107
FXCA62 TJSJ 102103
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
503 PM AST Wed Jul 10 2024

.SYNOPSIS...

Moisture is forecast to gradually decrease, with patches of more
humid and drier air reaching the region, through Friday night and
the establishment of mid to upper level ridging over the region.
More breezy easterlies, during the second half of the week, with
showers and possible afternoon thunderstorms are forecast, mainly
due to diurnal heating and local effects, tomorrow and Friday. An
elevated heat risk will also likely persist. Moisture from another
tropical wave is forecast to reach the islands by Friday night.
On Monday, another TUTT will approach the region.

&&

.SHORT TERM...Tonight through Friday...

The proximity of a Tropical Upper-Tropospheric Trough or TUTT low
north of the area and lingering moisture from a past tropical wave
prompted an active morning and early afternoon across the region.
Showers and thunderstorms moved from the Anegada Passage steered by
east-northeast winds during the early morning hours, affecting the
USVI, Vieques and Culebra. Under favorable conditions, this line of
showers and thunderstorms moved into eastern Puerto Rico by 8 to 9
AM AST. During the rest of the morning to early afternoon, this
activity moved across Puerto Rico. By 11 AM to noon, this activity
started to intensify and converge over southwestern and western PR,
prompting higher wind gusts, frequent lightning and flooding, as
well as lowering high heat indices that developed over western-
northwestern PR by that time. Because of this Flood products and
Special Weather Statements were issued. Several flood reports were
received for areas of Mayaguez, Cabo Rojo, San German and Sabana
Grande. The strongest activity moved offshore after around 2 PM AST.
Radar estimated precipitation above 2 inches was detected over most
municipalities in the SW quadrant of Puerto Rico. Radar estimated
precipitation above 3 inches was detected over Mayaguez, San German
and Sabana Grande. Highs were in the upper 80s to around the low 90s
across coastal sectors of the islands.

Current satellite derived precipitable water (PWAT) values shows an
above normal plume of moisture, around and above 2 inches, related
to the past tropical wave, over the islands. Lines of showers from
the northern USVI are currently reaching Culebra, eastern and
eastern interior Puerto Rico. A surface high pressure across the
Atlantic Ocean will promote light to moderate ENE winds tonight,
then increasing and veering to breezy easterlies by the latter part
of the workweek. Showers and potential t-storms will continue
reaching windward sectors of the islands tonight. Minimum
temperatures will be in the upper 70s to low 80s at lower elevations
of the islands. Convective activity is expected to decrease on
Thursday and Friday due to the arrival of patches with normal to
below normal PWAT values, the TUTT moving away and the
establishment of mid to upper level ridging over the region.
Another patch of moisture will move into the area tomorrow night.
Then, moisture from another tropical wave will start increasing
PWAT values to above normal Friday night. Showers and possible
thunderstorms are forecast each day, mainly due to diurnal heating
and local effects in the afternoon. An elevated to significant
heat risk will also persist, with heat indices in coastal, urban
and lower elevation areas reaching above 108 degrees.

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

./FROM PREV DISCUSSION Issued at 415 AM AST Wed Jul 10 2024/

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...

(18Z TAFS)

SHRA/TSTMS have cleared PR as wwd movg outflows move thru the Mona
Channel. E sfc wind flow is genly less than 10 knots. SHRA/TSRA to
diminish aft 10/22Z and behind the main band in the Mona channel
altho some redvlpmt is possible ovr interior PR. SHRA/TSRA may
redvlp E of PR aft 11/06Z and sprd wwd later. Aft 10/23Z winds will
bcm land breezes of less than 10 kt. Sea breezes of 10-20 kt will
return in E flow aft 11/14Z. Less SHRA and TSRA for wrn PR Thursday.

&&

.MARINE...

A surface high pressure anchored across the Atlantic Ocean will
promote light to moderate easterly winds tonight, 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 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...

For tonight and tomorrow, there is a moderate risk of Rip Currents
for the eastern beaches of St. Croix; low risk elsewhere. By
tomorrow night, the moderate risk of rip currents is forecast to
spread to the rest of the local islands. For additional
information, consult the Surf Zone Forecast (SRFSJU) issued by the
NWS San Juan Office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.

VI...None.

AM...None.

&&

$$

SHORT TERM....CAM
LONG TERM.....MRR
PUBLIC DESK...GRS