Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 230029

Area Forecast Discussion
National Weather Service San Juan PR
829 PM AST Thu Jun 22 2017

.UPDATE...Limited shower activity was observed during the afternoon
and early in the evening as an upper level ridge remains as the
main weather feature over the local islands. Only few showers
developed this afternoon mainly across the southwest section of
Puerto Rico, but this activity dissipated rapidly after sunset.
Latest satellite images continue to indicate a drier airmass
across the local islands. This dry air mass is expected to remain
over the region until at least Saturday, limiting the showers
development across the forecast area. A tropical wave is expected
to reach the region late Sunday and Monday, resulting in an
increase in cloudiness, showers and possible thunderstorms across
the region. No changes were needed to forecast grids tonight.


.AVIATION...Mostly VFR conditions expected to continue across the
local terminals throught the forecast period. Only VCSH possible
across the Leeward and, USVI and eastern Puerto Rico TAF sites as
small areas of moisture embedded in the trades move across the
region overnight and early Friday morning. Winds from the ENE
will diminish at around 10 KT overnight, but increasing once
again to around 15KT with occasional gusts after 23/14Z.


.MARINE...Small craft should exercise caution due to winds up to
20 knots across some coastal waters. Moderate risk of rip
currents across most of the local beaches in PR and the USVI.


.PREV DISCUSSION... /issued 408 PM AST Thu Jun 22 2017/

SYNOPSIS...Mid to upper level ridge will linger across the region through
Friday. Ridge aloft will erode over the weekend as a TUTT low will develop
and meander northeast of the region through early next week. Tropical
moisture is forecast to increase by Sunday with the arrival of a tropical
wave. A large area of Saharan dust is forecast to arrive across the
region by early next week. Broad surface high ridge will continue across
the west and central Atlantic through the upcoming weekend to promote
moderate east to southeast trade winds.

SHORT TERM...Rest of today through Saturday...A mid to upper
level ridge will prevail across the region through Friday. This
feature is expected to gradually erode beginning on Saturday, as
a mid to upper upper trough will develop and linger northeast of
the forecast area. Until then, the ridge aloft will support limited
moisture advection through Friday with recent model guidance
continuing to suggest precipitable water values below the normal
range. At lower levels, the broad high pressure ridge spread across
the Atlantic will persist through the forecast period resulting in
moderate east to southeast winds.

For the rest of the afternoon, diurnally induced showers will continue
to affect portions of the interior and southwest sections of Puerto
Rico. Expect this activity to diminish just around sunset leaving
partly cloudy to mostly clear skies. The remainder of the islands
should continue to experience mostly fair weather skies with only
a few quick passing showers.

Lesser trade wind moisture transport and shower activity is expected
on Friday, as the ridge aloft and trade wind inversion will limit
convection across the region. However, pockets of lingering
moisture along with local and diurnal effect will lead to
afternoon showers with a slight chance of a thunderstorm, mainly
over west and southwest Puerto Rico. Mostly fair and stable
weather conditions is so far expected elsewhere. On Saturday...a
weak surge of moisture in the trade winds will bring early morning
passing showers across the coastal waters and portions of the
islands. This will be followed by diurnally induced afternoon
showers, and possible isolated thunderstorms mainly over the west
sections of Puerto Rico.

LONG TERM...Sunday through Friday...
TUTT low northeast of the region will create a trof pattern across
the northeastern Caribbean and over the islands through early next
week with the upper trough to retrogress across the local islands
by Thursday and Friday.  A tropical wave is forecast to reach the
eastern caribbean and forecast area late Sunday or early Monday.
This will bring an increase in tropical moisture to the region. Upper
level support from the lingering TUTT/trof will further enhance convective
development especially across the mountain ranges and western sections
of PR. Another extensive area of Saharan dust is also forecast to reach
and spread across the region by early next week following the tropical
wave. This will result on a drier airmass and hazy skies. However,diurnally
induced afternoon convection will remain possible each day. Another
tropical wave is forecast to reach the region by Thursday and Friday.

AVIATION...VFR conds expected across the local terminals throught
the forecast period. VCSH possible as SHRA stream off the islands
this afternoon and on ISOLD tradewind showers overnight. Winds will
remain from the ENE at around 15 KT with sea breeze variations
through 23/00Z, becoming lighter overnight but increasing once again
to around 15KT with occasional gusts after 23/14Z.

MARINE...Small craft should exercise caution due to seas occasionally
of up to 6 feet and winds up to 20 knots. Moderate risk of rip currents
across most of the local beaches in PR and the USVI.


SJU  78  89  77  89 /  30  30  30  30
STT  79  89  78  88 /  30  30  50  50




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