Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 140116

Area Forecast Discussion
National Weather Service San Juan PR
916 PM AST Sun Jun 13 2021


Once again, calm weather conditions will prevail tonight with a
few passing showers across the local waters and windward sections
of the U.S. Virgin Islands and PR. Satellite data detected an
upper cloud layer over the region and an approaching tropical wave
near the Lesser Antilles. We are expecting calm weather overnight
and tomorrow morning. However, the tropical wave will reach the
islands by late tomorrow afternoon, increasing the available



VFR conditions will prevail overnight. Light and variable winds
14/12z, returning from the E at 10 to 15 knots afterward.
Afternoon convection will form between 14/17-22z across the
interior and west-PR.



Seas are relatively tranquil to slightly choppy. Winds will
remain easterly up to around 15 knots. Seas of 5 feet or less
are expected across the local waters.

Beachgoers, there is a moderate risk of rip currents for the
eastern coast of Culebra, east and south Vieques and St Croix,
and the southeast coast and north-facing beaches of Puerto Rico.


.PREV DISCUSSION... /issued 332 PM AST Sun Jun 13 2021/

A seasonable weather pattern will continue through Wednesday,
with passing showers during the early morning and afternoon
convection developing due to local and diurnal effects. Due to a
relatively drier air mass organized convection is not expected
through Wednesday.

SHORT TERM...Tonight through Tuesday...
Scattered showers and isolated thunderstorms developed across the
western areas of Puerto Rico this afternoon, this activity is
producing moderate to heavy rainfall and frequent lightning. A few
flood advisory are possible due to the moderate to heavy
rainfall. The showers and isolated thunderstorms will continue
through sunset, mainly across portions of the interior and
western areas of Puerto Rico. Dry Saharan air will advect over the
local waters and islands tonight through tomorrow, ahead of an
approaching tropical wave currently east of the Leeward Islands.
The dry Saharan air will inhibit organized convection from
developing on Monday, but local and diurnal effects will yield
shallow convection in isolated to scattered showers across the
islands. Global models keeps the tropical wave well south of the
local area; however, the sheared tropical wave mid and upper-
level clouds could pool over the local islands and cause cloudy

Tuesday, Dry Saharan air mass will linger over the region but,
shallow patches of moisture embedded in the easterly trades will
yield isolated to scattered showers during the early morning
across eastern and northern Puerto Rico and the U.S. Virgin
Islands. Then over portions of the interior and western Puerto
Rico during the afternoon.

PREV DISCUSSION... /issued 636 AM AST Sun Jun 13 2021/

LONG TERM...Wednesday through Monday...
A surface high pressure across the north central
Atlantic will yield east southeasterly winds across the local
islands midweek through the weekend. The forecast for the long-
term period continues mostly on track with Wednesday and early
Thursday looking warm and somewhat dry with a 500 mb high pressure
ridge and hazy skies due to the presence of Saharan dust
particulate. At lower levels, moisture will reach below normal
levels, however, enough will be present for daytime heating and
local effects to produce showers and possibly some isolated
thunderstorms mainly over the northwestern quadrant of Puerto Rico
during afternoon hours.

Slight discrepancies exist between the GFS and ECMWF models by
Thursday onwards. GFS shows two easterly waves affecting the area;
one arriving late Thursday through early Saturday and the other
moving in by Sunday through Monday. Conversely, ECMWF has a more
continuous transition between both easterly waves but much weaker
intensity affecting the area Friday through Monday. Both models
show 500 mb temperatures between -6 to -8 degrees, normal to
slightly above normal levels for this time of year. Dynamically
speaking, upper-level conditions are not extraordinary but enough
to aid in convection development with sufficient available deep
layer moisture. Precipitable water values will increase to more
than 2.0 inches with each easterly wave event, raising moisture to
above normal levels.

Despite model discrepancies, we could expect the wettest days of
the long-term forecast to occur by Friday through the weekend.
Thus, shower and thunderstorm activity may increase with each wave
passage, raising the available moisture. The moisture will
combine with diurnal heating and local effects to cause more
significant shower or thunderstorm development across portions of
Puerto Rico, but also the waves will bring shower activity to the
local area overall, affecting the USVI as well.

Mainly VFR conds expected across the local terminals.
SHRA/TSRA activity may be observed near TJBQ through 13/23Z,
resulting in brief MVFR conditions due to lower ceilings and reduced
visibility. Thereafter, SHRA will result in VCSH at TJSJ, USVI and
Leeward terminals. Winds will be from the E at 10-20 knots with sea
breeze variations through 13/23Z, becoming light and variable for PR
terminals through 14/13Z.

Tranquil marine conditions will continue through tonight, with
seas up to 5 feet and winds up to 15 knots. Monday winds will
increase to 20 knots and cause choppy conditions across the
Caribbean and Atlantic waters. Moderate risk of rip currents will
continue over the next several days across most of the local
beaches of Puerto Rico and the U.S. Virgin Islands.


SJU  88  78  89  78 /  30  30  30  20
STT  89  79  89  78 /  20  30  20  20




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