Area Forecast Discussion
National Weather Service San Juan PR
134 AM AST Mon Sep 28 2020
.SYNOPSIS...The tropical is moving across the local area,
bringing showers and thunderstorms to the general area.
The tropical wave will keep above normal moisture, which will
combine with a mid to upper-level trough (TUTT) that extends
northeast of the northern Leeward islands southwestward into the
eastern Caribbean. The tropical wave and TUTT combination will
continue to enhance the chances of showers and thunderstorms
today. On Wednesday, another tropical wave is expected to
approach the region, causing another increase in shower and
thunderstorm activity, followed by a slightly drier air mass by
the end of the week, perhaps with some Saharan dust.
.SHORT TERM...Monday through Wednesday...
A tropical wave in combination with a mid-to-upper level trough will
cause unsettled weather conditions across the islands today. Data
from 28/00Z sounding, as well as satellite-derived precipitable
water indicate values near 2.1 inches over the area. Additionally,
Doppler radar and surface observations indicated an area of showers
and thunderstorms affecting the U.S. Virgin Islands and eastern
Puerto Rico. In fact, the high resolution models show that this
activity should continue through the morning hours, affecting the
same general areas. Since some of these soils are already saturated,
urban and small stream flooding will be possible. In the afternoon,
additional showers will continue to be affecting eastern Puerto Rico
and the U.S. Virgin Islands, but also, local effects and diurnal
heating will support activity developing over the interior, western,
and northern Puerto Rico, including the San Juan metro area. Some of
these showers could bring heavy rain at times.
A somewhat drier air mass is expected to filter in later in the
evening and early Tuesday. Additionally, as the mid-level trough
departs from the area, conditions are not going to be as favorable
for showers development. However, at the surface, groups of clouds
will make their way into the islands, resulting in another round of
showers early in the day for the eastern half of the forecast area,
followed by afternoon convection in western Puerto Rico.
Late Tuesday into Wednesday, another round of active weather is
anticipated. A favorable weather pattern for rain will be
established over the area, with a mid-to upper level trough causing
instability aloft, as well as cold temperatures in the mid-levels.
Additionally, a tropical wave will advance into the islands,
providing an increase in moisture. Therefore, widespread shower and
thunderstorm activity is expected for the local waters and the
islands. First, as is usual, the eastern half of PR/USVI should see
the bulk of the activity, but as the day progresses, the potential
will increase for the production of convection over the interior,
western, and northern PR, and in forms of streamers from Vieques,
Culebra, El Yunque and USVI.
.LONG TERM...Thursday through Tuesday...
Somewhat drier air is expected to move in on Thursday into Friday,
perhaps with some Saharan dust, which could limit the shower and
thunderstorm activity across the area, especially when compared to
what is forecast on Wednesday. That said, there is a tilted upper
level trough that could help in the development of isolated
thunderstorms. This upper trough is forecast by the global models
to remain near the local islands through the long term period,
though it could put PR and the USVI in the subsident side late in
the weekend. There is also a strong SFC high pressure in the
central Atlantic that will dominate the local wind flow, keeping
us observing east to east southeast winds. The available moisture
is expected to be near to slightly above normal for most of the
long term period. So given the current setup, 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
.AVIATION...A tropical wave will continue to cause TSRA across TISX/TIST at
least through 28/10Z, with some reduction in VIS and lower ceilings
expected. VCTS will also be affecting TJSJ and the Leeward terminals
through the day, and TJBQ after 28/17Z. Winds will be out of the ESE
at 10-15 knots with stronger gusts, especially near thunderstorms.
.MARINE...Seas up to 5 feet and winds up to 20 knots are expected
today across most of the local waters. Showers and thunderstorms
causing locally higher seas and gusty winds are possible over the
waters as a tropical wave passes through. Seas will increase up
to 6 feet on Tuesday. Small craft operators are urged to exercise
caution. There is a moderate risk of rip currents across the north
beaches of PR and Culebra as well as eastern St Croix, low risk
.PRELIMINARY POINT TEMPS/POPS...
SJU 90 79 90 79 / 60 50 50 40
STT 88 79 89 79 / 40 40 40 60