Area Forecast Discussion
Issued by NWS San Juan, PR

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000
FXCA62 TJSJ 200032
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
832 PM AST Sat Aug 19 2017

.UPDATE...Doppler radar detected scattered to numerous showers
and isolated thunderstorms along and north of Cordillera Central,
across southeastern sections of Puerto Rico as well as across the
surrounding coastal waters this afternoon. Several Flood
Advisories and Special Weather Statement were issued for
municipalities across those sectors of the island. Several Marine
Weather Statements were also issued due to frequent lightnings
across the surrounding waters. For tomorrow, an area of low
pressure, Invest 92L, is expected to pass mainly north of the
local area. However the moisture provided by this system in
occasional seas up to 5 feet. Dominant Period 7 seconds. Isolated
combinations with daytime heating and local effects will aid in
the development of shower and thunderstorm activity across
central interior and western sections of Puerto Rico during the
afternoon. Scattered showers and a couple of thunderstorms are
also expected to affect Vieques, Culebra and the U.S. Virgin
Islands.

&&

.AVIATION...Mostly VFR conditions are expected overnight across
the local flying area. However, brief periods of SHRA will affect
the Leewards, USVI and TJSJ TAF sites. Winds from the ESE are
expected to decrease at 10-15 knots after 19/23Z, increasing again
at 15-20 Knots with higher gusts after 20/13Z. Periods of MVFR
conditions with mountain obscurations are expected across TJBQ
and TJMZ from 20/16Z through 20/22Z in SHRA/TSRA.

&&

.MARINE...Tropical Depression Harvey will continue to move
westward and away from the forecast area tonight. This will
continue to result in hazardous seas of 6 to 8 feet and winds up
to 20 knots particularly across the Caribbean waters and local
passages. Hazardous seas will continue to prevail across the
offshore Atlantic waters through Monday as an area of low pressure
east northeast of the Leeward Islands moves to the northeast of
the forecast area.

&&

.PREV DISCUSSION... /issued 531 PM AST Sat Aug 19 2017/

SYNOPSIS...Doppler radar and Satellite images continued to
depict areas and clusters of showers/thunderstorms on the
periphery of what is now Tropical Depression Harvey, propagating
northwards across the forecast area. This added moisture along
with good forcing and unstable conditions aloft aided in the
development of scattered to numerous showers and isolated
thunderstorms across the islands and coastal waters especially
over and around Puerto Rico. Expect this trend to continue through
at least the early evening hours, with a gradual diminishing of
the convective activity over much of the land areas by late
evening.

SHORT TERM...Overnight though Monday...Showers and thunderstorm
activity across the region should continue to diminish over land
areas and the coastal waters by late evening, as the recently
downgraded Tropical Depression Harvey will move farther westward
into the Central Caribbean waters. The trade winds are then expected
to become more east to southeast but will continue to transport an
elongated plume of low level moisture across the region, as another
tropical wave/area of low pressure lifts northwards and just northeast
of the region. This additional moisture and accompanying shower activity
will reach parts of the north and east sections of the islands during
the early morning hours on Sunday. For the rest of the day on Sunday,
expect good moisture convergence, in combination with daytime heating
and local effects, as well as the marginal instability aloft to support
periods of scattered to numerous showers with isolated thunderstorms
during the afternoon. The activity should be focused mainly across the
Cordillera Central and northwest sections of Puerto Rico including
parts of the San Juan metro area. By Monday afternoon, at this time,
showers and thunderstorms are expected to develop mainly north of the
Cordillera Central of Puerto Rico with only quick passing streamer like
shower activity expected elsewhere.

LONG TERM...Tuesday through Sunday...As moisture associated with
area of low pressure/invest 92L moves away, a drier air mass in expected
to encompass the forecast area on Tuesday. This drier air mass is expected
to be short-lived as a mid to upper level trough is forecast to establish
just north of the forecast area. This along with available moisture
will enhance convective development across the region Wednesday through
Thursday. By Friday and into the upcoming weekend, a mid level ridge
is then expected to build across the central Atlantic and into the
northeast Caribbean to result once again in moisture erosion and stable
conditions across the region. Therefore a more seasonal weather pattern
is expected to prevail with locally induced showers and thunderstorms
each day.

AVIATION...Mostly VFR conditions are expected overnight across the
local flying area. However, brief periods of SHRA will affect the
Leewards, USVI and TJSJ TAF sites. Winds from the ESE are expected
to decrease at 10-15 knots after 19/23Z, increasing again at 15-20
Knots with higher gusts after 20/13Z. Periods of MVFR conditions
with mountain obscurations are expected across TJBQ and TJMZ from
20/16Z through 20/22Z in SHRA/TSRA.

MARINE...Recently downgraded Tropical depression Harvey will
continue to move westward and exit the eastern Caribbean waters
later tonight. However expected hazardous seas between 6 to 8 feet
and winds between 15 to 20 particularly across the Caribbean waters
and local passages. Hazardous seas will continue to prevail across
the offshore Atlantic waters through Monday as an area of low pressure
east northeast of the Leeward Islands moves to the northeast of the
forecast area.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  79  90  80  89 /  40  60  40  30
STT  81  88  81  90 /  50  50  50  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 AM AST Sunday for Central Interior-
     North Central-Northeast-Northwest-Southeast-Southwest-
     Western Interior.

VI...None.
AM...High Rip Current Risk until 6 AM AST Sunday for Coastal Waters
     OF Southwestern Puerto Rico out 10 NM-Coastal Waters of
     Southern Puerto Rico out 10 NM.

     Small Craft Advisory until 2 AM AST Sunday for Caribbean Waters
     of Puerto Rico from 10 NM to 17N-Mona Passage Southward to
     17N.

     Small Craft Advisory until 2 PM AST Monday for Atlantic Waters
     of Puerto Rico and USVI from 10 NM to 19.5N.

&&

$$

SHORT TERM...JF
LONG TERM....99


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