295
FXCA62 TJSJ 230929
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
529 AM AST Mon Sep 23 2024

.SYNOPSIS...Light winds will continue well into the week with
showers and thunderstorms each day, but generally less on
Wednesday. A low risk of rip currents will be seen Monday and
Tuesday due to light winds. Winds will increase somewhat on
Wednesday through Friday when a mild tropical wave moves through
the area.

&&

.SHORT TERM...Today through Wednesday...

Overnight, mostly calm weather conditions prevailed across Puerto
Rico and the U.S. Virgin Islands. However, isolated to scattered
showers and thunderstorms developed over the waters and the U.S.
Virgin Islands Passage. Temperatures remained in the upper 60s to
lower 70s in the higher terrain and in the upper 70s to lower 80s
in the lower terrain (urban/coastal areas). The winds were
relatively light and variable across the islands.

As for today, citizens and visitors in Puerto Rico, Vieques,
Culebra, Saint Thomas, Saint John, and Saint Croix should experience
tranquil weather conditions through the morning hours. Nonetheless,
as the morning and day progresses, the heat indices will increase,
exacerbating the risk of heat illness due to the high humidity
and high temperatures. By the afternoon, we are forecasting the
development of showers across Puerto Rico, which will allow for
cooling where the rainfall occurs. This rainfall activity will be
associated with typical afternoon patterns and the presence of an
upper-level trough moving from the western Atlantic. The upper-
level trough will position itself to the north of the islands,
upward vertical motion and better moisture content through
Tuesday. Therefore, showers and thunderstorms will increase in
coverage once again, particularly 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 steep terrain, mainly along the Cordillera Central.
Across the USVI, a similar weather pattern is expected to continue
over the next few days, with diurnally induced afternoon showers
developing downwind of the islands.

High temperatures in the upper 80s to lower 90s will persist
across the lower elevations of the islands during the morning and
early afternoon hours. Therefore, excessive heat conditions can be
anticipated throughout the short-term period.

.LONG TERM...Thursday through Monday...

High pressure in the lower levels will be found just north of the
area and this will induce a more typical easterly surface flow.
That high pressure will move westward and a mild tropical wave
will move through the U.S. Virgin Islands, Puerto Rico and most
of the eastern Caribbean south of the area Friday afternoon and
Friday night. On Saturday a shearline will approach the islands
from the north but remain north of 20 north. This will maintain
easterly flow and given that the moisture does not really diminish
we will continue to see typical shower and thunderstorm formation
Saturday and Sunday. On Monday flow becomes southerly and very
light and temperatures will begin to climb a few degrees. This
will lead to stronger showers and thunderstorms in central and
western Puerto Rico.

At upper levels a weak TUTT low will be found just south of Puerto
Rico on Thursday. When it moves west, a second TUTT will dive in
from the northeast and will create a environment only somewhat
favorable to active convection given that 500 mb temperatures will
rise during the period--especially after the tropical wave
passes. Strong high pressure at upper levels will hold over Cuba
and then move over southern Florida adding to the stability over
the weekend and into Monday. With light southerly surface winds
beginning then, we will see a warming trend.

&&

.AVIATION...
(06Z TAFs)

VFR conditions across all terminals. Clds and SHRA are expected to
dvlp across the interior sections of PR, followed by SHRA/TSRA with
mtn obscurations and MVFR conds over the interior through 23/21Z.
TJSJ and TJBQ will experience brief MVFR conds, particularly after
18Z. Winds less than 10 kt with a few higher gusts along the
coasts and near TSRA till 23/22Z.

&&

.MARINE...An active low to our north will bring in 4-5 foot swell
THursday through Sunday. There may also be a northeasterly
component due to tropical storm formation--which the National
Hurricane Center is now giving a 70 percent chance of formation
during the next 7 days.

&&

.BEACH FORECAST...After a period of low risk on Monday and Tuesday
rip current risk will increase on north and northeast facing
beaches Wednesday through Friday. Rip current risk could be high
on Friday due to a 5 foot swell reaching the area from the north
northeast.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ001>004-007-
     013.

     Excessive Heat Warning until 5 PM AST this afternoon for PRZ005-
     008-010-011.

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

     Excessive Heat Warning until 5 PM AST this afternoon for VIZ002.

AM...None.

&&

$$

SHORT TERM/AVIATION...MMC
LONG TERM/MARINE....WS