National Weather Service San Juan PR
549 AM AST Thu Feb 20 2020
.SYNOPSIS...
Persisting surface high pressure to the north over the eastern
Atlantic is promoting continuing moderate to fresh trade winds
today. Patches of moisture are expected to stream across the area
embedded in these trade winds, aiding in the development of
afternoon showers, as well as some passing showers during the
morning and overnight hours. Decreasing winds and moisture are
forecast for Friday into the weekend, while Saharan dust will
likely see hazy skies over the area. Increasing moisture is
anticipated for the start of next week as a cold front approaches
the area, leading to a more active weather pattern.
&&
.SHORT TERM...Today through Saturday...
A broad surface high pressure over the eastern Atlantic will promote
moderate to fresh trade winds today. Areas of low level moisture
embedded in the trade winds will reach the islands from time to time
through the short term period. This will be enough to trigger
diurnally induced afternoon showers over portions of the interior
and west PR as well as from streamers developing off the USVI.
However, the overall moisture content decreases from 1.30 inches
early this morning to just under an inch of precipitable water
during the next couple of days. This is due to drier air moving in
from the east with Saharan dust. Although not a strong SAL event,
hazy skies are expected. Winds will gradually decrease on Friday and
Saturday, and turn more from the southeast as an area of low
pressure develops over the western Atlantic and an associated
frontal boundary stalls to the north of the region during the
weekend.
.LONG TERM...Sunday through Friday...
By Sunday, a cold front will be approaching the region from the
northwest. This front will likely not pass completely over the
region, stalling near the northwest of Puerto Rico. The central-
Atlantic ridge ahead of this frontal system is expected to continue
to pull away from the region to the northwest. These factors will
combine to continue the relatively light winds lasting into Monday
at least. Moisture associated with the aforementioned front will
push over the local islands starting late Sunday. Meanwhile, the
remnants of an old frontal boundary embedded in the trade winds will
also be making their way westward towards the area. The combination
of factors is also likely to lead to moisture convergence early in
the week, and this is when the highest rainfall amounts are likely.
Associated with these two boundaries, moisture levels at or above
seasonal normals are expected to persist into the end of the work
week next week. Therefore, shower activity is likely to be near or
above normal levels for February through the week. However,
conditions aloft are likely to be generally stable, with ridging
seen at both the mid- and upper-levels by late Monday; the 500 hPa
temperatures support this conclusion, generally staying warmer than
climatological normals for this time of the year through out the
long term period. As such, convection is unlikely, and rainfall
totals are not likely to be spectacular. Late in the week, late
Thursday into Friday, a trend towards drying conditions is
anticipated. Though the 0Z run of the GFS has slowed this drying
trend somewhat compared to previous runs, the trend does remain, and
still seems likely; as such, it is reflected in the forecast.
&&
.AVIATION...
Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, diurnally induced afternoon showers
over western Puerto Rico could cause brief MVFR conds at TJMZ. HZ due
to Saharan dust is expected but vsby should remain P6SM. Sfc winds
increasing 15-20 kt from the east with higher gusts and sea breeze
variations after 14z.
&&
.MARINE...
Moderate to fresh trade winds continue to impact the area. This will
support persisting choppy and hazardous seas, with heights up to 7
feet across much of the regional waters. Winds, and therefore seas,
will begin to subside somewhat tonight into tomorrow. Conditions are
expected to improve enough to no longer meet small craft advisory
criteria by early tomorrow morning, except in the outer Atlantic
waters, where hazardous conditions are forecast into the afternoon.
Relatively tranquil conditions are then expected as we make our way
into the weekend. However, another northerly swell is anticipated
for Sunday, and will bring a return of hazardous marine
conditions.
&&
.PRELIMINARY POINT TEMPS/POPS...
SJU 86 73 85 72 / 50 20 20 20
STT 84 72 82 71 / 30 30 30 20