Area Forecast Discussion
Issued by NWS San Juan, PR

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FXCA62 TJSJ 081500
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1100 AM AST Sat May 8 2021

.UPDATE...

The ridge is eroding, giving way to the arrival of a mid to upper-
level trough. A surge of moisture will interact with local effects
and sea breeze variations resulting in afternoon convection across
the interior and north of the Cordillera Central. Showers will
also develop downwind from USVI and El Yunque to affect eastern PR
and the San Juan Metro Area. Thunderstorms will develop over the
interior and northwest quadrant. A tropical wave is still forecast
to cross the east Caribbean late tonight and into PR/USVI late
Sunday.

&&

.AVIATION...

SHRA will develop across E-PR and west of the USVI through this
evening and could affect JSJ/IST/ISX at times. Clouds will
increase over the interior and NW quadrant giving way to the
formation of SHRA/TSRA between 08/17-09/00. This activity could
impact the terminals of JBQ. Winds will continue from the E-ESE at
10 to 20 knots with local sea breeze variations and higher gusts;
winds will drop below 10 kt after 08/23z.

&&

.MARINE...

Calm marine conditions will persist across most local waters, with
seas below 5 feet and winds around 15 knots. However, an
approaching tropical wave could produce locally higher winds and
choppy conditions, in addition to the formation of thunderstorms.
Therefore, mariners should exercise caution.

&&

.PREV DISCUSSION... /issued 425 AM AST Sat May 8 2021/

SYNOPSIS...A diurnal weather pattern will continue to prevail
today through Sunday afternoon, with passing showers affecting
portions of the eastern Puerto Rico and the USVI during the
overnight and morning hours followed by afternoon convection
across northwestern Puerto Rico. By Sunday night into Monday, a
tropical wave is forecast to move across the area, with more
widespread shower and isolated thunderstorm activity expected.

SHORT TERM...Today through Monday...

A broad surface high pressure across the central Atlantic will
promote light to moderate east to southeast trades through Sunday.
Winds will increase briefly and turn more from the east between late
Sunday night and early Monday, as another surface high over the
western Atlantic merges with the high over the central Atlantic and
a tropical wave moves across the region. Meanwhile, a polar
trough will move over Hispaniola today and remain west and over
portions of the local area through early Monday.

For today, the combination of the available low-level moisture with
day time heating and the sea breeze convergence will result in
afternoon showers over west/northwest PR and from streamers
developing off the USVI and eastern mountains of PR. Maximum
temperatures are expected to range from the mid 80s to low 90s
across the lower elevations and urban areas of the islands.
Increasing moisture content and unstable conditions aloft will
promote better chances for showers and isolated thunderstorm
development across portions of the islands and regional waters from
late Sunday into Monday morning. Urban and small stream flooding can
be expected with the heaviest showers.

LONG TERM...Tuesday through Saturday...

Weather conditions at this time are expected to improve by Tuesday
as the tropical wave moves away from the area and the moisture
content diminishes with low to mid-levels of the atmosphere drying
out. This is in response to a mid-level ridge expected to build
in from the western tropical Atlantic into the area. Therefore,
shower activity is expected to be limited across most of the local
area. The mid-level ridge is expected to hold through the end of
the long term period, limiting the potential for organized and
widespread convective activity from materializing. Meanwhile, at
lower-levels, a series of surface high pressures will move across
the Atlantic basin north of the area. This will result in a
moderate easterly wind flow, which will push patches of low-level
moisture into the area from time to time. Therefore, the typical
diurnal weather pattern is expected with some passing showers
during the overnight and morning hours across the USVI and eastern
Puerto Rico, followed by the development of afternoon convection
across the western half of Puerto Rico. The afternoon convection
across western Puerto Rico may be heavy enough to result in some
urban and small stream flooding.

AVIATION...Mainly VFR conditions expected to prevail across all
terminals. However, SHRA is expected to develop in and around
TJSJ/TJBQ btw 16z-22z, this could cause MVFR conds. VCSH mainly
across the USVI terminals. Low-level winds ESE at 10-15 kts with sea
breeze variations aft 14z.

MARINE...Mostly tranquil seas of between 2 and 5 feet are
expected to prevail across the regional waters today and Sunday.
More choppier seas are possible by early next week as winds
increase due to passage of a tropical wave, however, no small
craft advisory conditions are expected at this time. With the
passage of the tropical wave, shower and isolated thunderstorm
activity is expected to increase across portions of the local
waters. For beachgoers, there is a moderate risk of rip currents
for some of the northern beaches of Puerto Rico as well as the
eastern tip of Culebra, Vieques and Saint Croix.

FIRE WEATHER...Latest surface observations continue to indicate
very dry soils across the southern coastal plains of Puerto Rico
with KBDI values over 700 in Guanica and Camp Santiago. Very
similar weather conditions are expected today compared to previous
days, with little to no rainfall activity across the southern
coastal plains. Relative humidity values are forecast to drop into
the mid 40s to low 50s across most areas with winds expected to
peak at around 15 mph with higher gusts. Therefore, the
combination of these factors will once again result in an elevated
fire danger threat across the southern coastal plains and a Fire
Danger Statement was issued accordingly.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  89  74  87  75 /  30  20  60  70
STT  86  78  86  78 /  30  20  50  50

&&

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

$$

SHORT TERM...CAM
LONG TERM....CVB
PUBLIC DESK...ERG


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