Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 190831

Area Forecast Discussion
National Weather Service San Juan PR
431 AM AST Sat Jan 19 2019

Surface high pressure across the northeast Atlantic and a weakening
low pressure system north of the area moving eastward across the
western Atlantic will help to maintain light to moderate east to
northeast wind flow. A mid to upper level ridge will continue to
build northwards across the region from the central Caribbean while
maintaining stable conditions aloft. A cold front and the associated
area of low pressure is forecast to enter and move across the western
Atlantic Sunday through Monday.


.SHORT TERM...Today through Monday...

Drier than normal air mass will prevail over the local area for the
next few days. There will be a few patches of moisture across the
area that will bring slightly higher moisture, but it is not
expected to have that big of an impact other than scattered showers
over the local waters and around the USVI. In the afternoon hours
for the next few days, showers are expected to develop across
western PR, but mainly locally induced. The high resolution models
indicate decent amounts of rain today and on Monday, but less rain
on Sunday because of even drier air moving in. Nevertheless, some
showers are forecast each afternoon across portions of western PR.
There is no particular synoptic feature at the surface or the upper
levels that would help enhance shower development. The forecast
soundings indicate that the little moisture there is will be limited
to the lower levels, and the other instability parameters indicate a
fairly stable atmosphere is expected. Therefore, the main pattern
will be mostly fair weather with a few brief and mainly light
showers across the local waters, near the USVI and eastern PR in the
morning and overnight hours, then locally induced showers across
portions of western PR in the afternoons.

.LONG TERM....Tuesday through Saturday...

Overall dry and stable conditions is forecast to continue at
least through Wednesday, as the mid to upper level ridge will
build and extend northward across the region. This in turn will
erode low- level moisture convergence and at the same time strengthen
the trade wind cap inversion. Model guidance also continued to suggest
decreasing layered precipitable water values to less than an inch
at least until Wednesday. By the latter part of the week the local
pressure gradient is to tighten across the region as the surface
high builds north of the forecast area while spreading across the
west and central Atlantic. This will cause an increase in the east
to northeast trade winds and consequently increase the moisture
advection from an old frontal boundary which is forecast to sink
southward into the tropical Atlantic. As a result, an increase in
moisture transport is forecast for the latter part of the week and
into the weekend with breezy conditions and periods of quick
passing clouds and showers during the overnight and early morning
hours. During the daytime, isolated to scattered afternoon showers
will remain possible mainly over parts of central and west to
northwest sections of PR, and mostly downwind of the USVI.


.AVIATION...VFR conds expected for the local terminals for the next
24 hours, with the exception of possible MVFR conds at some point
between 19/19Z and 19/22Z at TJMZ. Winds will be light and variable
through 19/13Z, thereafter winds will become mainly easterly at
around 10-15KT with sea breeze variations.


.MARINE...Marine conditions will remain unchanged through the
weekend with seas of 2 to 4 feet, with easterly winds of 10 to 15
kts. A northerly swell will continue to fade across the Atlantic waters
and local passages today while another long Dominant period northwesterly
swell is forecast to arrive across the local Atlantic Waters by Tuesday.
A low to moderate risk of rip currents for the northern beaches
of Puerto Rico and the U.S. Virgin Islands.


SJU  83  73  85  74 /  10  20  20  20
STT  82  72  83  73 /  10  20  20  20




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