Area Forecast Discussion
Issued by NWS San Juan, PR
FXCA62 TJSJ 280947
Area Forecast Discussion
National Weather Service San Juan PR
547 AM AST WED SEP 28 2016
.SYNOPSIS...Patch of drier air is expected to move into the local
area today, however there should still be enough moisture to cause
showers and thunderstorms in the afternoon across western and
central PR. SFC high pressure across the north central Atlantic
will keep a moderate easterly wind today. Strong tropical wave
known as Invest 97L is now about 150 miles east of Barbados and
it is expected for the invest to continue moving west at about
15-20 mph. National Hurricane Center expects Invest 97L to become
a tropical cyclone later today. As the tropical wave moves into
the Caribbean waters, the local marine conditions will deteriorate
this afternoon through Friday increasing both winds and seas.
Weather conditions may also deteriorate on Thursday.
.DISCUSSION...For today...a small area of relatively drier air is
expected to move into the local islands, decreasing the
precipitable water values to roughly 1.5 inches by this afternoon.
However, there is still enough moisture to cause the development of
showers and thunderstorms across the interior and western sections
of PR in the afternoon once the moisture combines with the diurnal
heating and local effects. The big story though is that the
expected weather and marine conditions are expected to deteriorate
tonight across the local area due to the proximity of Invest 97L
which at this time is still a strong Tropical Wave. Invest 97L is
expected to bring a significant increase in sustained wind speeds
and shower and thunderstorm activity. The local winds are expected
to increase across the local waters late in the afternoon with
coastal winds across the Anegada Passage and the Offshore
Caribbean waters possibly observing winds of 20-25 knots, then
late tonight, shower activity is expected to increase across the
local islands, affecting the USVI and eastern PR. This increase in
shower and thunderstorm activity is expected to continue through
Thursday, particularly affecting the eastern and northern sections
of PR on Wednesday night and through Thursday, while western PR
may observe the activity mainly in the late morning into the
afternoon. Passing showers and thunderstorms will move around the
USVI, some of which will affect the islands. The overall moisture
will increase significantly on Thursday but improve by Thursday
night into Friday morning, which may cause the shower and
thunderstorm activity to diminish at that time. Also, the
sustained winds are currently forecast to be up to around 25 knots
or slightly higher starting late tonight into Thursday, but there
is a chance that winds would be slightly higher even as guidance
suggested as much as 36 knots over the waters. We decided to tone
down the wind speeds for our forecast since there is currently no
observation that would confirm that the models are initializing
well, every surface observation we saw suggested at least 10 knots
under what the models were indicating for this time, so our
forecast is currently under what the latest guidance is
suggesting, appropriate changes to the forecast will be made if
Depending of the track and intensity of this tropical
disturbance, we could see an increase in the overall moisture
during the weekend and into the early part of the upcoming week.
So at this time there is considerable uncertainty as to what would
happen this weekend and early next week, but everything seems to
indicate that a persistent area of deep moisture will drape over
the local islands through at least Wednesday. This set up would cause
good shower and thunderstorm activity over the local forecast area
for several consecutive days.
.AVIATION...VFR conds expected across all TAF sites through 28/12z.
MVFR conds possible at TJMZ with MTN Top obscurations near
convection along interior and west PR between from 28/17Z-22Z. Winds
incrg aft 28/14Z bcmg E 15 to 20 with gusty winds up to 30kt.
.MARINE...Small craft advisories will go into effect this
afternoon across many of the local waters and then most of the
local waters will be under a small craft advisory starting this
evening. Seas are expected to start deteriorating this afternoon
with seas gradually increasing to 7 feet late this afternoon and
then up to 8 feet by this evening, continuing to deteriorate,
possibly up to 14 feet with occasional higher seas across the
offshore Caribbean waters. There is a moderate risk of rip
currents across most of the local beaches but that will change
quickly to high risk of rip currents this evening as the local
seas and winds increase.
.PRELIMINARY POINT TEMPS/POPS...
SJU 91 80 89 78 / 20 50 50 30
STT 92 80 89 79 / 20 60 60 40
PR...Small Craft Advisory from 8 PM this evening to 8 PM AST Friday
VI...Small Craft Advisory from 8 PM this evening to 8 PM AST Friday
for St.Thomas...St. John.. and Adjacent Islands.
AM...Small Craft Advisory from 2 PM this afternoon to 8 PM AST Friday
for Anegada Passage Southward to 17N-Atlantic Waters of
Puerto Rico and USVI from 10 NM to 19.5N-Caribbean Waters
of Puerto Rico from 10 NM to 17N-Coastal Waters of Northern
USVI and Culebra out 10 NM-Coastal Waters of Southern USVI
Vieques and Eastern Puerto Rico out 10 NM.
Small Craft Advisory from 8 PM this evening to 8 PM AST Friday
for Coastal Waters of Northern Puerto Rico out 10 NM-
Coastal Waters of Southern Puerto Rico out 10 NM-Mona
Passage Southward to 17N.