Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 120829
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
429 AM AST Wed Aug 12 2020

.SYNOPSIS...

Increasing shower activity is expected today, with the arrival of
a tropical wave, which will linger over the area into tonight.
Widespread showers are likely for the afternoon, especially in
western and interior Puerto Rico, as well as isolated
thunderstorms. Behind the wave, drier air will move in place, with
some Saharan dust along with it. Some showers are still possible,
but generally fair weather is expected. Another tropical wave is
anticipated for this weekend, and we may also see some indirect
impacts from Tropical Depression Eleven, which is forecast to pass
by to the north.

&&

.SHORT TERM...Today through Friday...

Fair weather is expected to prevail in the morning hours across the
local forecast area. A tropical wave will move in today, gradually
affecting the local waters and islands. The wave and its associated
moisture will cause showers and isolated thunderstorms across, in,
and around the U.S. Virgin Islands and eastern Puerto Rico
starting this morning. This activity will continue to spread
westward and affect the north, interior and western areas of
Puerto Rico during the afternoon. Therefore, numerous showers and
isolated thunderstorms are in the forecast for this afternoon
across many locations in the local forecast area. Some of the
showers and thunderstorms may be persistent enough to cause small
stream and urban flooding, especially in areas with poor drainage.
By this evening, most of the moisture and shower activity is
expected to have moved away, and the latest guidance is suggesting
drier air for tonight and much drier air with some Saharan dust
for Thursday. That said, isolated to scattered brief shower
activity will not be ruled out for Thursday, as the high res model
guidance still likes some shower activity, but the global models
favor a much drier solution on Thursday. Near normal moisture is
expected on Friday, and the mid to upper levels do not look
particularly favorable for deep convection, therefore the forecast
calls for scattered showers, mainly locally induced over Puerto
Rico and in the afternoon hours, while the USVI stays with mainly
fair weather and a slight chance of brief showers.

.LONG TERM...Saturday through Thursday...

The forecast for Saturday, and, indeed, the rest of the weekend and
into Monday, is quite dependent on the development and trajectory of
Tropical Depression Eleven. Should the system strengthen, as is
presently expected, it would be likely to pull northward, away from
the rest of the tropical wave, as it neared the Caribbean. In this
scenario, a dry slot forms between the tropical cyclone and its
parent wave. That being said, there does remain some uncertainty, as
we are still several days out. A shift in the strength or trajectory
would shift the timing and location of any possible dry slot. A less
likely scenario involves the system not strengthening, or being
short-lived. This has the potential to lead to more moisture being
brought through the area. While this currently does not look likely,
it cannot be fully ruled out. Either way, it does look to be a
fairly rainy weekend, increasingly so as we move into Sunday.
Widespread showers are likely by Sunday, with isolated to locally
scattered thunderstorms. Conditions aloft are expected to be more
favorable for convective activity, but not very favorable. This will
still help contribute to the afternoon showers and thunderstorms,
though.

Moisture will remain in the area for Monday, when there will likely
be a continuation of Sunday, though slightly weaker in terms of
activity. Dynamic forcing aloft remains relatively weak, and its
influence will wane somewhat. Decreasing moisture is expected during
the evening and into the night. And by Tuesday, conditions aloft
will become decreasingly favorable for convection, with drying
forecast for the mid-levels. A large, but weak, developing
disturbance will help bring additional moisture into the area on
Tuesday into Wednesday; moisture levels will be near-normal levels,
climatologically speaking. A typical pattern of afternoon showers in
the west and passing showers overnight and into the morning in the
east will prevail through midweek. Further drying is expected
Wednesday night into Thursday, which will act to inhibit some of the
typical shower activity across the local islands.

&&

.AVIATION...

VFR conds expected across the local terminals expected
through the early morning hours. Approaching tropical wave will
start bringing SHRA/TSRA to the local area by 12/12Z. This will
cause VCSH/VCTS through much of the afternoon with brief periods of
SHRA/TSRA at the local terminals, except TJPS. Winds will be from
the east, but increase after 12/13Z to 10-15KT with occasional
gusts. Winds will be stronger near showers and thunderstorms.

&&

.MARINE...

Winds up to 20 knots are forecast for the majority of the local
waters over the next couple of days, associated with a tropical
wave passing through the area today. Building seas are also
anticipated, and heights of up to 6 feet are possible through the
end of the workweek. Conditions are expected to remain below Small
Craft Advisory criteria for now, though. That is likely to change
this weekend, as another tropical wave pushes through the area,
with the potential for a tropical cyclone passing by a couple
hundred miles to the north. There is a moderate risk of rip
currents at northern beaches of Puerto Rico, as well as some
south-central beaches and southeastern beaches, and also most
beaches of Vieques, Culebra, and St. Croix.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  80  90  80 /  60  40  20  20
STT  90  81  90  80 /  60  40  20  20

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM...JA
LONG TERM....CRS
PUBLIC DESK...99


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