Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 201739

Area Forecast Discussion
National Weather Service San Juan PR
139 PM AST Sun Aug 20 2017

.UPDATE...Moisture associated with a surface of low pressure
located a couple of hundred miles north of Puerto Rico and St
Thomas induced showers and thunderstorms late this morning into
the afternoon hours. Flooding, frequent lightning and gusty winds
have been observed across the local waters and portions of PR and
the U.S. Virgin Islands. This activity is expected to continue
at least until evening hours. No changes were made to the
inherited forecast package.


SHRA/TSRA expected to affect TJSJ/TJBQ/TJMZ at least until 20/23z
as well as the VCTY of TIST/TISX/TJPS. This activity is associated
with a surface low pressure a couple of hundred miles to the
north of Puerto Rico. SHRA expected to slowly diminish overnight.
E to ESE Winds at 10 to 15 knots with higher gusts in/near
SHRA/TSRA dropping at 10 knots or less after 20/23z.


Hazardous marine conditions are expected to continue across the
the local waters due to thunderstorms, gusty winds and lightning.
Mariners can expect seas up to 7 feet across the Atlantic Offshore
waters, elsewhere at 6 feet or less.


.PREV DISCUSSION... /issued 515 AM AST Sun Aug 20 2017/

SYNOPSIS...Weak waves will continue to pass through the area even
as high pressure weaken after Monday. High pressure will return to
the northeast central Atlantic to reinvigorate the easterly trade
winds. Bands of good moisture will accompany the tropical waves
bringing scattered showers to the area. The best moisture of the
week is expected today.

SHORT TERM...Today through Tuesday...
Scattered showers were observed across the U.S. Virgin Islands,
Vieques, Culebra as well as across the eastern and northern
sections of Puerto Rico during the overnight hours. This activity
was associated with the leading edge of an area of low pressure
which was located north of the U.S. Virgin Islands early this
morning. This area of low pressure is producing a big area of
showers and thunderstorms extending for a few hundred miles
eastward. Latest satellites images indicated plenty of moisture
associated with this system and that will continue to propagate
westward throughout the day. As a result, the combination of the
available moisture with strong daytime heating and orographic
effects will be sufficient to produce scattered to numerous
showers with thunderstorms this afternoon across most of the
islands. The heaviest activity will occur across the San Juan
metropolitan area and across the Cordillera Central of Puerto
Rico. As prevailing winds shift more southeasterly, the shower
and thunderstorm activity will move northwest affecting mainly the
northern half of Puerto Rico later in the afternoon hours.
Periods of heavy rainfall with frequent lightning and strong gusty
winds can be expected. Urban and small stream flooding will be
likely this afternoon across these areas.

By late tonight into Monday, a drier air mass is expected to move
across the region from the east. Therefore, limited shower activity
is expected to occur during this period. Shower and thunderstorm
development can be expected once again Monday afternoon, but only
across the western interior and west sections of Puerto Rico. At
this time, looks like the rest of Puerto Rico will only see some
passing showers from time to time. An area of low level moisture is
expected to affect the region Tuesday.

LONG TERM...Wednesday through Sunday...
A north-south oriented TUTT will move through the southern
Caribbean during the last part of the week but winds aloft will
remain mostly below 20 knots in the forecast area. The next
tropical wave moves through on Wednesday and another band of
moisture moves through on Saturday followed by another wave on the
Sunday after. This, local effects, and generally favorable low-
level moisture will keep scattered showers and thunderstorms in
the forecast area-wide with showers and thunderstorms likely each
afternoon in western Puerto Rico, but upper level dynamics will
remain limited through the period, offering little support for
more organized convection. Current GFS does not develop any more
tropical cyclones east of the forecast area through Sunday.

AVIATION...Mainly VFR conditions will prevail across most TAF sites
through at least 20/16Z. Periods of MVFR conditions can be expected
in and around TNCM and TKPK through at least 20/18Z. Meanwhile,
Periods of MVFR conditions with mountain obscurations can be
expected across TJSJ, TJMZ and TJBQ from 20/16Z through 20/22Z in
SHRA/TSRA. VCSH can be expected elsewhere. Winds becoming east
southeast below FL130 til 21/00Z. Max winds till then E at 25 kt
from FL320-400.

MARINE...Seas still at or above 7 feet at the outer buoy and
likely extending south in the forecast area this morning, but will
subside this afternoon. Seas will reach a minimum by Thursday and
are expected to be below 4 feet everywhere then. Winds increase
somewhat behind a passing ridge on Thursday causing seas to


SJU  79  89  79  88 /  40  30  30  30
STT  81  90  81  90 /  50  40  40  30


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



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