National Weather Service San Juan PR
515 AM AST Mon Jun 22 2020
.SYNOPSIS...
Suspended African dust particles will peak today, creating very
hazy and low visibilities over the islands. Overnight
temperatures will remain in the low-80s, making it easier to rise
into the 90s during the afternoon hours. These conditions will
prevail through at least Tuesday. However, the aerosols leftovers
will linger through much of the workweek. A tropical wave is
forecast to reach the islands by Friday.
&&
.SHORT TERM...Today through Wednesday...
Saharan dust concentrations are expected to peak today and tomorrow.
Visibilities near and even below 6 miles are likely across the
region, with the dust concentrations declining during the day on
Wednesday, though still remaining quite high. Along with the dust, a
very dry airmass has invaded the region, with precipitable water
values well below average at 1.3 to 1.5 inches. Furthermore high
pressure will dominate the region at all levels, resulting in
subsidence and further warming/drying aloft. This is plainly evident
in model soundings with warmer than average 500 mb temperatures and
an inversion at 850 mb. With these specific conditions lasting today
and tomorrow, very little rainfall is expected. High resolution
model guidance does suggest a chance for some showers in west to
northwest Puerto Rico tomorrow and Tuesday afternoons. However,
these showers will have very minimal impact. Winds will stay fairly
breezy due to surface high pressure strengthening the central
Atlantic.
Heading into Wednesday, some moisture is expected to drop in from
the northeast during the daytime associated with an old front. The
low level inversion is expected to erode, so conditions should be
more conducive for shower and isolated thunderstorm development
for western Puerto Rico in the afternoon. Furthermore, an upper
level trough will begin developing to our west during the day,
which may aid in enhancing thunderstorms.
.LONG TERM...Thursday through Monday
An upper-level trough will amplify over the Windward Passage and
slowly drift eastward Thursday into the weekend. The leftovers of
today`s African dust event will make its way into the long-term
period in combination with patches of low level moisture embedded
in the trade winds. Moisture will slowly ramp-up over the
Northeast Caribbean producing a few passing showers and afternoon
convection each day. However, at this time, no significant
rainfall event is anticipated over the islands until the arrival
of the next tropical wave on Friday.
Both GFS and ECMWF are indicating the arrival of a tropical wave
on Friday. This tropical perturbation will interact with an
upper-level trough centered over Jamaica and extending into our
region. If the models are right, the aforementioned upper-level
feature could enhance upper-level ventilation hence thunderstorm
formation and downpours over the county warning area. After this
wave, a fast-drying trend is forecast on Saturday. Then, another
trade wind perturbation will be advecting over the islands by
Sunday. A second tropical wave is forecast to swing by the region
on Tuesday. Keep in mind that sometimes models have high
uncertainty in the humidity field, especially during the long-
term.
&&
.AVIATION...
The Saharan dust event will continue to impact visibilities at all
terminals through tomorrow at least, ranging from 4-8 miles. E winds
will pick up to 10 to 20 kts by 15z, and slow down to 5 to 15 kts
by 00z. VCSH possible for TJBQ, otherwise dry conds will prevail.
&&
.MARINE...
Mariners can expect moderate to fresh winds between 15 and 20 kts
and therefore choppy seas between 3 and 6 feet across the
regional waters through at least the middle of the workweek. Fresh
winds will create seas up to 7 feet across the Mona Passage this
evening. Therefore, a Small Craft Advisory in in effect until at
least Tuesday morning. Elsewhere, small craft operators should
exercise caution. Visibilities will be limited between 4 and 7
nautical miles in a Saharan dust episode through Wednesday.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 92 83 91 80 / 10 10 10 20
STT 92 83 91 79 / 10 10 10 20
&&

