National Weather Service San Juan PR
508 AM AST Fri Aug 16 2019
.SYNOPSIS...The Saharan dust will clear out of the area tonight,
leaving only traces for the next several days. A weak low level
trough will move through on Saturday and increase showers and
thunderstorms. Then moisture will increase Sunday through Monday
with the passage of a tropical wave on Monday. Localized urban and
small stream flooding are possible on Monday. Some moisture will
continue Tuesday. Then, sporadic showers and isolated
thunderstorms will follow patches of moisture through the area for
the remainder of the week.
&&
.SHORT TERM...Today through Sunday...
Hazy skies due to Saharan dust are expected to continue through
tonight. The Saharan dust will also continue to produce stable
weather conditions across the region with very limited shower
activity expected. However, patches of low-level moisture riding
in the northeasterly trade wind flow may continue to spark some
passing showers across the regional waters and affecting northern
and eastern portions of Puerto Rico and the USVI through the early
morning hours. Then, showers and isolated thunderstorms are
expected across southwest Puerto Rico this afternoon as the
available moisture combines with strong daytime heating and local
effects. However, the activity that does develop should be short-
lived and only result in light rainfall amounts.
A broad upper-level low is expected to be situated just north of the
area with an inverted trough at low to mid-levels for Saturday.
Therefore, conditions are expected to become favorable for the
development of more significant shower and thunderstorm activity to
materialize, especially across interior and western Puerto Rico
during the afternoon hours when the available moisture combines with
strong daytime heating and sea breeze convergence. Therefore, there
is a better potential for urban and small stream flooding to occur
in areas that receive a prolonged period of heavy rainfall activity.
Showers in the form of streamers may also develop downwind of the
USVI and El Yunque, affecting the San Juan metro area.
A drier air mass ahead of a tropical wave, currently located near 50
degrees west Longitude as of this morning, will move over the area
Saturday night through early Sunday afternoon. Therefore, mainly
fair weather conditions can be expected. Afternoon convection is
expected across northwest Puerto Rico due to local effects. Then, as
the moisture associated with the tropical wave begins to increase
late Sunday afternoon into Sunday night, showers are expected to
increase in coverage across the Caribbean waters and affect portions
of the USVI and eastern Puerto Rico.
.LONG TERM...Monday through Friday...
Sunday night will bring the entrance of a strongly tilted
tropical wave into the forecast area from the southeast. This will
spread moisture over the area that will peak on Monday and then
steadily decline through Wednesday. Areas of moisture then move
through the forecast area Wednesday night and Friday morning.
The GFS has found this wave difficult and has had some fairly wide
variation in intensity and timing from run to run. For the local
area, Monday afternoon may still be the most uncertain period. For
having moisture in place and good convergence in lower levels and
modest divergence aloft the low probabilities of precipitation
over land areas and the placement of near 100 percent POPs north
of the Saint John in the outer Atlantic waters seems questionable
so have left better chances for rain where orographic lifting
will be taking very moist air and generate convection over land
that will not be completely devoid of heating. After clouds clear
somewhat on Tuesday, lingering moisture should be able to generate
another day of good shower and thunderstorms activity. Wednesday
through Friday will see passing showers and recurring afternoon
showers and thunderstorms, however warm mid levels forecast by the
models on Thursday to be minus 2 to minus 4 degrees at 500 mb,
seem to indicate that strong convection will not occur.
Upper levels in the atmosphere show only minor changes in the
divergence field except as the leading edge of the tropical wave
passes on Monday. At that time, areas of higher divergence aloft
are seen to pass over Puerto Rico, but not so much over the U.S.
Virgin Islands.
&&
.AVIATION...Mainly VFR conds with hazy skies expected to prevail
across the area during the forecast period. Sct SHRA/TSRA expected
across SW PR between 16/16z and 16/22z, affecting the VCNTY TJMZ.
However, no significant impacts to operations are expected as the
heaviest activity should remain to the south of the terminal.
Low-level winds from the ENE-NE below 10 knots through 16/12z,
increasing to 10 to 20 knots with higher gusts and sea breeze
variations after 16/14z. Max winds ENE 20-25 kt btwn FL050-180 and
W 20-25 kt btw FL340-400.
&&
.MARINE...Marine conditions are expected to remain below small
craft advisory criteria through the next 7 days. Thunderstorms
Monday through Wednesday could generate higher winds and seas.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 89 78 88 78 / 40 40 30 20
STT 91 78 89 79 / 20 30 30 30