Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 191756

Area Forecast Discussion
National Weather Service San Juan PR
156 PM AST Thu Jan 19 2017

.SYNOPSIS...A gradual warming and moistening of the air mass will
occur beginning Friday. A weak pre-frontal boundary will pass
through on Saturday with a front approaching, and perhaps
entering, the forecast area on Sunday. Southerly flow will begin
Sunday with good moisture advection, followed by a stronger front
from the northwest on Wednesday/Wednesday night of next week.

At upper levels...A weak short-wave trough will pass through on
Friday afternoon with a secondary trough coming through early
Saturday morning. By Monday a ridge will be over the local area. A
weak trough will pass through on Tuesday followed by another weak
trough on Thursday.

At mid levels...Northwesterly flow will prevail until high
pressure now over the western Caribbean spreads across the entire
basin by Monday. Weak troughing will pass north of the area on
Tuesday. Mid-levels are almost completely dry except for a brief
shot of moisture on Tuesday.

At lower levels...Gradients will continue to be weak across the
area as high pressure over the Bahama Islands drifts into the west
central Atlantic. Strong low pressure over the southeastern United
States and the weak high over the west central Atlantic will
commence winds with a southerly component later on Sunday until a
likely frontal passage Wednesday. Trade wind flow will increase
Wednesday and Thursday.


.DISCUSSION...No showers and only a few clouds were seen in the
area today. A few clouds were beginning to form over the higher
terrain of the Cordillera Central. The visible satellite image
showed a prefrontal band sweeping down from 28 north 60 west to 21
north 70 west with a cold front north of Puerto Rico around 26
degrees north. Currently models are expecting these bands to
remain intact for Saturday and Sunday passages respectively,
although the old boundary will linger over the area Sunday through
Tuesday. The increase in moisture expected Tuesday will be mainly
associated with moisture advected from the south with the
southerly flow ahead of the next front. The GFS has this moisture
about 55 miles farther west than it did 24 hours ago and over
eastern Puerto Rico as well as the USVI. Although rain chances
will increase--mainly on the north side of the Island and in the
northern U.S. Virgin Islands both Saturday and Tuesday, these
systems are not expected to bring more than slight accumulations
of rain. Another front from a stronger low pressure system will
pass through on Wednesday/Wednesday night, but it loses strength
quickly as it approaches. Mid-levels will have already dried so
that moisture associated with this front will be very limited.


.AVIATION...Mainly VFR conditions are expected to continue across
all terminals during the next 24 hours. SKC overnight across all PR
terminals. Sea breezes should continue through at least 22z. Light
and variable winds at the surface overnight...increasing from the
north around 5-10 kts after 20/14z.


.MARINE...Tranquil conditions exist with no advisories or
precautions. A northerly swell will enter the area Friday night
and increase seas up to 6 feet every 10 to 11 seconds by early
Saturday morning. Seas do not increase again after that until
Sunday afternoon. Then seas will rise to 6 or nearly 7 feet by
Monday night in north northeast swell of 12 to 14 seconds.


SJU  69  82  72  82 /   0  10  40  40
STT  74  82  74  82 /  10  10  20  30




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