266
FXCA62 TJSJ 222000
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
400 PM AST Sun Sep 22 2024

.SYNOPSIS...

Fair weather conditions are expected to return a few hours after
sunset and continue throughout the night. Showers with isolated
thunderstorm activity are likely each afternoon, heightened
between Monday and Tuesday due to the proximity of an upper-level
trough and increased moisture levels. Excessive heat will continue
to pose a risk to vulnerable individuals, particularly those
outdoors without proper cooling or hydration. Additionally, the
moderate risk of rip currents will return by midweek.

&&

.SHORT TERM...Tonight through Tuesday...

Generally fair, warm-to-hot conditions, with skies becoming
progressively partly to variably cloudy by midday. Shower with
isolated thunderstorm activity began early in the afternoon,
spreading from the interior northward into coastal areas of
northwestern and northeastern Puerto Rico. So far, radar
estimates indicated the highest rainfall totals, around 1.5-2.0
inches, from Gurabo northward across Trujillo Alto and Carolina.
Surface observations report consistently hot temperatures up to
92-95F, with heat indices rising into the mid-100s, while higher
elevations recorded temperatures in the lower 80s. Winds were
predominantly from the east to east-southeast at 5-15 mph, with
gusts up to 20 mph along coastal areas.

A weak low-level high pressure over the Tropical Atlantic will
continue to provide a southerly component in the winds through the
rest of the short term period. However, light and variable winds
will prevail through at least Monday, influenced by sea/land
breezes. A relatively drier air mass is over the area today, as an
upper level-low continues to move east over the Leeward Islands and
an upper-level ridge extends from the central Caribbean. This will
be replaced by an upper-level trough that is moving from the western
Atlantic. This will promote troughiness and better moisture content
from late Monday through Tuesday. Therefore, showers and
thunderstorms will increase in coverage once again, especially on
Tuesday afternoon. Weak steering winds will also lead to higher
rainfall accumulations over the same areas, increasing the potential
for flooding and mudslides in areas of steep terrain. Particularly
along the Cordillera Central.

Across the USVI, a similar weather pattern is expected to continue
during the next few days with diurnally induced afternoon showers
developing downwind of the islands. However, for the rest of today
and through the night isolated thunderstorms can develop over the
waters near the islands due to the proximity of the upper-level low
over the Anegada Passage.

High temperatures in the upper 80s to low 90s will continue across
the lower elevations of the islands during the morning and early
afternoon hours across all the islands. Therefore, excessive heat
conditions can be anticipated each day. Warm temperatures in the low
80s will continue during the nights across coastal areas.

.LONG TERM...Wednesday through Saturday...

From previous discussion issued at 504 AM AST Sun Sep 22 2024

For the long term, we anticipate calm and warm weather conditions
with the development of showers and thunderstorm activity,
particularly in the afternoon hours.

On Wednesday, the steering wind flow will gradually recover to around
10-15 knots as the deep-layered trough continues to exit the
region. The latest model guidance suggests that all the available
moisture should remain below 700 mb through at least Thursday,
meaning that patches of moisture will move across the forecast
area from time to time, resulting in showers reaching the coastal
areas of Puerto Rico and the Virgin Islands each day. The
strongest shower and thunderstorm activity is expected each
afternoon across the Cordillera Central, with the activity then
drifting toward the coast. From Friday into the upcoming weekend,
we foresee that a tropical wave will move well south of Puerto
Rico and the U.S. Virgin Islands. However, we anticipate a slight
increase in moisture across the area, which could enhance shower
development. With the proximity of this tropical wave and the
everyday afternoon shower activity, we forecast occasional ponding
of water on roads and poorly drained areas through Saturday.

Temperature-wise, above-normal temperatures are expected to
persist over the region, with the threat of excessive heat each
day. This heat will mainly impact those without effective cooling,
adequate hydration, or prolonged exposure.


&&

.AVIATION...
(18Z TAFs)

Mainly VFR conditions expected through the forecast period. However,
SHRA/TSRA over mainland PR may cause brief MVFR conds at TJSJ/TJBQ
thru 22/22z. Mostly VCSH expected at TIST/TISX. Light winds with a
southerly component will continue to prevail thru Monday.


&&

.MARINE...

A surface low pressure to the northeast of the region over the
Central Atlantic will maintain light and variable winds into the
early part of next week. Showers and isolated thunderstorms will
develop from time to time across the regional waters, and some will
move into the coastal waters of Puerto Rico each afternoon.


&&

.BEACH FORECAST...

The rip current risk will remain low across the islands through at
least Tuesday night, when the moderate risk returns for the
northern coastal areas of Puerto Rico.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>005-007-
     008-010>013.

VI...Heat Advisory until 5 PM AST this afternoon for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...DSR/RAM
LONG TERM....ICP
AVIATION...DS/RAM
PUBLIC...GRS/ICP