799
FXCA62 TJSJ 172015
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
415 PM AST Fri May 17 2024

.SYNOPSIS...

Hazy skies due to suspended Saharan dust and hot weather conditions
are expected through the weekend. However, shower development is
also anticipated each afternoon across western portions of Puerto
Rico and downwind of the local islands. A wet and unstable weather
pattern continues to be expected by midweek of next week as a deep-
layered trough approaches from the west and combines with tropical
moisture.

&&

.SHORT TERM...Tonight through Sunday

In the early afternoon, a streamer downwind of El Yunque developed,
bringing moderate to heavy showers that briefly affected portions of
Canovanas and Carolina. Later, convective activity emerged over the
interior and western/northwestern regions of Puerto Rico. This
activity was caused by marginal instability aloft, resulting from a
weak mid-level trough combined with diurnal heating and sufficient
low-level moisture. As the heating effects subside, the activity
should diminish by this evening. Hot weather conditions prevailed
today as well, with heat index values reaching above 105F across
coastal and urban areas of the local islands.

A similar weather pattern is expected through early in the weekend
as the weak mid-level trough persists over the local area under an
east-southeast steering wind flow. However, dry air intrusion at
mid-to upper levels will limit the areal coverage of convective
activity. The moisture content at lower levels will vary between
1.5 and 1.7 inches throughout the forecast period, remaining
within the 50th and 75th percentiles of typical climatological
levels, or adhering to seasonal norms.

Hi-res models do indicate more shower and thunderstorm activity on
Saturday as cold air advection at 500 mb cools temperatures to
almost -7 degrees Celsius. Therefore, Saturday will be the wettest
day of the short-term forecast, but activity will develop in a
typical manner, with moisture content combining with sea breeze
convergence and surface heating during the afternoon hours.
Therefore, showers and isolated thunderstorms will develop over
the interior and western/northwestern regions of Puerto Rico.
Streamers downwind of El Yunque into the San Juan metro area and
downwind of the smaller islands cannot be ruled out. A limited to
elevated flooding threat will prevail in those areas with the
heaviest showers. Sunday looks more stable as the weak mid-level
trough erodes and is replaced by ridging aloft. Still, some
isolated showers can develop in localized areas, particularly
during the afternoon hours.

Hazy skies and hot weather conditions will persist as a wide area of
suspended Saharan dust particulates continues to be advected over
the local area, while southeasterly winds continue to bring warmer
temperatures into the northeastern Caribbean. Heat advisories may be
necessary each day, particularly across the coastal and urban
areas.

&&

.LONG TERM...Monday through Friday...
.PREV DISCUSSION... /issued 454 AM AST Fri May 17 2024/

Surface high pressure system extending from the central and northeast
Atlantic will maintain southeasterly winds across the region during
the initial phase of the period. These winds will aid in transporting
patches of tropical moisture to the region, and therefore increase
the likelihood of shower development over land areas particularly
during the afternoon hours. Shower development should be focused
across portions of the interior and west to northwest sections of
Puerto Rico as well as in isolated areas in the San Juan metro.
Mostly sunny skies and hot an humid conditions will persist through
Monday. As a result these conditions will increasing maximum heat
indices, posing a limited to elevated heat threat especially for
the northern coastal and urban areas of Puerto Rico and the adjacent
islands including the U.S. Virgin Islands.

Recent model guidance although there is some discrepancy between
the them both point toward an increasing moist and unstable
weather pattern with increased low level moisture convergence and
instability aloft for most of the period. The GFS model guidance
continued to be the most aggressive and suggests a wetter pattern
during the period. It has so far initialized well with the
present and ongoing pattern and continued to suggest the formation
of a deep layered trough forecast to deepen and move across the
western Atlantic while becoming amplified west of the region while
moving eastward. So far the strongest divergence and most favorable
side of the trough and its associated jet max should reach the forecast
area by Tuesday and continue to affect the region through Thursday
and Friday. With increasing instability along with cooler advective
temperatures aloft and abundant tropical moisture convergence, the
forecast layered precipitable water (PWAT) values are still expected
to exceed 2 inches during the latter part of the period. Consequently,
this wet and unstable period is forecast for the region, with the
moderate to high probability of showers and thunderstorm development
with areas of widespread rains likely, especially during the overnight
and afternoon hours. The anticipated period of excessive and heavy
rainfall will therefore increase the risk of urban and river flooding
especially from Tuesday onwards with the most favorable impacts being
Wednesday through Thursday but activity could continue thorough the
end of the work week. Stay tuned and be prepared, as we will continue
to monitor how this pattern unfolds and make the necessary adjustments
to the long term forecast.

&&

.AVIATION...
(18Z TAFS)

Mainly VFR conditions are expected to prevail across all terminals
during the next 24 hours. However, SHRA/TSRA in and around TJBQ
may cause tempo MVFR conditions thru 17/23z. Low-level winds will
continue from the east-southeast at 10 kt or less, lighter land
breeze variations expected overnight.

&&

.MARINE...

A surface high pressure across the central and northeast Atlantic
will result in light to moderate east to southeast through the
weekend. Overall, tranquil marine conditions are forecast throughout
the forecast period. Afternoon showers and isolated thunderstorms
could develop and move from western Puerto Rico into the coastal
waters each day.

&&

.BEACH FORECAST...

There is a low rip current risk the rest of today across all area
beaches of Puerto Rico and the U.S. Virgin Islands. The risk will
likely remain low through the weekend. However, life-threatening
rip currents can often occur in the vicinity of groins, jetties
and piers. In addition, with possible maximum heat indices between
102-107 degrees or higher, along the local beaches, heat exhaustion
will be likely with prolonged exposure. Stay hydrated!

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...Heat Advisory until 5 PM AST this afternoon for PRZ005.

VI...None.

AM...None.

&&

$$

SHORT TERM...CVB/ICP
AVIATION...DSR
PREV LONG TERM...RAM
PUBLIC...YZR