375
FXCA62 TJSJ 110926
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
526 AM AST Thu Jul 11 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 in the forecast, mainly
due to diurnal heating and local effects, today and Friday. Another
pulse of suspended Saharan dust particulates will reach the islands
on Friday and persist through Sunday. An elevated heat risk will
also likely persist. Moisture from another tropical wave is forecast
to reach the islands by Friday night leading to an increase in showers
and thunderstorms throughout the weekend. On Monday, another TUTT
will approach the region enhancing instability.

&&

.SHORT TERM...Today through Saturday...

Today, moisture levels will decrease slightly to near-normal levels.
Meanwhile, the TUTT that impacted the area the previous day will
continue to move westward as ridging aloft settles in. This will
promote a less active day, with some showers reaching windward
coastal areas during the morning hours. In the afternoon, diurnal
heating and local effects will promote showers and isolated
thunderstorms across western and northwestern portions of Puerto
Rico under an east-southeast steering flow. However, this activity
is expected to have less areal coverage and last for a shorter
period.

For the rest of the short-term forecast, weather conditions will
vary. On Friday, another pulse of suspended Saharan dust
particulates will reach the islands, promoting hazy skies for the
most part. However, the leading edge of a tropical wave will also
arrive, increasing moisture content to above-normal levels,
particularly during the afternoon hours. This raises the potential
for isolated to scattered showers and some isolated thunderstorms in
localized areas, mostly across interior and western Puerto Rico. By
Saturday, model guidance suggests the arrival of a wind surge, which
could bring squally weather for a very short period. Therefore,
after 12z, expect an increase in thunderstorm activity with the
potential for frequent lightning and gusty wind conditions. Behind
the wind surge, a broad area containing higher concentrations of
suspended Saharan dust particulates will engulf the forecast area.
This could shorten the period of shower activity in some areas,
bringing back hazy skies by late Saturday evening.

Warmer-than-usual conditions will persist in the coming days.
Consequently, some urban and coastal areas may experience a limited
to elevated excessive heat threat, increasing the possibility of
heat exhaustion with prolonged exposure, particularly on Friday and
Saturday.

&&

.LONG TERM...Sunday through Wednesday...

Based on the latest models, during the beginning of the long-term
period, we will continue to have the presence of Saharan dust
across the region exacerbating health conditions for immunocompromised
individuals, vulnerable populations, and those sensitive to air
quality issues. During Sunday afternoon and evening, the Saharan
dust should be receding. Residual moisture from a tropical wave
will keep precipitable water mainly above normal values, with a
slight decrease to normal on Sunday. On Monday, PWAT should
increase again as a Tropical Upper- Tropospheric Trough (TUTT)
will approach the region and for now looks like it could be the
more active day of this forecast period. 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 and Wednesday. Early next week, enough
instability will be available to generate active afternoons across
the interior and western Puerto Rico. Temperatures in the 500 MB
are expected to drop to around -7 degrees Fahrenheit on Sunday and
Monday and that could be beneficial for thunderstorm development.
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.
Potential impacts range from ponding water on roads to flooding in
poorly drained areas and urban and small stream flooding.

In terms of temperatures, warmer-than-usual conditions are
expected for most of the period. On Monday, if the rain
materializes, the temperature should stay a few degrees lower.
Generally, 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 coastal and urban
areas may soar above 108 degrees Fahrenheit.

&&

.AVIATION...
(06Z TAFS)

VFR conditions will likely prevail across all terminals through the
morning hours. However, afternoon SHRA/TSRA may result in MVFR to
brief IFR conditions, with reduced visibility and lower ceilings,
particularly for TJBQ. The remaining terminals may experience the
occasional VCSH/VCTS. Light to calm and variable winds will become
more E-ESE and increase to 14-18 knots between 11/13-23Z, accompanied
by higher gusts and sea breeze variations. Higher wind gusts are
likely near the SHRA/TSRA.

&&

.MARINE...

A surface high pressure across the Atlantic Ocean will promote
moderate to fresh easterly to southeasterly winds for the next
several days. An approaching drier airmass will result in less
rain chances across the waters today, but some thunderstorms could
develop and move across the western waters of Puerto Rico during
the afternoon hours. Moisture from another tropical wave is
expected to reach the northeastern Caribbean by Friday night
leading to an increase in showers and thunderstorms throughout the
weekend.

&&

.BEACH FORECAST...

There is a moderate risk of rip currents for beaches of Saint Croix
today, low risk elsewhere. However, by tonight, the moderate risk
of rip currents is forecast to spread to the rest of the local islands
and remain moderate through the upcoming weekend. For additional
information, consult the Surf Zone Forecast (SRFSJU) issued by the
NWS San Juan Office.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for PRZ001-003-005-007-008-010-011.

VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon
     for VIZ002.

AM...None.

&&

$$

SHORT TERM/AVIATION...CVB/CAM
LONG TERM/MARINE/BEACH FORECAST...YZR