776
FXCA62 TJSJ 170913 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
513 AM AST Wed Jul 17 2024

.SYNOPSIS...Tutt low and associated low level trough will continue
to influence the local weather pattern today but will gradually
weaken while moving westward away from the region. Low level winds
will be generally from the southeast as high pressure builds across
the west and central Atlantic. Hot and humid conditions will continue
the rest of the workweek with an extensive layer of Saharan dust
forecast to spread and linger across the region through the end of
the week promoting hazy skies. A tropical wave will bring additional
moisture and instability Friday through Saturday. Moderate to locally
strong east to southeast winds will bring breezy conditions later
today and continue through the weekend.

SHORT TERM...Today through Friday...

Current satellite derived precipitable water (PWAT) show normal to slightly
above normal values (above 2 inches) across the islands. Overnight low
temperatures were in the upper 70s to low 80s at lower elevations and
in the upper 60s to low 70s at higher elevations of Puerto Rico. East-
southeast to southeast winds steered showers, that left radar estimated
rainfall since midnight, across southern, eastern interior and eastern
Puerto Rico, St. John, Vieques and Culebra. A prominent Saharan Air
Layer (SAL) will reach the islands later today prompting hazy skies
and decreased air quality for the rest of the short term period. Aerosol
Optical Depth (AOD) values can reach around 0.40 to 0.60. These conditions
can exacerbate respiratory conditions in immunocompromised, vulnerable
and sensitive groups.

A TUTT low will linger north of the region today while gradually moving
away, another upper low will remain well east of the area through the
second half of the workweek. A mid to upper level ridge will also gradually
build across the eastern Caribbean during the period. Normal to above
normal PWAT values will continue through Thursday morning. Model guidance
suggests PWAT values decreasing to up to high end normal values from
Thursday morning through Friday morning. By Friday afternoon onwards,
the moisture field of an approaching tropical wave will once again
promote increasing PWAT values to up to above normal to end the period
as the wave crosses Friday night into the long term period. The nearby
presence of the TUTT low, an induced surface trough and a tropical wave
will instability during the period, which alongside with daytime heating,
sea breeze convergence and local effects, will aid in enhancing afternoon
shower and thunderstorm development. Afternoon showers and thunderstorms
today will concentrate over the northwestern quadrant of Puerto Rico,
as well as possibly downwind of El Yunque. Surface winds will generally
be from the east- southeast to southeast during most of the period,
increasing to become more breezy for the second half of the workweek,
as the upper trough retrogrades westward and a surface high-pressure
continues to builds over the central Atlantic. Patches of moisture,
showers and potential t-storms will affect windward sectors of the
islands. Strong showers and thunderstorms can lead to periods of heavy
rainfall that will cause urban and small stream flooding. Daytime
highs will be in the upper 80s to around the low 90s in lower
elevations of the islands. Saharan dust will also inhibit nighttime
cooling,prompting warmer than normal nights. Heat indices in coastal,
urban and lower elevation areas can reach above 108 degrees Fahrenheit
in part due to east- southeast winds and available moisture.

LONG TERM...Saturday through Thursday...

Strong surface high pressure will remain the dominant feature across
the Atlantic and north of the region. This will maintain moderate
east to northeast winds through Saturday as a tropical wave and associated
moisture field enters the eastern Caribbean and crosses the forecast
area. This increasing moisture will bring unstable conditions to the
region followed by a wind surge and moderate to strong southeasterly
winds by Saturday afternoon and early Sunday as the tropical wave
exits the region. Trailing the wave an extensive layer of dense
Saharan dust will quickly spread across the region the rest of the
weekend and linger through Monday then diminish from Tuesday onwards.
Low level winds are then expected to become more easterly as another
Tutt and induced surface trough is forecast to approach the region
from the east. All in all Hot, humid and hazy conditions will likely
be the dominant weather pattern through the weekend and into the early
part of the following week with some locally and diurnally induced
afternoon convection each day.

Recent model guidance continued to initialize well and suggests the
most impactful period to be Friday through Saturday due to increasing
moisture convergence and instability. That said there will be a better
chance for enhanced overnight and afternoon convection as the tropical
wave and somewhat cooler advective temperatures aloft are across the
region. As previously mentioned considerably hazy conditions and fairly
drier airmass will follow due to the dense layer of Saharan dust particulates.
Local temperatures will remain slightly above normal with maximum daytime
heat indices forecast to be elevated to significant at times especially
along the coastal and urban areas. Marine conditions area also forecast
to deteriorate through the weekend due to the tightening of the local
pressure gradient and a wind surge which will increase the tradewinds.
Precautionary statements and or small craft advisories may therefore
continue for portions of the local waters and passages due to the
choppy wind driven conditions.

Sunday through Monday are forecast to be the driest period with typical
summertime weather and hot and hazy conditions. Isolated to scattered
locally and diurnally afternoon convection expected mainly over the
west and northwest sections of Puerto Rico. By late Monday through
the rest of the period, the upper ridge will erode as a Tutt low is
forecast to retrograde across the region and low level moisture gradually
increases. This will again increase the chance for more frequent overnight
passing showers followed by afternoon convection across parts of the
islands.

Based on the Tropical Weather Outlook from the National Hurricane
Center...Tropical cyclone formation is not expected during the next
seven days.

&&

AVIATION...
(06Z TAFS)

Mainly VFR conditions across the terminals. Winds will be ESE to SE
at up to 12 to 18 kts with sea breeze variations and higher gusts.
VCSH and VCTS are forecast for the interior and NW PR, including
TJBQ, at around 17/17Z-22Z with streamers that can also affect TJSJ
at that time. This can promote brief MVFR conditions. Brief VCSH
across other terminals possible during the period. Increasing
Saharan dust after 17/18Z.

&&

MARINE...
A surface high pressure anchored across the west central Atlantic
and an upper trough and associated induced low level trough crossing
the region will aid in maintaining a light to moderate east southeast
wind flow promoting occasional showers and isolated thunderstorms across
the local waters and passages. Winds will become more southeasterly
and increase to moderate to locally fresh later today. As the wind
increases, choppy seas will be likely across the local waters and
passages, resulting in occasional seas up to 7 feet and winds up
to 20 knots. Precautionary statements will continue for the local
waters and Small craft advisories may be required for some areas
by late Thursday.
&&

BEACH FORECAST...
For the remainder of the week, there will be a moderate risk of
rip current for most beaches and therefore life-threatening rip
currents will be possible in the surf zone.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for PRZ001-002-005-008-012-013.

     Excessive Heat Warning from 11 AM this morning to 5 PM AST this
     afternoon for PRZ010.

VI...Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for VIZ001-002.

AM...None.
&&

$$

SHORT TERM...MRR
LONG TERM....RAM
AVIATION...MRR