260
FXCA62 TJSJ 100815
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
415 AM AST Wed Jul 10 2024

.SYNOPSIS...

Moisture from a past tropical wave and the proximity of a TUTT
low to the north will keep the potential for showers and t-storms
across the forecast area through tonight. A Saharan Air Layer
(SAL) over the islands will maintain hazy skies and decreased air
quality through this afternoon, but another round of Saharan dust
is anticipated by Friday afternoon. An elevated heat risk will
also likely persist. Another tropical wave is forecast to reach
the islands by Friday night.

&&

.SHORT TERM...Today through Friday...

An overnight radar and satellite imagery analysis indicated
increased showers with isolated thunderstorms over the local waters,
with some generating measurable rainfall amounts to around half an
inch in portions of the San Juan metropolitan area and a quarter of
an inch in Vieques and the northern U.S. Virgin Islands. Although
slightly lower than previous days, reports again highlighted warm
overnight low temperatures above 80 degrees Fahrenheit in coastal
areas of eastern and southern Puerto Rico and the local islands. In
contrast, higher elevations have reported low temperatures in the
upper 60s. Winds were mainly from the east-northeast at 5-10 mph but
light to calm and variable further inland.

Due to lingering moisture from a departing tropical wave and the
nearby presence of a TUTT low to the north, showers and isolated
thunderstorms are expected to persist today and likely continue into
late tonight. Precipitable water values remain above average,
ranging from 2.0 to 2.3 inches, and temperatures at the 500 MB level
are lower than previous days and within normal thresholds, creating
favorable conditions for deep convection. The bulk of the shower and
thunderstorm activity will transition from local waters in the
morning to inland regions during the afternoon. Notably, areas
downwind of mountains toward the west-southwest of Puerto Rico are
expected to be most affected, driven by east-northeasterly winds.
The expected activity will likely produce frequent lightning, strong
gusty winds, and intense downpours, prompting concerns about
flooding. Currently, west-southwestern Puerto Rico faces a moderate
risk of excessive rainfall, while other areas have a limited risk.
Possible effects include water ponding on roads and areas with poor
drainage and urban and small stream flooding. Localized flash floods
from the heaviest and longest-lasting rainfall are also possible.

While convective activity is expected to decrease on Thursday and
Friday due to the arrival of a drier-than-normal airmass and the
establishment of mid-level ridging over the region, a surge of
tropical moisture linked to the leading edge of a tropical wave
will reach the northeastern Caribbean by Friday night. Meanwhile,
showers and possible thunderstorms are anticipated each day,
primarily driven by diurnal heating and local effects in the
afternoon.

Warmer-than-usual conditions will persist in the coming days.
Daytime highs will range from the lower 90s in lower elevations to
around 80 degrees in higher elevations, with heat indices in many
coastal and urban regions soaring above 108 degrees Fahrenheit.
Since these conditions present significant health risks, Heat
Advisories have been issued for most coastal areas of Puerto Rico
today. It is crucial for residents and visitors, especially those
participating in outdoor activities, to take necessary precautions
to stay safe.

&&

.LONG TERM...Saturday through Tuesday...

A tropical wave will approach the region on Saturday, bringing an
increase in moisture to the region. This wave does not look
particularly strong, so a significant rainfall event is not
anticipated. In general, the weather will be driven by a low to mid
level ridge centered over the central Atlantic. Once the wave
departs later in Saturday, dry air at the mid levels will filter in.
This air mass will contain Saharan dust, so skies will be hazy
through early next workweek.

On Monday, a Tropical Upper-Tropospheric Trough (TUTT) will
approach the region. This feature will also be evident in the mid
and low levels of the atmosphere, with winds first from the
northeast on Monday, and then shifting from the southeast on
Tuesday. These days, enough instability will be available to
generate active afternoons across the interior and western Puerto
Rico. Additionally, the TUTT will weaken the pressure gradient,
with a steering flow below 10 knots anticipated. Trade winds will
carry some showers across portions of Puerto Rico and the Virgin
Islands too, but the higher risk of flooding and mudslides will be
for the interior and west of Puerto Rico.

In terms of temperatures, 925 mb temperatures are expected to be
above normal, so the users are advised to remain hydrated, and to
take frequent breaks from the sun. This level of heat affects most
individuals sensitive to heat, specifically those without effective
cooling or adequate hydration. Impacts possible in some health
systems and in heat sensitive industries are possible.

&&

.AVIATION...
(06Z TAFS)

SHRA/TSRA may lead to MVFR to brief IFR conditions, with reduced
visibility and lower ceilings, primarily affecting TJSJ, TJBQ, and
USVI terminals throughout the forecast period and TJPS between 10/17-
23Z. The remaining terminals may experience occasional VCSH/VCTS.
Light to calm and variable winds will shift to E-ENE and increase to
14-18 knots between 10/13-23Z, with higher gusts and sea breeze
variations. Stronger wind gusts are expected near SHRA/TSRA.

&&

.MARINE...

A surface high pressure anchored across the Atlantic Ocean will
promote light to moderate easterly winds today, then increasing to
moderate to fresh for the latter part of the week. Residual
moisture from a departing tropical wave will keep the potential
for showers and thunderstorms across the local waters today and
tonight, with some improvement tomorrow due to an approaching
drier airmass. Moisture from another tropical wave is expected to
reach the northeastern Caribbean by Friday night leading to
showers and possible thunderstorms.

&&

.BEACH FORECAST...

There is a moderate risk of rip current for beaches of Saint Croix
today, low risk of rip currents elsewhere. Similar conditions are
expected to persist Thursday, then increase to moderate for most
beaches at the end of the week through the weekend. For more details
about the latest forecast please refer to the Surf Zone Forecast
(SRFSJU) issued by the NWS San Juan Office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001-003-005-007-008-010-011.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ002.

AM...None.

&&

$$

SHORT TERM/AVIATION...ICP
LONG TERM...ERG/GRS
MARINE/BEACH FORECAST...YZR