Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 220025

Area Forecast Discussion
National Weather Service San Juan PR
825 PM AST Sun Jul 21 2019

.UPDATE...The shower and thunderstorm activity that affected the
area during the afternoon hours has dissipated. For the overnight
hours and into the morning hours of Monday, passing showers are
expected across the regional waters, with some of the activity
affecting portions of eastern Puerto Rico and the USVI. Given that
these showers will be brief, only minor rainfall accumulations are
expected. Then, by the late morning into the afternoon hours of
Monday, more enhanced showers and thunderstorms due to strong
daytime heating and local effects are expected to develop across
the northwest quadrant of Puerto Rico as the low and mid-level
flow changes from the southeast. Additional showers in the form of
streamers are also possible downwind of the USVI and around the
San Juan metro area.


.AVIATION...Mainly VFR conditions should prevail through 22/16z.
VCSH due to passing SHRA are possible across the Leeward Islands,
USVI, and eastern PR terminals. Between 22/16z and 22/23z,
SHRA/TSRA expected across the northwest quadrant of PR, affecting
the vicinity of TJBQ and TJMZ. TEMPO MVFR conditions are possible.
Light and variable winds expected overnight, increasing to 10 to
20 knots with higher gusts and sea breeze variations after 22/14z.


.MARINE...Fairly tranquil seas of up to 5 feet are expected. Winds
overnight will generally be below 15 knots. On Monday, winds are
expected to increase up to 20 knots across the local passages,
offshore Caribbean waters, and the nearshore Atlantic and
Caribbean waters. Therefore, small craft operators need to
exercise caution. Rip current risk for Monday will be moderate
across several beaches along the northern and southern coast of
PR, as well as across Vieques, Culebra, and Saint Croix.


.PREV DISCUSSION... /issued 452 PM AST Sun Jul 21 2019/

SYNOPSIS...Patch of moisture will prevail over the local area
before a tropical wave moves just south of the local area late
Monday into Tuesday, increasing moisture locally. Upper low will
move west and across the local area, and will be just west of the
local area by Tuesday. Surface high pressure will promote easterly
winds for the next several days.

SHORT TERM...Tonight through Tuesday...

Lingering moisture has helped in the development of showers and
thunderstorms across the local area in the afternoons as it
combines with the diurnal heating. The near normal moisture will
continue through Monday, causing a similar weather pattern on
Monday afternoon, showers and thunderstorms across much of the
local area. A tropical wave will move south of the local islands
late Monday into Tuesday, increasing the available moisture and
shower activity even though the deep moisture will remain south of
the islands. Shower activity will increase across the local
waters, USVI and eastern PR overnight monday and through most of
Tuesday. The western and northwestern sections of PR is expected
to observe showers and thunderstorms every afternoon through

LONG TERM...Wednesday through Monday...

..From previous discussion...

Little to no change to the weather conditions expected during the
long-term forecast, favoring two independent periods of active
weather. This will be the result of a series of tropical waves and
trade wind perturbations moving across the region, that will
increase moisture content and the potential for widespread shower
and thunderstorms activity. The first wave and associated plume of
moisture is expected to affect the area late Wednesday night into
Thursday. This feature will be quickly followed by an extended
trade wind perturbation that will enhance another round of showers
and thunderstorms on Friday. Favorable upper-level conditions
with a TUTT pattern aloft and mid-level temperatures dropping to
or below -8C will enhance thunderstorm activity across the region
by the latter part of the week into the weekend. Another tropical
wave is expected to arrive on Monday, which will generate another
period of active weather. Relatively drier air accompanied by
suspended Saharan dust will briefly move in from the east in the
wake of each wave, but lingering moisture supported by diurnal
heating and local effects will enhance the potential for locally
induced afternoon convection each day.

AVIATION...SHRA/TSRA should end before sunset over western PR.
Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. East winds will continue btw 10-15 kt, but
bcmg light and variable overnight.

MARINE...Generally benign marine conditions as seas will be up to
5 feet. However, at times the coastal winds will be up to 20 knots
and thus small craft operators are urged to exercise caution.
There is a moderate risk of rip currents across some of the local
beaches, low risk across most of the local beaches.


SJU  79  89  79  88 /  40  50  60  60
STT  79  89  81  89 /  50  40  60  30





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