Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 181725
Area Forecast Discussion
National Weather Service San Juan PR
125 PM AST Wed Jan 18 2017
.SYNOPSIS...Fair weather with isolated showers will prevail for
the next several weeks. Southerly flow beginning on Monday will
mark a warming trend for several days.
At upper levels...Weak short-wave troughs will pass Friday and
Saturday through the prevailing northwest flow downstream from
the ridge over the western Caribbean. That ridge will drift slowly
east and pass through the area to the Leeward Islands on Monday.
Another weak trough is expected Wednesday of next week.
At mid levels...High pressure over and west of the western
Caribbean will gradually replace the trough extending from the
northeast over the eastern Caribbean Sunday through Tuesday of
next week. High pressure will then strengthen over the western
Atlantic and Caribbean. Moisture will be lacking during the entire
period at mid levels.
At lower levels...High pressure extends across the sub-tropical
Atlantic from Florida to the Central Atlantic while lower pressure
persists across the southwest Caribbean. A low north of Puerto Rico
will deepen rapidly tonight and tomorrow as it moves northeast,
weakening the high to our north and gradients in general in the
local area. That weakened high pressure will then migrate eastward
across the Atlantic over the weekend. Strong low pressure will
exit Virginia and North Carolina into the Western Atlantic Monday
night bringing southerly flow Monday and Tuesday to Puerto Rico
and the U.S. Virgin Islands. After the low passes far to the
north, high pressure will begin building over the Bahama Islands
causing the flow to shift to the northeast again by Wednesday
.DISCUSSION...Drier air in the mid levels is slowly filtering
down to clear the skies and shoo away the showers that have been
so persistent during the last week or so. Low pressures are
strong over the United States and, as they develop, across the
Atlantic, well north of the area, but models are currently
keeping most of the moisture associated with fronts that dissipate
over the area to the north, or, in the case of the southerly flow
mentioned above on Monday and Tuesday, just east of Culebra and
Vieques, possibly affecting the U.S. Virgin Islands. Gradients
that have been so strong will weaken considerably as the strong
low moves northeast tonight through Saturday allowing winds to
diminish. Such strong winds do not return this week or next even
though modest high pressure rebuilds over the sub-tropical
.AVIATION...Mainly VFR conditions are expected to continue across
all terminals through the forecast period. Limited shower activity
is forecast across the local waters. Low level winds will continue
from the ENE at 10-15 kts with lighter land breezes after 19/00z.
Maximum winds will continue from FL310-460 50-60 kt becoming more
.MARINE...Seas are slowly subsiding and the high risk of rip
currents will end tonight. Although northerly swell enters the
area on Saturday and Sunday it should be no more than 3 to 4 feet.
Small craft advisories are not expected this week or next week in
the San Juan marine forecast area.
.PRELIMINARY POINT TEMPS/POPS...
SJU 73 84 73 84 / 10 10 10 20
STT 72 84 72 84 / 10 10 20 20
PR...High Rip Current Risk until 6 PM AST this evening for Culebra-
Mayaguez and Vicinity-North Central-Northeast-Northwest-San
Juan and Vicinity-Southeast-Western Interior.