National Weather Service San Juan PR
425 AM AST Wed Sep 30 2020
.SYNOPSIS...A tropical wave is passing through the local area
today, causing an increase in shower and thunderstorm activity
today across the local area. Surface high pressure across the
north central Atlantic will cause moderate to locally fresh east
to east southeast winds for the next several days. Upper trough
will gradually strengthen over the next few days, causing an
increase in instability across the local area.
&&
.SHORT TERM...Wednesday through Friday...
A surface high pressure over the western Atlantic will continue to
strengthen, tightening the gradient over the islands through the
next few days. This will cause winds to increase, especially over
the local waters and coastal areas. For today, A tropical wave will
continue to approach the local islands. This feature will bring
plenty of moisture across the area, with precipitable water values
around 2.0 inches. At the upper levels, there is a trough to the
northeast of the area that will bring some divergence and cold
temperatures aloft. As the wave progresses, some showers will move
across the U.S. Virgin Islands and Puerto Rico. Then, in the
afternoon, these ingredients will combine with local effects to
result in showers and thunderstorms over the interior and western
Puerto Rico, as well as from El Yunque toward the San Juan metro
area. Isolated to scattered activity will be possible for the rest
of the area.
For tomorrow, as the wave exits toward the central Caribbean, some
Saharan dust will reach the area and will continue through the rest
the end of the workweek. However, another area of moisture is
expected to reach the islands from the south later on the day. This
moisture should be enough to fuel another round of convection during
the afternoon hours. Then, on Friday, the air mass is expected to be
drier than on Thursday, thus limiting the intensity and coverage of
the showers over the area.
In terms of temperatures, increased amounts of moisture in the
atmosphere will result in elevated heat indices at least through
tomorrow. Urban and areas at low elevations across Puerto Rico and
the U.S. Virgin Islands are expected to observe heat indices around
100 to 107 degrees.
.LONG TERM...Saturday through Thursday...
Long range models are suggesting drier than normal air mass this
weekend, perhaps with some lingering Saharan dust, which normally
limits the shower and thunderstorm activity across the area.
However, there is a tilted upper level trough that could help in
the development of thunderstorms over the general area. This upper
trough is forecast by the global models to remain near the local
islands through the long term period; moving from north of the
local islands on Saturday into the western Caribbean by Wednesday.
There is also a strong SFC high pressure in the north-central
Atlantic that will dominate the local wind flow, keeping us
observing east to east-southeast winds. The SFC high will move
into the northeastern Atlantic by early next week, but the winds
will remain from the east to east-southeast. The available
moisture is expected to increase to near or slightly above normal
from Monday onward. Given the forecast pattern, we can expect
morning and overnight isolated to scattered showers with isolated
thunderstorms over the waters, near the USVI and eastern PR; while
each afternoon, showers and thunderstorms can be expected across
portions of central and western PR and areas of sea breeze
convergence.
&&
.AVIATION...VFR conditions are expected to prevail through most
of the forecast period. VCSH are expected at TISX/TIST and the
Leeward Terminals through 30/22Z. VCTS are expected at TJSJ/TJBQ
after 30/17Z. This could lead to TEMPO groups with MVFR and brief
IFR conditions due to reduced VIS and low ceilings. Winds will
continue out of the ESE at 10 to 20 knots with stronger gusts.
&&
.MARINE...Moderate to locally fresh winds up to 20 knots will
cause choppy seas up to 6 feet today and early Thursday. The local
winds could increase to 20-25 knots on Thursday night across
portions of the Caribbean waters and the Anegada passage on
Thursday night, causing seas to become choppy to hazardous. There
is a moderate risk of rip currents for most of the local beaches
today and tonight.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 90 80 90 79 / 50 40 40 40
STT 89 80 89 81 / 60 40 30 20