Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 130804

Area Forecast Discussion
National Weather Service San Juan PR
404 AM AST Wed Dec 13 2017

.SYNOPSIS...Surface high pressure across the east-central
Atlantic and Low pressure over the northwestern Atlantic will
continue to cause light southerly winds today and possibly until
Thursday while moisture also increases. Frontal boundary will
continue stalled to the west of the local islands until Thursday,
before it starts to dissipate late Thursday. Local pattern will
likely cause isolated to scattered showers across portions of the
local forecast area.


.SHORT TERM...Today through Friday...

Mostly clear skies and a few passing showers were observed across
the forecast area overnight and early this morning. Minimum
temperatures were in the low to mid 70s across the lower elevations
and in the 60s across the higher elevations. Winds were light and

A surface front will continue to stall west of the forecast area
while it dissipates. The surface front to the west and a surface
high across the central Atlantic will continue to yield a light
south southeast wind flow through at least Thursday. As the surface
front dissipates, the surface high will then spread across the area,
tightening the local pressure gradient. Therefore, moderate easterly
winds are expected to return on Friday. At upper levels, a ridge
pattern will continue to slowly build, resulting in erosion of low
level moisture on Friday. Meanwhile, latest guidance continues to
indicate precipitable water values near the normal range through
Thursday under the south southeast wind flow. As a result, still
expect a mainly fair weather pattern with trade wind showers at
times as well as locally induced showers due to sea breeze
convergence across the northwest quadrant of the island today and
Thursday. Limited shower activity is expected Friday.

.LONG TERM...Saturday through Thursday...

Upper level ridge will continue over the local islands through the
weekend, then an upper trough moves near the eastern Caribbean but
stays to our east, which means that we will stay in the subsident
side of the trough, inhibiting thunderstorm development over the
local area for early next week. The local pressure gradient is
expected to tighten this upcoming weekend into next week as a
surface high pressure moves north of the local islands. This setup
will cause a breezy easterly wind flow from Saturday into at least
Wednesday, winds decreasing a little thereafter. The available
moisture will be near normal with patched of higher moisture
moving in, causing a brief increase in shower activity over the
local area. Overall a seasonable pattern is expected in the
extended forecast. Temperatures also are expected to be seasonable
with highs in the low to mid 80s across the lower elevations and
in the 70s across the higher elevations, lows in the low to mid
70s across the lower elevations and in the 60s across the higher
elevations...some isolated areas possibly slightly cooler.


.AVIATION...VFR conditions are expected to prevail at all TAF sites
through the forecast period with some mtn obscurations and brief
SHRA possible in and around TJBQ/TJMZ/TJSJ between 18z and 22z.
VCSH elsewhere. A light southerly wind flow is expected to prevail
with some sea breeze variations.


.MARINE...Relatively tranquil marine conditions are expected until
the arrival of a northerly swell late Thursday. Seas will be up
at 5 feet today and Thursday during the day, as the northerly
swell moves in late Thursday into early Friday, the local seas
will increase up to 6 feet through Saturday then 7 feet or higher
on Sunday and into early next week.


SJU  87  74  86  75 /  10  20  30  30
STT  83  75  85  74 /  20  40  40  30




LONG TERM....JA is the U.S. government's official web portal to all federal, state and local government web resources and services.