Area Forecast Discussion
Issued by NWS San Juan, PR

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908
FXCA62 TJSJ 220047
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
847 PM AST Sat Sep 21 2019

.UPDATE...Mainly fair weather conditions with very limited shower
activity were observed across the local area throughout the day.
For tonight and during the day on Sunday, a band of moisture
associated with Tropical Storm Jerry located well north of the
area will move across the region. This will generate scattered
showers across the regional waters during the overnight and
morning hours of Sunday with some of this activity possibly
affecting portions of eastern and southern PR and the USVI. At
this time, significant rainfall accumulations are not expected.
Then, during the afternoon hours on Sunday, this moisture will
combine with daytime heating and sea breeze convergence to result
in a round of showers and thunderstorms across northwest Puerto
Rico.

A drier air mass will follow later Sunday through most of Monday,
with more limited shower activity expected. Attention then turns
to a strong tropical wave, located approximately 100 miles east
of Barbados as of this evening. This disturbance is expected to
move westward and then northward during the next couple of days,
reaching the local area by Tuesday. There is still a high degree
of uncertainty as to whether this disturbance will be tropical
wave or a weak tropical cyclone as it approaches. The National
Hurricane Center is now giving the wave a 70% of development into
a tropical cyclone during the next 5 days. Independently as to
whether or not this wave develops into a tropical cyclone, the
main threat at this time appears to be heavy rainfall activity
affecting a majority of the forecast area Tuesday and into
Wednesday.

&&

.AVIATION...Mainly VFR conditions expected across most of the
area. VCSH are possible across the Leeward Islands, USVI, and
eastern PR terminals through 22/14z. SHRA/TSRA expected across
western and northwestern PR between 22/16z and 22/22z, possibly
affecting the vicinity of TJMZ and TJBQ. Winds are expected to be
light and variable overnight, increasing to 10 to 15 knots from
the ESE-SE with sea breeze variations developing after 22/14z.

&&

.MARINE...Northerly swell associated with Tropical Storm Jerry
located well north of the forecast area continues to produce
hazardous marine conditions with seas continuing to run greater
than 7 feet with 12-14 second period. Even though the swell is
expected to gradually diminish overnight and during the day on
Sunday, seas will continue to run near 7 feet, especially across
the Atlantic waters. Therefore, small craft and high surf
advisories will continue in effect through Sunday. There will also
be a moderate to high risk of rip currents across most of the
local beaches, except for those beaches located near Mayaguez and
Vicinity and most of the southern coast of Puerto Rico.


&&

.PREV DISCUSSION... /issued 1134 AM AST Sat Sep 21 2019/

SYNOPSIS...The risk of coastal flooding and high surf will
gradually diminish through the weekend. Jerry continues to move
northwest and drier air will move in behind it. A tropical wave is
showing signs of development, but considerable uncertainty remains
as to its final intensity once it moves into the local area.
Generally moist conditions persist through the rest of next week.

SHORT TERM...The rest of today through Monday...
The outer rain bands of Jerry are not showing in the local area
and Tropical Storm Jerry continues to move northwest and away from
the area. Models are very aggressive with developing the moisture
in the feeder band that extends into the storm across the Anegada
Passage, but currently little has formed in the area and no rain
is present over land. The best showers in the area are south of
Saint Croix. Tops are just less than 30 kft. The current sounding
shows modest instability with a minus 4 lifted index and a
forecast surface based CAPE of 1800 j/Kg, but moisture in the
21/12Z sounding is limited from 5-16 kft and a dry layer overlays
this for nearly 10 kft more. Models are widely divergent on
rainfall with some very aggressive in bringing numerous showers
over the area to the southeast, over the U.S. Virgin Islands and
along the north coast of Puerto Rico this afternoon, while the
high resolution models seem to only favor convection over the
north coast and scattered across the waters to the east of Puerto
Rico. The WRF was the least aggressive and given the amount of
convection now visible have limited the shower activity to
scattered this afternoon. Upper levels have areas of weak
divergence aloft, so this will not be a major factor. Some heavy
showers will be possible across the north coast where winds should
turn northerly as sea breeze influences overcome the general
southerly flow. Also the southerly flow is bringing very warm
temperatures to much of the north coast of Puerto Rico. The record
for the date in san Juan is 94 degrees and temperatures may indeed
approach that high value.

The models all continue to bring in considerable amount of
moisture across the eastern half of our forecast area tonight,
and some shower activity appears to be approaching the Windward
Islands. The showers that were approaching the Leeward Islands
have dissipated for the most part despite the precipitable water
values of two or more inches.

Therefore have toned down expectations for numerous showers or a
vigorous feeder band into Jerry like Dorian had from a similar
location. Rainfall that does occur in the U.S. Virgin Islands and
eastern Puerto Rico should be manageable and even beneficial.
There is a dry area approaching behind the main moisture area
approaching tonight and Sunday from above 850 mb to above 600 mb
that should greatly limit convection over the area except in the
northwest portion of Puerto Rico.

LONG TERM...Monday night through Saturday...
The long-term period is unsettled as a result of a vigorous
tropical wave that is forecast to move into the southern Caribbean
Caribbean waters Sunday night. The tropical wave and its
associated moisture is expected to propagate to the northwest and
bring an increase in showers and thunderstorms to eastern Puerto
Rico and the U.S. Virgin Islands late Monday into Tuesday. The
moisture will continue to spread across the interior and western
areas of Puerto Rico Tuesday afternoon. Guidance shows the bulk
of the moisture associated with the tropical wave arriving
Tuesday. Moisture trails through the area until Wednesday night.
Timing is quite uncertain and there is, according the the
National Hurricane Center a 50 percent chance of tropical cyclone
formation. Several inhibiting factors are currently working in our
favor that would limit development. First, the tropical wave will
be moving into a environment of higher shear (up to 40 knots
between 200-250 mb), and a drier air mass is currently ahead of
the tropical wave. The confidence in the long-term forecast and
the resulting precipitation is low for Tuesday and Wednesday due
to the uncertainty regarding the tropical wave. The end of the
workweek shows a northerly wind flow with tropical moisture that
was originally in the Caribbean waters being pulled back across
the forecast area. This will result in an extended wetter than
usual period across the area in any case.

AVIATION...Mainly VFR conditions are expected across PR, the
USVI, and the Leeward Islands. Brief MVFR conds are possible
across the northwest portion of PR. Southerly flow at the surface
will be dominated by sea breeze influences and generally 6-12
knots until 21/21-22Z. Maximum winds NE-E around 20 kt between
FL420-540.

MARINE...Swell off of Tropical Storm Jerry will continue to
maintain hazardous seas through at least Sunday night, although
some improvement is expect tonight and Sunday. Seas from a system
approaching from the southeast may also bring 7 foot seas into
the Caribbean waters and local passages Monday and Tuesday.
Mariners should monitor condition there for possible tropical
cyclone development.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  80  92  80  91 /  30  30  10  30
STT  81  89  82  89 /  50  30  10  50

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late Sunday night for Culebra-
     Mayaguez and Vicinity-North Central-Northeast-Northwest-San
     Juan and Vicinity-Vieques.

     High Surf Advisory until 6 AM AST Sunday for Culebra-Mayaguez
     and Vicinity-North Central-Northeast-Northwest-San Juan and
     Vicinity.

VI...High Surf Advisory until 6 AM AST Sunday for St.Thomas...St.
     John...and Adjacent Islands.

     High Rip Current Risk through Sunday afternoon for
     St.Thomas...St. John...and Adjacent Islands.

AM...Small Craft Advisory until 6 PM AST Sunday for Coastal Waters of
     Northern Puerto Rico out 10 NM.

     Small Craft Advisory until 8 AM AST Monday for Anegada Passage
     Southward to 17N-Atlantic Waters of Puerto Rico and USVI
     from 10 NM to 19.5N.

     Small Craft Advisory until 2 AM AST Sunday for Coastal Waters of
     Northern USVI and Culebra out 10 NM-Mona Passage Southward
     to 17N.

&&

$$

SHORT TERM...GL
LONG TERM....CAM



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