Puerto
Rico, as well over the San Juan metro area from El Yunque
streamer. Showers in the form of streamers are expected to develop
downwind from the U.S. Virgin Islands this afternoon.
&&
.Short Term...Today through Friday...
Upper level low north of Hispaniola will continue to move further
west as an upper level ridge builds from the east and moves over
the forecast area by the end of the week. This will limit
widespread shower activity across the area through the short term
period. However, a weak surface trough moving across the areas
today and the proximity of the upper level low to the northwest,
will result in showers across the USVI and over portions of
eastern PR during the morning hours. In addition, forecast
sounding for this afternoon continues to indicate enough
instability with a lifted index of -8. Therefore, expect another
round of scattered to locally numerous showers and thunderstorms
to develop over the interior and western sections of PR, as well
over the San Juan metro area from El Yunque streamer.
On Thursday and Friday, under ridge building aloft, shower activity
should decrease gradually each day. However, diurnally induced
afternoon showers should continue over portions of the interior and
western PR with isolated thunderstorms expected on Thursday
afternoon. A drier air mass is forecast to move from the east on
Friday morning, lowering the chances for thunderstorms to develop
during the afternoon over western PR.
.LONG TERM...Saturday through Thursday...
An mid-upper level high pressure ridge will hold through early
next week. At low levels, a drier air mass with patches of
moisture will result in a seasonal weather pattern with overnight
and early passing showers across the windward sections of Puerto
Rico and the U.S. Virgin Islands, followed by afternoon convection
across the interior and west portion of Puerto Rico as well as
downwind from the U.S. Virgin Islands each day.
A TUTT-low will replaced the upper level ridge by Monday or
Tuesday inducing several surface trough early next week. This
upper level feature will retrograde across the region through at
least Thursday. Model guidance are suggesting a Tropical Cyclone
approaching the region by the end of the work week. However, the
weather pattern after mid-week will be highly influenced by the
evolution of two systems; 1) Tropical Cyclone Florence, which is
forecast to move well north of the local area into the western
Atlantic Ocean, and by 2) a broad area of low pressure located a
couple of hundred miles south southwest of the Cabo Verde Islands.
This area of low pressure is being monitored by the National
Hurricane Center and the Local Forecast Office as it could become
a tropical cyclone later this week. Refer to the Tropical Weather
Outlook (TWOAT) for additional details.
Based on the current track of Florence, long period east to
northeast swell could impact the north and east coast of Puerto
Rico and the U.S. Virgin Islands by the end of the work-week.
&&
.AVIATION...
Mainly VFR conditions are expected to prevail across all
the terminals through the forecast period. However, SHRA/TSRA
developing over the interior and western PR btw 16z-22z can create
tempo MVFR conds at TJSJ/TJBQ/TJMZ. Surface winds will continue from
the east-southeast at 7-15 kt with sea breeze variations after 14z
and higher gusts with SHRA/TSRA.
&&
.MARINE...
Seas will range from 1 to 4 feet across most of the local waters
with easterly winds generally between 5 to 15 knots. A long period
east to northeast swell is forecast to move across the Atlantic
Waters and into the local waters by tomorrow Thursday into the
weekend.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 80 / 30 30 30 30
STT 88 79 89 80 / 20 30 30 30
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