Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 120139

Area Forecast Discussion
National Weather Service San Juan PR
939 PM AST Tue Aug 11 2020

.UPDATE...Drier air ahead of a tropical wave has moved across the
region, the SJU upper air sounding indicated just 1.52 inches of
precipitable water and dry air above 800mb. This should maintain
fair weather conditions overnight across the islands. However, the
tropical wave is expected to move around sunrise over the USVI
and across the rest of PR through Wednesday afternoon. This will
increase shower activity in general with isolated thunderstorms
developing as the wave propagates across the region. Urban and
small stream flooding is expected with the wave passage across PR.
Minor changes were made to the previous forecast package.


.AVIATION UPDATE...Mainly VFR conditions are expected to prevail
across all terminals early in the fcst period. A tropical wave
east of the Leeward Islands will reach the USVI by Wednesday
morning and spread across the rest of PR through the day. This
will bring scattered SHRA with iso TSRA that may cause tempo MVFR
conds across all terminals. The 12/00z TJSJ sounding indicated
east winds up to 22 kts blo 3000 ft.


.MARINE UPDATE...No change to previous marine discussion at this


.PREV DISCUSSION... /issued 414 PM AST Tue Aug 11 2020/

SYNOPSIS...Thunderstorms over western PR should end around
sunset. A tropical wave will increase shower activity between
Wednesday and Thursday. Another tropical wave, with higher
moisture content is forecast to move close to the region during
the weekend.

SHORT TERM...Rest of Today through Thursday...
Tranquil weather prevailed this morning with a few brief showers
developing over eastern Puerto Rico. This morning TJSJ sounding
showed a precipitable value of 1.39 inches. This value ranks near
the 25th for August climatology. A ridge of high pressure is
expected to hold south of the area for the next several days.
Later today, locally and diurnally induced showers and
thunderstorms will develop over portions of the interior and
western areas of Puerto Rico. This activity could cause urban and
small stream flooding, especially in areas that received rain
yesterday. Also, can`t rule out a few streamers developing over
portions of eastern Puerto Rico and the U.S. Virgin Islands.

Later tonight through Wednesday, a weak tropical wave currently 300
miles east of the Lesser Antilles will affect the outer waters of
the Atlantic and Caribbean waters. The wave and its associated
moisture will cause showers and isolated thunderstorms across
portions of the U.S. Virgin Islands and eastern Puerto Rico
during the overnight through Wednesday morning. This activity will
continue to spread westward and affect the interior and western
areas of Puerto Rico during the afternoon. Also, the instability
will depend on the amount of mid to upper-level clouds that will
remain over the region from the overnight hours. Additionally, a
drier air mass is currently in place and it will take some time
for the wave moisture to moisten the mid-levels of the atmosphere.
Currently, the GFS20-Prico sounding and relatively humidity
imagery shows drier air in the mid-levels of the atmosphere
ranging from 3 to 10 percent through midday on Wednesday. The GFS
shows moisture pushing the drier air away from Puerto Rico by
Wednesday afternoon, mid-level relatively humidities are expected
to range from 50 to 60 percent during the afternoon.

Thursday, the wave is forecast to move west of the area however,
moisture is expected to linger across Puerto Rico and the U.S.
Virgin Islands. The lingering moisture coupled with the local and
diurnal effects will induce showers and isolated thunderstorms
during the afternoon. Also, isolated to scattered showers across
eastern Puerto Rico and the U.S. Virgin Islands during the early
morning hours.

PREV DISCUSSION... /issued 400 AM AST Tue Aug 11 2020/

LONG TERM...Friday through Wednesday...
A region of dry air is expected to persist on Friday and into the
day on Saturday, with patches of near-normal levels of moisture
streaming across the area, embedded in the trade winds. Aloft,
conditions do not look to be very favorable for convective activity,
with a mid- to upper-level ridge stretching across the region. This
ridge will be relatively weak in the mid-levels, and so there is the
potential for some plumes of moisture into the lower mid-levels
during this time frame. On the whole, however, a typical pattern of
afternoon showers due to local effects and diurnal heating and
passing showers during the overnight and morning hours is expected,
though activity will be somewhat inhibited.

There remains some uncertainty with respect to what the weekend will
bring. It is likely that the affects of a tropical wave will be felt
across the area during the weekend, though. It is also likely that
this feature will be an open wave during its passage as well. As for
the timing, the arrival of the wave and its moisture is likely to be
during the latter part of the day on Saturday, but could be as late
as Sunday morning. Moisture will persist across the area into
Monday, but the duration remains a bit uncertain as well.
Regardless, wet conditions are quite possible for Saturday, and
likely for Sunday, with fairly good potential that they will last
into Monday. With significant questions remaining with respect to
the development of this feature, though, the true extent and nature
of the impacts to the region remains uncertain at this time.
Conditions aloft are forecast to be marginally favorable for
convection during this time frame, mostly on Sunday, with the ridge
aloft weakening, and its influence over the area waning. As such,
widespread showers are expected, with isolated to occasionally
scattered thunderstorms.

The wave is forecast to exit the region by Tuesday morning. A drying
trend is anticipated for Tuesday into Wednesday. A decrease in
shower activity is likely. Aloft, conditions will become
increasingly unfavorable for convective activity. Thunderstorms will
be less likely, and afternoon showers will be less widespread and
vigorous. The typical pattern of afternoon showers in western and
interior Puerto Rico, and passing showers in the east during the
night and morning is still anticipated, as there will remain
sufficient moisture, especially in patches of moisture carried in on
the trade winds.

VFR conds durg prd. SCT ocnl BKN lyrs nr FL025...FL050
ovr regional waters and ovr islands. SHRA with Isold TSRA psbl ovr W
PR and TJMZ/TJBQ til 11/22Z with VCSH at other terminals.  No other
sig operational AVN wx impacts attm. Expect incr clds SHRA/Isold
TSRA ovr Nrn Leewards and regional waters E of PR and USVI aft
12/04Z due to approaching tropical wave. SFC wnds fm E 10-15 kts
with sea breeze variations and ocnl hir gusts...bcmg mainly E- NE 10
kts or less aft 11/23Z.

MARINE...Seas and winds will gradually build up to 6 feet with
the passage of a tropical wave on Wednesday through Thursday. However,
seas should remain below Small Craft Advisory criteria. A moderate
risk of rip currents will continue for the eastern, northern and
southern beaches of the islands. Isolated thunderstorms are
expected to continue through early this evening over the western
waters of PR. Hazardous marine conditions are possible across the
Atlantic waters during the weekend as an active tropical wave is
forecast to move north of the region.


SJU  88  79  89  79 /  20  20  60  60
STT  90  79  90  80 /  20  30  60  60




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