020
FXCA62 TJSJ 081530
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1130 AM AST Mon Jul 8 2024

.UPDATE...Outflow boundaries--perhaps not so noticeable on the
ground--swept through the U.S. Virgin Islands and brought some
quick passing showers with less than 5 hundredths of an inch and
also through eastern Puerto Rico where up to 36 hundredths of an
inch fell on the eastern slopes. Showers were generally moving
from the east northeast and have tapered off but not disappeared
entirely. Heating from the partly sunny skies will fuel new
convection over western and interior Puerto Rico with showers and
thunderstorms likely there. Otherwise the outflow boundaries have
cleared the area behind them. The convection associated with the
tropical wave has made little progress due to its link to the
upper level trough anchored to an upper level low 1000 miles north
of Martinique in the Windward Islands. The trough that initially
enhanced the wave is rotating away from the local area.
Nevertheless, the band of moisture about 450 miles across, associated
with the tropical wave is moving westward and the leading edge is
now over central Puerto Rico. This band has deeper moisture with
precipitable water values of 1.9-2.15 inches. It is expected that
this additional moisture will bring good activity to the area that
should continue well into the night. Current expectations are that
Saharan dust will arrive in the area after midnight and become
somewhat oppressive by dawn.

No other changes to the forecast grids. Temperatures are behaving
as expected with lower 90s in the lower elevations and mid 70s to
mid 80s in the higher elevations. heat indices have reached the
100-110 degree range and will hold on to the heat advisory.

&&

.PREV DISCUSSION... /issued 508 AM AST Mon Jul 8 2024/

SYNOPSIS...

A tropical wave will reach the islands increasing the potential
for showers and thunderstorms and bringing a break from Saharan
dust today, but another Saharan Air Layer (SAL) is expected to
reach the islands tomorrow and last until mid week. Lingering
moisture and slightly unstable atmospheric conditions will
continue to support shower and thunderstorm development for the
same period. In addition, an elevated to significant heat risk
will likely persist. Another tropical wave is forecast to reach
the islands by next weekend.

SHORT TERM...Today through Wednesday...

An overnight radar and satellite imagery analysis indicated clear to
partly cloudy skies, with passing showers brushing Puerto Rico`s
northern coast, Vieques, Culebra, and the northern U.S. Virgin
Islands. Reports again highlighted warm overnight low temperatures
above 80 degrees Fahrenheit in coastal areas of the San Juan
metropolitan area, eastern and southern Puerto Rico, and the local
islands. Coastal observation sites and local buoys have also
recorded warm low temperatures, hovering around 83-84 degrees
Fahrenheit. Winds were mainly light to calm and variable.

Satellite and radar data also indicated a tropical wave moving
through the northern Leeward Islands, accompanied by increased
cloudiness and persistent thunderstorm activity. As this system
advances through the eastern Caribbean, it will bring substantial
tropical moisture, with precipitable water levels exceeding seasonal
moisture thresholds through most of the forecast period. This will
elevate the likelihood of showers and thunderstorms. On Tuesday, a
brief break is expected as a drier air mass and a dense Saharan Air
Layer follow the tropical wave. However, persistent lingering
moisture and slightly unstable atmospheric conditions will continue
to support shower and thunderstorm development throughout the
forecast period. This weather pattern may result in heavy rainfall,
frequent lightning, and strong winds, leading to a moderate to high
risk of excessive rainfall, particularly in eastern and far western
Puerto Rico. Potential impacts include water ponding on roads,
flooding in poorly drained areas, and urban and small stream
flooding. Localized flash floods are also possible. Additionally,
breezy conditions, driven by high surface pressure over the North
Atlantic, are likely, especially in southern and northern Puerto
Rico and the nearby islands.

Even with the anticipated high rain chances, 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 abundant moisture in the air, heat indices
in many coastal and urban areas may soar above 108 degrees
Fahrenheit. These conditions present significant health risks,
leading to the issuance of Heat Advisories. It is crucial for
residents and visitors, especially those participating in outdoor
activities, to take necessary precautions to stay safe.

LONG TERM...Thursday through Monday...

Model guidance suggests a broad surface high pressure will remain
anchored across the Atlantic Ocean, promoting easterly winds across
the islands throughout the long-term period. Thursday seems to be
a transition day where we still have residual humidity from the
tropical wave, but some patches of drier air are beginning to
enter along with the wind flow, resulting in a mix of clouds and
sunshine and the arrival of quick passing showers. The latest
Precipitable Water (PW) models suggest values decreasing from
above-normal to normal climatological levels that day. Still,
afternoon convection will remain possible with available moisture,
diurnal heating, and local effects. By the end of the week, the
NASA Dust Extinction guidance is suggesting the presence of
another Saharan Air Layer across the islands which probably will
limit rain development that day. However, during the weekend, the
next tropical wave should approach our region. If the dry air mass
surrounding it does not negatively impact it, we may see a wet
and unstable pattern with a better chance of organizing convection
on Saturday or Sunday.

Temperature-wise, an elevated to significant heat risk will likely
persist across lower elevations and coastal areas of Puerto Rico
and the rest of the islands, as models suggest the pattern will
continue with warmer-than-normal temperatures at 500 MB and 925 MB.

AVIATION...

(06Z TAFs)

VFR conditions will likely prevail across all terminals through
the morning hours. However, increased SHRA/TSRA from a passing
tropical wave may result in MVFR to brief IFR conditions, with
reduced visibility and lower ceilings, at most terminals. Light to
calm and variable winds will become more easterly and increase to
14-18 knots between 08/13-23Z, accompanied by higher gusts and
sea breeze variations. Higher wind gusts are likely near the
SHRA/TSRA.

MARINE...

A surface high pressure anchored across the Atlantic Ocean will
tighten the pressure gradient across the Northeast Caribbean and
promote moderate to fresh easterly winds with choppy seas. A
tropical wave will move across the islands today, increasing the
potential for showers and thunderstorms across the local waters.
Higher winds and seas near thunderstorms.

BEACH FORECAST...

There is a moderate risk of rip currents for southwestern,
northwestern, northern and eastern Puerto Rico, as well as for
Culebra, St. Thomas, St. John, and St. Croix. Low risk elsewhere.
Mainly low risk is forecast through mid-week, then moderate risk
returns.For additional information, consult the Surf Zone
Forecast (SRFSJU) issued by the NWS San Juan Office.

&&

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

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

AM...None.

&&

$$

SHORT TERM/MARINE...WS
PUBLIC/COOP...RC
UPPER AIR...RVT/MNG