356
FXCA62 TJSJ 210912
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
512 AM AST Sat Sep 21 2024

.SYNOPSIS...

Slightly drier conditions will prevail across the islands today.
A typical pattern of calm nights and mornings, followed by warm-
to- hot afternoons and evenings driven by diurnal heating and weak
steering winds, will prevail for the next few days. 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 persist over
the next few days.

&&

.SHORT TERM...Today through Monday...

Currently, radar and satellite imagery show some isolated showers
over the local waters. Most clouds and showers have dissipated over
land except for some tops that have blown south from earlier
thunderstorms over the Atlantic waters. Low temperatures this
morning are running from the mid 70s to the lower 80s in the lower
elevations and from the upper 60s to the mid 70s in the mid to upper
elevations. Winds have quieted down to 5 mph or less with many areas
becalmed.

The TPW product shows a drier strip of air crawling very slowly
northward. It has reached Ponce and Saint Croix and will spread
across Saint Thomas and some of Puerto Rico during the day. Lower
level winds are quite light with winds 10 kts or less below 25 kft.
This is due to high pressure over the Canary Islands being blocked
by a pair of low pressures between 25 and 30 north and between 40
and 60 west. Weak high pressure can be seen north of the Mona
Passage and across the local area. At mid levels a broad area of
high pressure extends across the tropical Atlantic. At upper levels
the TUTT low over the area tonight is edging east and will allow a
30 knot jet from the north northwest to push across the area Saturday
through Sunday. Low pressure will bring troughing to our area Monday
night, but its associated low will remain north of 30 north and will
just barely be able to impinge on the high pressure over the west
central Caribbean Sea. Flow at lower levels and specifically the
mean flow between 1000 and 700 mb will be southerly and will bring
the drier air to our south over the area and allow temperatures to
warm more than yesterday. For this reason Excessive Heat Warnings
for most of the lower elevations of Puerto Rico and Saint Croix have
been issued. Heat Advisories have been issued for the remaining
lower elevations including the interior valleys in eastern Puerto
Rico.

All of these warm-to-hot conditions will fade a little on Sunday
when better moisture arrives, and precipitable water values peak
around 7 PM AST each day of the short term period when showers and
thunderstorms grow over interior Puerto Rico. But warm to hot
conditions will persist through the short term and into the long-
term before meaningful relief occurs.

&&

.LONG TERM...Tuesday through Friday...

The inherited forecast remains on track. According to the latest
model guidance, a deep-layered trough is expected to move just
north of the Caribbean islands until at least late Tuesday into
Wednesday. This weather feature will result in a weak steering
wind flow, with speeds ranging from 1 to 5 knots and gradually
recovering to around 10 knots after its passage. Tuesday is
expected to be a day with variable weather conditions, as the
trough will be closest to the islands allowing the surface
moisture to slightly filter across all levels of the atmosphere.
The 500 mb temperatures are expected to be around -7 to -8 degrees
Fahrenheit on Tuesday, which could lead to an increase in
instability and the potential for thunderstorm development
particularly during the afternoon hours. Despite the passage of
the trough, occasional patches of moisture are anticipated to move
across the forecast area throughout the period, 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. Due to weak winds, it is
difficult to determine which area will be more affected, aside
from the interior, which should receive the bulk of the activity
each day. Towards the end of the workweek, a tropical wave will
reach the islands. We anticipate occasional ponding of water on
roads and poorly drained areas from Tuesday into Friday.
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...
(06Z TAFs)

Mainly VFR conditions ovr land. Aft 21/15Z Clds and SHRA to dvlp ovr
interior PR followed by SHRA/TSRA with mtn obscurations and
MVFR/IFR conds over interior and wrn PR aft 21/17Z till arnd 21/21Z.
TAF sites will have only brief unfavorable conds. Winds less than 10
kt with a few hir gusts in mostly sea breezes and nr TSRA till aft
21/22Z. SHRA/TSRA ovr land will dissipate btwn 21/21-23Z. High
density altitudes at TAF sites are expected today due to heat and
humidity.

&&

.MARINE...

A weak surface low pressure to the northeast of the region will
maintain weak and variable winds throughout early next week. Showers
and isolated thunderstorms will persist across the regional waters,
particularly off the western and southern coastal waters of Puerto
Rico each afternoon.

&&

.BEACH FORECAST...

The moderate rip current risk will persist for the northern and
eastern coast of Puerto Rico and Culebra. Elsewhere is low.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001-
     003-005-007-008-010-011.

     Heat Advisory until 5 PM AST this afternoon for PRZ002-004-012-
     013.

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

     Heat Advisory until 5 PM AST this afternoon for VIZ001.

AM...None.
&&

$$

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