272
FXCA62 TJSJ 170820
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
420 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 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 trade winds.
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...None.
VI...None.
AM...None.
&&

$$

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