567
FXCA62 TJSJ 152041
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
441 PM AST Sat Jun 15 2024

.SYNOPSIS...

An Excessive Heat Watch is in effect from 10 AM to 6 PM tomorrow
for the western, northern, and eastern coastal areas of Puerto
Rico including Culebra, Vieques, and the USVI. Please follow the
recommendations from the local health department to avoid heat
exhaustion. Passing showers followed by afternoon convection is
expected tomorrow as plenty of moisture remains in the area,
brought in by the southeasterly lower level wind flow.

&&

SHORT TERM...Tonight through Monday...

A surface trough will continue to move north of the region
tonight with trailing bringing passing showers overnight with the
prevailing southeast low level winds. A mid to upper level ridge
will hold just to our west, keeping conditions somewhat stable.
Expect this overall pattern to persist through tomorrow along with
a light to moderate plume of Saharan Dust and suspended
particulates expected to continue crossing the region causing slightly
hazy conditions tomorrow. Although drier air will filter in
across the region tonight along with the Saharan dust, there will
enough moisture available to aid in the development of afternoon
showers and isolated thunderstorms during the afternoon hours.
With fairly light winds and local sea breezes, most of the
expected rainfall should be focused across eastern, central and
northwest sections of Puerto Rico steered by the southeast wind
flow.

The suspended Saharan dust particulates, along with the warm
southerly flow and presence of the mid to upper level ridging will
continue to favor subsidence and support above normal daytime high
temperatures in the low 90s especially along the north and west
coastal areas. That said, maximum heat indices are expected to
exceed 100 degrees once again, resulting in excessive heat impacts
especially along the west, north and east coastal and urban areas.
These conditions will also remain likely on Sunday. Therefore
a Heat Watch will remains in place for tomorrow. Please refer to
the latest Non-Precipitable Weather Message(NPWSJU) issued by WFO
SJU PR for additional information on these heat impacts.

&&

LONG TERM...Tuesday through Saturday...
/from previous discussion/

A deteriorating weather trend is forecast for the first part of
the long-term period. A trough will build just over the western
Atlantic at the upper and mid- levels. The presence of the mid- to
upper-level feature will decrease the 250 MB height and turn the
temperatures at 500 MB colder, ranging from
-7 to -8 degrees. This increases the potential for more
widespread lightning activity on Tuesday into Wednesday. According
to the global model guidances (GFS and ECMWF), surface conditions
will also be favorable for shower activity due to plenty of
tropical moisture embedded in the east-southeasterly wind flow
across the region. Given the expected conditions from Tuesday into
Wednesday, the islands will experience passing showers in the
morning hours over the eastern sections, the U.S. Virgin Islands,
and the local waters, followed by widespread shower activity
across the western interior. So far today, the rainfall threat is
mostly concentrated across the western interior, with ponding of
water in roadways and poor drainage areas. From Thursday into
Saturday, plenty of surface moisture will continue to stream
across the local islands, keeping the potential for convection in
the afternoon, especially across the mountains and the western
interior. Although plenty of moisture will remain in place, more
stable conditions will be present at the upper level, provided by
an upper-level ridge.

A similar temperature pattern is expected across the islands by the
first part of the long term. According to the model guidances,
925 MB temperature, the trend calls for warmer conditions,
resulting in heat indices rounding 108 to 112 F degrees. Residents
and visitors are urged to stay informed of possible advisories or
warnings due to Excessive heat.

&&

.AVIATION...
(18Z TAFS)

Scattered SHRA expected to prevail across northern half of PR and
TJBQ through 22Z, with periods of reduced VIS and low ceilings
possible. Additional activity expected to increase after 18Z for the
Caribbean waters and USVI terminals as well. Winds are from south
above the SFC to FL050 on 15/12Z. Hz due to Saharan dust is expected
to increase after 15/16Z for the USVI, but VIS should remain at P6SM.

&&

MARINE...

Surface high pressure over the Atlantic and an induced trough north
of the area will promote light to moderate east to southeast winds.
Moist air will allow showers and isolated thunderstorms to develop
over portions of the local waters tonight, resulting in localized
hazardous marine conditions. Winds will continue to diminish and
veer, becoming more southeasterly through tomorrow.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Watch from Sunday morning through Sunday
     afternoon for PRZ001-002-005-008-010-012-013.



VI...Excessive Heat Watch from Sunday morning through Sunday
     afternoon for VIZ001-002.



AM...None.

&&

$$

ERG/YZR/RC