298
FXCA62 TJSJ 160931
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
531 AM AST Tue Jul 16 2024

.SYNOPSIS...
Tutt and associated low level trough will continue to cross the
region today. Fragments of trade wind moisture will affect the area
as the low level winds become more east southeast then southeasterly
by Wednesday. Hot and humid conditions will continue the rest of the
period with an elongated plume of Saharan dust forecast to affect
the area into the upcoming weekend promoting hazy skies. Good daytime
heating along with local sea breezes and the proximity of the upper
trough will aid in enhancing afternoon shower and thunderstorm development
each day. A tropical wave will bring additional moisture and instability
to the islands Friday through Saturday. Breezy conditions with moderate
to locally strong east to southeast winds will return to the region
by Wednesday and continue into the weekend.

&&

.SHORT TERM...Today through Thursday...

Current satellite derived precipitable water (PWAT) values indicate
a dry air mass over the region. Overnight minimum temperatures were
in the upper 70s to low 80s at lower elevations and in the upper 60s
to low 70s at higher elevations. Steering winds gradually veered to
become east-southeast. Showers moved across the area during the
overnight hours, leaving mostly minimal accumulations over eastern
Puerto Rico, Vieques and St. Croix since midnight. Radar estimated
rainfall indicate up to 0.68 to 0.95 inches at sectors of Ceiba
since midnight.

A TUTT low will linger north of the region today while moisture
gradually increases. More normal PWAT values will be seen late this
morning. A patch of moisture related to an induced surface trough
will increase PWAT values to above normal, above 2 inches, this
evening and tomorrow. The presence of these features will promote
instability, and, 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 during 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. Model
guidance suggest PWAT values staying at normal values for Thursday.
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.

Lingering Saharan dust will be present today. However, a more
prominent Saharan Air Layer (SAL) will approach the area by tomorrow
morning, 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 0.55. Highs will be in the upper 80s to around
the low 90s in lower elevations of the islands. The Saharan dust can
also inhibit nighttime cooling (especially with more prominent dust
like on Wednesday), 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...Friday through Wednesday...

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 Friday into Saturday while a tropical wave
will enter the eastern Caribbean and cross the forecast area. This
is expected to bring increasing moisture and instability to the
region along with a wind surge and moderate to strong southeasterly
winds by Saturday afternoon and on Sunday, as the tropical wave
exits the region. Accompanying and trailing the wave is an
extensive plume of dense Saharan dust which will quickly spread
across the region the rest of the weekend through Monday before
diminishing from Tuesday onwards. Winds are then to become more
easterly while another Tutt and induced surface trough approaches
from the east. Hot,humid and hazy conditions will likely persist
through the weekend.

Model guidance continued to initialize fairly well so far and still
suggests the most impactful period to be Friday through Saturday due
to increasing moisture convergence and instability with enhanced
overnight and afternoon convection expected due to the tropical wave
and cooler advective temperatures aloft. This will be followed by
hazy conditions as mentioned and a drier airmass due to the dense
layer of Saharan dust particulates which will maintain unseasonably
hot and humid conditions. Local temperatures are expected to continue
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 Friday and into
the weekend due to the tightening of the local pressure gradient
and a wind surge which will increase the trade winds. Precautionary
statements and small craft advisories may be required for all or
portions of the local waters and passages through Saturday.


Sunday through Monday are expected to improve with typical summer
time weather and hot and hazy conditions. So far only 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 in turn will increase the chance
for more frequent overnight passing showers and afternoon convection
across parts of the islands.

Otherwise based on the tropical weather outlook for 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. VCSH
and VCTS are forecast for the interior and NW PR, including TJBQ, at
around 16/17Z-22Z. Streamers can also affect TJSJ at that time. This
can promote brief MVFR conditions. Brief VCSH across other terminals
possible during the period. Lingering saharan dust present,
visibility P6SM.

&&

.MARINE...
A surface high pressure centered across the west central Atlantic and
an upper trough and associated induced low level trough crossing the
region will maintain a light to moderate east to southeast wind flow
with occasional passing showers across the local waters and passages.
Winds will become more southeasterly and increase to moderate to locally
fresh by Wednesday becoming breezy and thus creating choppy conditions.
Small craft operators should exercise caution across the offshore
waters and local passages today.

&&

.BEACH FORECAST...
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...Excessive Heat Warning from 11 AM this morning to 5 PM AST this
     afternoon for PRZ001-005-008-010.

     Heat Advisory from 11 AM this morning to 5 PM AST this afternoon
     for PRZ002-003-007-011>013.

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