National Weather Service San Juan PR
447 AM AST Sat May 29 2021
.SYNOPSIS...A mid and upper level ridge moving in on Sunday will
make today the wettest of the next three days. But, Tuesday
will mark the beginning of a moistening trend that will not end
until Saturday. Our most unstable conditions will occur Thursday
and Friday and we expect a noticeable increase in thunderstorms
accompanied by heavy rains then.
&&
.SHORT TERM...Today through Monday...
For today, a surface high pressure extending across the north
Atlantic will maintain a generally moderate east to east-southeast
wind flow across the region. At mid to upper levels, a short-wave
trough to the west, with an axis extending southward over
Hispaniola, will maintain somewhat favorable conditions aloft. This
along with near-normal moisture levels fluctuating between 1.5 and
1.7 inches, diurnal heating and local effects will support another
active afternoon with shower and isolated thunderstorm development.
Under a southeasterly component of the steering flow, the bulk of
the activity is expected to favor the interior and northwestern
quadrant of Puerto Rico, as well as portions of the San Juan
metropolitan area and eastern Puerto Rico. Until afternoon
convective development begins by the late morning and early
afternoon hours, expect showers moving from the waters into portions
of eastern Puerto Rico and the US Virgin Islands. Although today`s
activity is expected to be slightly faster and more focused than
previous days, prolonged periods of moderate to heavy rains over
already saturated soils will result in urban and small stream
flooding and possible mudslides near areas of steep terrain. Daytime
high temperatures are forecast to reach the mid-80s to near 90
degrees across coastal areas, with heat indices as high as 100
degrees across coastal areas of northern Puerto Rico.
For Sunday and Monday, a weak mid-level ridge is forecast to roll in
as the aforementioned short-wave trough lifts to the east-northeast
and away from the region. This will sustain a trade winds inversion
that may be strong enough to inhibit deep convection, but not strong
enough to suppress shallow development. As the plume of moisture
meanders away, intermittent pockets of relatively drier air and
shallow moisture, with moisture fluctuating between 1.10 to 1.80
inches, will dominate the local weather conditions. If the timing of
enhanced moisture coincides with peak afternoon hours, which is,
so far, anticipated, intense diurnal heating and local effects
will aid in the development of showers and possible isolated
thunderstorms. Since the local wind flow is expected to display an
east-southeast and an east-northeast component on Sunday and
Monday, respectively, expect the bulk of the activity to shift
accordingly. Localized urban and small stream flooding remains
possible, especially with the heaviest rain expected each
afternoon.
.LONG TERM...Tuesday through Saturday...
Troughing at upper levels over Cuba and the Bahama Islands
develops a cut-off low on Wednesday. The trough will continue in
the same general area while the cut-off low weakens and lifts to
the northeast. This in turn will cause a relatively weak
southwesterly jet, again at upper levels, to brush the area and
increase the instability. At the same time 500 mb temperatures
will lower to nearly minus 8 degrees. As mentioned precipitable
water values will drop to around 1.10 inches on Monday and will
increase until Saturday. Moisture between the 850 and 700 mb
levels is the weakest part of the profile during the period and
this will likely suppress some of the rain over the area, but in
general rainfall will increase--especially on Thursday and periods
of heavy rainfall are expected each day over western and interior
Puerto Rico. Showers will also increase for the U.S. Virgin
Islands, but amounts will be limited compared to Puerto Rico.
Areas of urban and small stream flooding will become more
widespread Wednesday and Thursday, but will continue through the
end of the period, sustained by the high moisture content of the
atmosphere over the area.
&&
.AVIATION...Mainly VFR conditions are expected across all terminals.
However, brief MVFR conditions can be expected with VCSH affecting
TJSJ and USVI terminals through 29/14Z and TSRA/SHRA affecting
mostly TJSJ/TJBQ with VCSH/VCTS at TJMZ/TJPS between 29/15-23Z due
to low ceilings and reduced visibility. Winds will be light and
variable through 29/12Z, bcmg ESE at 10-15 knots with higher
gusts through 29/23Z. Maximum winds W 50-62 kts btwn FL425-495
strongest at FL470.
&&
.MARINE...Moderate trade winds over the local area will allow
seas to remain 5 feet or less through the next 7 days.
Thunderstorm activity will increase on Thursday and continue into
Friday.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 77 88 77 / 40 50 40 50
STT 87 77 87 76 / 40 40 30 30