674
FXCA62 TJSJ 162059
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
459 PM AST Thu May 16 2024

.SYNOPSIS...

A seasonal weather pattern with some troughiness aloft will prevail
through early next week. However, shower and thunderstorm activity
is expected to increase substantially across the forecast area by
midweek of next week onwards as a deep-layered trough impacts our
area, leading to a return to a wet and unstable weather pattern and
increasing the flooding threat across the forecast area.

&&

.SHORT TERM...Tonight through Saturday...

Afternoon convection across the interior and western Puerto Rico
will gradually diminish through the early evening hours due to the
loss of daytime heating. Following this, weather conditions will
remain tranquil tonight through Friday morning, with some brief
passing showers across portions of the local waters. Some of this
activity may reach eastern Puerto Rico and portions of the USVI.

The chance for shower and thunderstorm activity is expected to
increase somewhat by Friday afternoon into Saturday as a weak mid-
level trough crosses over the northeastern Caribbean. At lower
levels, a surface high-pressure system across the central Atlantic
will induce more east-southeast winds from tomorrow (Friday) through
the forecast period.

Under the influence of troughiness aloft and moisture carried by
east-southeast winds, shower, and thunderstorm development is
expected across west and northwest Puerto Rico each afternoon.
Additionally, frequent passing showers are possible across eastern
areas of Puerto Rico as well as the US Virgin Islands. Consequently,
there is an increasing potential for localized urban and small-
stream flooding.

The east-southeast winds will promote 925 mb temperatures to be well
above normal for this time of year. The high temperatures, combined
with higher moisture content, could cause heat indices to exceed
100F in coastal and urban areas, possibly reaching above heat
advisory criteria. Additionally, the GEOS-5 model indicates a broad
area of suspended Saharan dust particulates approaching the Leeward
Islands, with the highest concentrations reaching the local islands
by this weekend. Therefore, in areas with limited showers, hot and
hazy skies can be expected for the rest of the short-term forecast.

&&


LONG TERM...Sunday through Thursday...

.PREV DISCUSSION... /issued 530 AM AST Thu May 16 2024/

A 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 each
day, but particularly in the afternoon. Most showers should be
focused across the interior and northwestern quadrant of Puerto
Rico and and in isolated areas around the San Juan metro area.
There will be plenty of sunshine and hot and humid conditions
through the weekend and into early next week. These conditions
will yield increasing heat indices, posing a limited to elevated
heat threat for the northern coastal and urban areas of Puerto
Rico and the adjacent islands including the U.S. Virgin Islands.

Moisture convergence and instability is however expected to
gradually increase by late Sunday and throughout the forecast
period as a deep layered trough is forecast to move across the
western Atlantic and become amplified west of the region while
moving eastward. Recent model guidance continued to suggest, the
strongest divergence and most favorable side of the deepening TUTT
(Tropical Upper Tropospheric Trough) and its associated jet
stream will reach the forecast area from Monday and continue to
affect the region through Thursday. With increasing instability
aloft along with colder temperatures and abundant tropical moisture
convergence,precipitable water (PWAT) values are forecast to
exceed the climatological normals by over 2 inches. Consequently,
a wet period is still forecast for the islands and coastal waters,
with the increasing probability of showers and thunderstorm development
with areas of widespread rains likely, especially during the late
morning and afternoon hours. This so far anticipated period of
excessive and heavy rainfall will increase the risk of urban and
river flooding from Tuesday through the rest of the period. Stay
tuned as we will continue to monitor how this unfolds and make
adjustments to the long term forecast if needed.

AVIATION...
(18Z TAFS)

Mainly VFR conditions are expected to prevail across all terminals
through the forecast period. Mostly VCTS/VCSH expected at TJBQ/TJPS
thru 16/22z. Winds will continue from the east around 10 kt, becoming
light and variable overnight due to land breeze variations.

.MARINE...

A surface high pressure across the western and central Atlantic will
result in light to moderate easterly winds for the upcoming days.
Therefore, tranquil marine conditions will prevail throughout the
forecast period. Afternoon showers and isolated thunderstorms could
move from western Puerto Rico into the coastal waters each day.

.BEACH FORECAST...

There is a low rip current risk 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 heat indices between 102-107
degrees or higher, along the local beaches, heat exhaustion will
be possible with prolonged exposure. Stay hydrated!

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...None.
&&

$$

SHORT TERM/MARINE/BEACH FORECAST...CVB
AVIATION...DSR/ICP
PUBLIC...YZR