911
FXCA62 TJSJ 080137
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
937 PM AST Tue May 7 2024

.UPDATE...

Mostly stratified rainfall prevailed across portions of southern
and eastern Puerto Rico during the past few hours, while heavier
showers and thunderstorms were observed across the Caribbean and
Atlantic waters, as well as across the Anegada Passage. The latest
SJU sounding suggest easterly steering winds at less than 10 kt
and 2.34 inches of precipitable water, well above normal levels.
Therefore, showers could still redevelop across portions of eastern
Puerto Rico and the U.S. Virgin Islands through Wednesday
morning.

Flooding rains are expected to return once again on Wednesday
afternoon across the same areas. Therefore, the threat for major
river flooding, flash flooding, and mudslides in areas of steep
terrain remains high.

&&

.AVIATION...

(00Z TAFS)

Mainly VFR conditions expected overnight. SCT/BKN lyrs will
continue mainly abv FL040. However, SHRA/TSRA developing across
the local waters may cause -RA/VCSH across the area terminals.
Tempo MVFR to brief IFR conds are possible btw 08/16z-22z at
TJPS/TJSJ. The 08/00z TJSJ sounding indicated ESE winds up to 14
kt blo FL050


&&

.HYDROLOGY...

It was another active afternoon with rainfall estimates between 4
and 8 inches across the interior of Puerto Rico. Excessive runoff
from this activity has caused flooding across major rivers and
tributaries, mainly along the north central and southwestern
municipalities of Puerto Rico. As of 9:30 PM AST, Flood
Warnings (FLWSJU) were still in effect for the Rio Cibuco, Rio de
la Plata, Rio Grande de Manati, Rio Grande de Arecibo, Rio Loco,
and Rio Grande de Anasco. These rivers will slowly subside through
the early morning hours, and FLWs could be extended later
tonight.


&&

.PREV DISCUSSION... /issued 450 PM AST Tue May 7 2024/

SYNOPSIS...Abundant moisture and increasing instability from an
approaching mid-to-upper-level trough will maintain a wet and
unstable weather pattern during the next few days. Due to the
soil saturation and the much above-normal streamflows, any period
of moderate to heavy rains may result in sudden urban and small
stream flooding to localized flash floods and mudslides near areas
of steep terrain. Severe thunderstorms may produce wind impacts,
including waterspouts, and in rare instances, weak isolated
tornadoes. Residents and visitors alike are advised to stay
weather-aware and informed about these potential hazards and
closely monitor the forecast for any updates.

SHORT TERM...Today through Thursday...

Doppler radar indicated showers with isolated thunderstorms
affecting eastern Puerto Rico early this morning, and around 9-10
AM AST, convective development began over central Puerto Rico. By
noon, shower activity intensified and spread across most of Puerto
Rico. So far, radar-estimated rainfall accumulations have
exceeded 5 inches in an area centered around Ponce, Adjuntas,
Jayuya, Ciales, Orocovis, and Barranquitas. Various Flood
Advisories, Flash Flood Warnings, and River Flood Warnings were
issued during this period. Most coastal sites have had high
temperatures in the mid 80s to lower 90s. Winds were generally
easterly with sea breeze variations at 10-15 mph, but gusty of up
to 20-30 mph near the shower and thunderstorm activity.

The wet and unstable weather pattern is forecasted to persist
until Wednesday evening due to above-normal precipitable water
values, ranging from 2.0 to 2.2 inches, coupled with an advancing
upper-level trough from the west and below-normal mid-level temperatures,
around -8 degrees Celsius. While a weak surface-induced trough
over the northeastern Caribbean and eastward-moving high pressure
across the Atlantic will sustain light to moderate easterly winds
of 5-15 mph at lower levels, winds from the west to southwest will
dominate around the 700 mbar layer.

Anticipate a showery weather scenario tonight into Wednesday,
characterized by slow-moving showers and a likelihood of severe
thunderstorms, which could exacerbate flooding and lightning
risks. According to the latest model guidance, the activity will likely
concentrate primarily across the local waters into eastern Puerto
Rico, Vieques, Culebra, and the U.S. Virgin Islands during
nighttime, shifting to inland and downwind regions from
mountainous areas during the day. However, rainfall could affect
most land areas at any given time.

Given the saturated soils and significantly elevated river
streamflows, any additional moderate to locally heavy rain poses a
risk of dangerous flooding, including sudden mudslides and
landslides in steep terrain areas. Additionally, thunderstorms may
produce strong winds capable of causing tree falls, power line
disruptions, and movement of unsecured objects. Waterspouts, and
in rare instances, weak isolated tornadoes, could also occur with
the most intense storms. Therefore, residents and visitors alike
are advised to stay weather-aware and informed about these
potential hazards and closely monitor the forecast for any
updates.

Temperature-wise, a warm trend will continue, with daytime high
temperatures ranging between the upper 80s to lower 90s along the
coastal/urban areas and in the upper 70s to low 80s along the
higher Terrains. With abundant moisture across the region, heat
indices may exceed 102 degrees Fahrenheit, primarily across
coastal areas of Puerto Rico, the local islands, and other urban
areas.

LONG TERM...Friday through Tuesday

FROM PREVIOUS DISCUSSION...446 AM AST Tue May 7 2024

Model guidance suggests a mid- to upper-level ridge is trying to
build over the Northeast Caribbean Friday into the weekend,
promoting stable conditions and potentially bringing relief from
the current weather conditions.

Once again, GFS Total Precipitable Water guidance suggests
returning the climatological values for this time of year, with
the advection of occasional patches of increased moisture.
Combining high moisture content with the typical upper 80s to low
90s maximum air temperatures will result in heat indices between
100-107 Fahrenheit or even higher, especially during the daily
maximum heating, across the USVI and PR`s urban and coastal areas
where no significant rain is observed.

Although we expect variable weather conditions with a mixture of
sunshine/clear skies and clouds each day, local effects and sea
breeze fluctuations may promote strong afternoon convection each
day. Furthermore, the advection of a cooler air mass across the
above-normal warmer sea surface temperature may result in frequent
nighttime showers, affecting the windward sections and local
waters each night.

The NASA aerosol optical thickness (AOT) guidance suggests the
arrival of an air mass with possible African Dust Particles by the
upcoming weekend, which may extend into next week.

AVIATION...
(18Z TAFS)

Mostly VFR conditions expected for the USVI and TJPS terminals this
afternoon, but SHRA and TSRA expected to linger through 22Z around
the Cordillera Central and near TJSJ and TJBQ. Periods of reduced
VIS and low ceilings are expected through this time. SHRA are
expected to decrease aft 22Z across land areas, but will continue
across Atlantic waters. Some may reach the USVI and TJSJ terminals
at times. Winds will be light, shifting from the ESE east aft 06Z at
5-10 kts.

MARINE...

The surface high-pressure building north of the islands from the
Western to Central Atlantic will maintain light to moderate trade
winds. Pulses of northeasterly swell will spread across the
regional waters later tonight onward. Unsettled weather conditions
due to strong thunderstorms are likely, especially during the
afternoon and evening, today and tomorrow.

BEACH FORECAST...

FROM PREVIOUS DISCUSSION...446 AM AST Tue May 7 2024

Expect increasing seas of 3 to 4 feet across most of the local
beaches in the Atlantic, mainly from the northeast, at 6 to 8
seconds. Across the Caribbean, seas will range around 2 feet from
the east to southeast at 6 to 8 seconds. Therefore, we expect the
risk of rip currents to increase to moderate today.

Pulses of northeast swells, with wave periods of 10 to 13 seconds,
will increase the risk of rip currents to moderate from late
tonight into next weekend across the north and east-facing beaches
of PR and the USVI. There is still a possibility of a high risk of
rip currents during that period; it`s crucial to monitor the beach
forecast for possible updates to the expected conditions as the
coastal hazards can change rapidly.

&&

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

$$

UPDATE/AVIATION/HYDROLOGY...DSR