339
FXCA62 TJSJ 021459
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
1059 AM AST Thu May 2 2024

.UPDATE...

Minor changes introduced to the forecast. A Flood Watch has been
issued for Puerto Rico and the U.S. Virgin Islands from Friday
morning through Sunday afternoon. For more details please refer to
FFASJU & FFASPN. Unstable conditions (due to an upper-level
trough approaching the area from the west and an induced surface
trough), increasing moisture, and lighter winds will promote
periods of showers and thunderstorms across the local islands.
Rainfall accumulations are expected to range from 2 to 4 inches,
especially for the eastern half of Puerto Rico and the U.S. Virgin
Islands with isolated higher amounts possible between 6 and 8
inches. Elsewhere, 1 to 3 inches are expected.

&&

.PREV DISCUSSION... /issued 505 AM AST Thu May 2 2024/

SYNOPSIS...

Unstable conditions and increasing moisture levels will likely
persist during the rest of the workweek into the upcoming
weekend, as a mid to upper level trough lingers over the western
Atlantic and moisture from the Caribbean is steered by east to
southeast winds into the area. There is an elevated threat for
flooding across all the islands, particularly on Friday and
Saturday. The precipitable water content is forecast to remain at
normal to above normal levels through the long term period.

SHORT TERM...Today through Saturday...

Overnight, mostly calm weather conditions prevailed over land, with
a few showers developing along the eastern half of Puerto Rico and
the U.S. Virgin Islands. Nonetheless, showers and thunderstorms were
the dominant weather feature over the offshore Atlantic waters
through the night. The rainfall accumulations across the islands
were around half an inch. Minimum temperatures were in the low to
mid-70s across coastal areas and the mid-60s across the higher
terrains.

The inherited forecast remains unchanged as wet and unstable
conditions continue to evolve. This pattern, driven by an upper-
level trough approaching the area from the west, an induced surface
trough developing north of the region, and the introduction of
additional moisture from the Caribbean, will continue to enhance
shower and thunderstorm development across the local area. Later
today, expect more moisture to filter across the islands. At this
time, the forecast leans toward Friday into Saturday being the most
active days, with showers and thunderstorms being the dominating
features, particularly along the eastern half of Puerto Rico and the
U.S. Virgin Islands. By the end of the workweek, an upper-level
maxima will move across the islands with winds around 80 knots. On
Saturday, the yet maxima shouldremain to the north of the area.
Nonetheless, winds around 70knots will still reach the region.
The Precipitable Water (PWAT) model guidance shows values above
normal climatological levels by the early part of the weekend. The
500 mb temperatures also suggest values around -8 degrees Celsius
on Saturday, meaning that there is an increase in the potential
for some isolated thunderstorm activity across Puerto Rico and the
U.S. Virgin Islands. As saturated soils prevail, any brief period
of heavy showers or persistentlight rainfall could lead to urban
andsmall-streamflooding, localized flashflooding and
mudslides, particularly across the the interior, the eastern half
of Puerto Rico, and the U.S. Virgin Islands. We encourage
citizens and visitors to be aware of any additional updates in the
forecast.

Temperature-wise, they will stay in the upper 80s to low 90s along
the coastal/urban areas and in the upper 70s to low 80s along the
higher terrains.

LONG TERM...Sunday through Thursday...

Lingering moisture and the departing upper level-trough will continue
to promote shower and thunderstorm development on Sunday. At
lower-levels, a surface high pressure is expected to build over
the western Atlantic during the first part of the week. This will
promote moderate winds from the east to northeast through the long
term period. In addition, the remnants of the previous surface
trough to our north or just across the northeastern Caribbean are
expected to merge with a frontal boundary across the central
Atlantic and linger over the local area through at least Thursday.
Also, at upper levels, another trough will pass mainly to our
north from Tuesday onwards, maintaining unstable conditions in
general across the region. The highest precipitable water (PWAT)
values are expected during this period, with global guidance
suggesting between 2.10-2.25 inches of PWAT from Monday morning
through Thursday afternoon. Therefore, any period of moderate to
heavy showers across the islands would lead to quick urban and
river flooding, as well as mudslides in areas of steep terrain.
Residents and visitors are encourage to monitor the evolution of
the weather conditions during the next 5 to 7 days, and stay alert
for the issuance of any possible flood watch/warning products.

AVIATION...

(06Z TAFS)

SHRA/TSRA were observed across the offshore Atlantic waters
overnight but dissipated around 02/07Z. VCTS could develop later
this afternoon near TJBQ/TJSJ/TIST/TISX, causing tempo MVFR to brief
IFR conds. ESE winds at 06-16 kt with stronger gusts near showers.

MARINE...

Light to moderate trades and a subsiding northerly swell will
promote seas up to 6 feet across the Atlantic waters, and portions
of the local passages today. Therefore, small craft operators
should exercise caution across these waters. Also, thunderstorm
development is likely across the Atlantic waters and the Mona
Passage. Lighter winds from the east to southeast and increasing
shower and thunderstorm activity is expected during the next
several days.

BEACH FORECAST...

A subsiding long period northerly swell will continue to promote a
high risk of rip currents along the north-facing beaches of Puerto
Rico, including Culebra today. A High Rip Current Risk (CFWSJU)
is in effect through late this afternoon.

HYDROLOGY...

An upper-level trough and an induced surface trough should promote
wet and unstable conditions throughout the weekend, particularly
on Friday and Saturday. There is an elevated flood threat due to
saturated soils across portions of the islands. Please refer to
the latest Hydrologic Outlook (ESFSJU) for more information.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk until 6 PM AST this evening for PRZ001-002-
     005-008-012.

     Flood Watch from Friday morning through Sunday afternoon for
     PRZ001>013.

VI...Flood Watch from Friday morning through Sunday afternoon for
     VIZ001-002.

AM...None.
&&

$$

MRR/LIS/OMS/RC/RVT