Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 200904

Area Forecast Discussion
National Weather Service San Juan PR
504 AM AST Tue Mar 20 2018

.SYNOPSIS...Mid to upper level ridge is forecast to build and hold
over the northeast Caribbean today through Wednesday. This will limit
any significant convection across the region. The ridge aloft is to
then erode by the latter part of the work week and into the upcoming
weekend, as a polar trough will build and spread across the western
Atlantic. This will be supported by a cold front which is expected
to approach the region by the end of the week. The associated frontal
boundary is forecast to bring an increase in low leve moisture and
allow a better chance for shower development across the islands and
coastal waters.


.SHORT TERM...Today through Thursday...
A mid to upper level ridge is forecast to build over the islands
through mid-week. Low level winds will continue from the southeast
today, in response to a dissipating shear line north of the region.
TPW values will remain near the normal for March at least until
Wednesday. Although little or no shower activity is expected during
the morning, under a light southeasterly wind flow sea breezes are
expected to aid in the development of showers mainly across the
interior and northwest portion of Puerto Rico during the afternoon.
Also, shower development would remain possible downwind from the
U.S. Virgin Islands.

Winds are forecast to back from the east on Wednesday, as a surface
high pressure intensify over the Eastern Atlantic. The ridge aloft
will inhibit shower activity through much of the day, but the
available moisture will be enough to result in isolated to scattered
showers across the interior and western areas of Puerto Rico.

Thursday, the vertical stacked ridge will continue to promote fair
weather earlier on Thursday. Then, an approaching cold front with a
pre-frontal trough will combined with a warm tropical air mass to
increase the chance of shower development across the northern coast
of Puerto Rico.

.LONG TERM...Fri thru Wed...
Weakly induced surface trough will maintain a light southeasterly
wind flow across the region through Friday as a cold front will
continue to move across the western Atlantic and approach the region
from the northwest. Light southeast wind along with available moisture
convergence and diurnal effects will give way to afternoon showers
mainly over parts of the central interior and northwest sections of

By Saturday and Sunday winds are to become more east to northeast then
dominantly northeasterly Monday through Wednesday, as a surface high
pressure ridge will build and spread across the western Atlantic. Meanwhile,
the remnants of the aforementioned cold front/frontal boundary and associated
moisture field is so far forecast to sink southwards across the
region over the upcoming weekend and bring an increase in cloudiness
and shower activity.

The mid to upper level high pressure ridge is forecast to erode by Friday
and through the weekend as a polar trough will deepen and move across
the west and central Atlantic. This will create a high zonal/westerly
wind flow and provide good ventilation and instability aloft, while
eroding the trade wind cap inversion over the upcoming weekend. Therefore
for the weekend, expect the combination of the prevailing northeast
winds, upper level instability and low level moisture convergence to
keep the islands and regional waters in an unsettled weather pattern
with increasing showers especially during the early morning and afternoon

Tuesday thorough Wednesday mid to upper level ridge will build across
the southwest atlantic, while a surface high pressure is forecast to
set up north of the region. This will tighten the local pressure gradient
and thus increase the northeast trade winds resulting in occasional
patches of shallow low level moisture advection across the coastal
waters and parts of the islands. The shower activity should however
be mainly during the early morning and late afternoon hours with
mostly fair weather skies to prevail once again during the daytime
as the building ridge aloft will suppress convective development.


.AVIATION...VFR conds are expected to prevail through the fcst
period. Showers and cloud are expected to form across the interior
and NW coast of PR btwn 20/16-22z. Which could bring SHRA/-SHRA and
brief MVFR conds at TJBQ and VCSH at TJMZ. Sfc winds will be calm to
light and vrb becoming from the SE at 10 kt with sea breeze
variations aft 20/14Z.


.MARINE...Tranquil marine conditions will prevail across the local
waters during the next few days with seas of 2 to 5 feet and east to
southeast winds between 10 to 15 knots. Winds and seas are expected
to increase by the upcoming weekend as a surface high pressure ridge
builds north of the region and a northerly swell reaches the regional
waters and some of the local passages.


SJU  87  75  88  75 /  10  10  20  50
STT  85  74  86  76 /  10  10  20  50




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