303
FXCA62 TJSJ 070144
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
944 PM AST Mon May 6 2024

.UPDATE...

It was an active afternoon across mainland PR, the Doppler radar
estimated between 4 and 6 inches along the Cordillera Central,
between 1 and 2 across the lower elevations, and around half an
inch across the USVI. Although rainfall diminished during the past
few hours across land areas, additional showers and thunderstorms
were developing across the Atlantic and eastern waters of the
islands. This activity could move over portions of the islands
through Tuesday morning, increasing the potential for flooding
once again.


&&

.AVIATION...

(00Z TAFS)

Mainly VFR conditions expected overnight. However, SHRA/TSRA
across the Atlantic and eastern waters of the islands may cause
VCTS or tempo MVFR conds across the USVI terminals and TJSJ. The
07/00z TJSJ upper air sounding indicated ESE winds up to 12 kt blo
FL070.

&&

.HYDROLOGY...

Excessive runoff from this afternoon rainfall caused river rises
across the Guanajibo, Cibuco, Indio, and Rio Grande de Manati
rivers this evening. Flood warnings are in effect through at least
midnight tonight, and could be extended later tonight if needed.
Road #2 in Manati, road 160 in Vega Baja, roads 620/647 in Vega
Alta, and road 347 in San German were impassable due to flood
waters.



&&

.PREV DISCUSSION... /issued 408 PM AST Mon May 6 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 along PR`s
principal rivers, 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.
Additionally, due to the high moisture content, above-normal heat
indices surpassing 102 degrees Fahrenheit.

SHORT TERM...Today through Wednesday...

Doppler radar indicated showers developed early this morning over
northeastern Puerto Rico. As the day progressed, shower development
was gradually observed downwind from the local islands, and around
10 AM AST, convective development began over central Puerto Rico and
downwind from El Yunque National Forest. By noon, shower activity
intensified and spread across most Puerto Rico. So far, radar
estimates recorded around 3 to 5 inches of rain in and around Coamo,
Adjuntas, San German, Jayuya, and Morovis. During this time,
various Flood Advisories and one Flash Flood Warning for Coamo
were issued. High temperatures have been in the mid 80s to lower
80s across most coastal sites. Winds were generally easterly with
sea breeze variations at 10-15 mph, but gusty of up to 20-25 mph
near the shower and thunderstorm activity.

The wet pattern will persist during the forecast period, with
precipitable water values holding at above-normal moisture levels
around 2.0 to 2.4 inches. While an eastward moving high pressure
will maintain light to moderate east to east-northeast winds at 5-15
mph at lower levels, conditions aloft will remain favorable for deep
convective development with a weak trade wind cap inversion and the
passage of an upper-level trough Tuesday into Wednesday. This wet
and unstable scenario will not only support an active weather
pattern, but the expected slow-moving showers will likely support an
elevated flooding risk. Since soils are saturated and river
streamflows are running much above normal, any period of moderate to
locally heavy rain could lead to dangerous flooding impacts, as well
as sudden mudslides and landslides in areas of steep terrain.

For tonight, prolonged showers and isolated thunderstorms following
the afternoon convection will continue to affect mainland Puerto
Rico through the evening hours. As the night progresses, the shower
and isolated thunderstorm activity will shift to the local waters
and coastal areas of northern and eastern Puerto Rico, Vieques,
Culebra, and the U.S. Virgin Islands. With the proximity and then
passage of an upper-level trough, weather conditions will likely
deteriorate Tuesday into Wednesday. Based on the model guidance, an
increase in shower and thunderstorm activity is expected throughout
the period, with peak activity likely on Tuesday afternoon and
around midnight on Wednesday, when mid-level temperatures will drop
to around -8 degrees Celsius and precipitable water values will
peak around 2.3-2.4 inches. For that reason, we encourage
residents and visitors to remain weather-aware and informed about
these potential hazards and to monitor the forecast for future
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. Combined with above-than-normal moisture, heat indices may
exceed 102 degrees Fahrenheit across coastal areas of Puerto Rico
and the local islands, as well as other urban areas.

LONG TERM...Thursday through Monday

FROM PREV DISCUSSION... /issued 402 AM AST Mon May 6 2024/

The mid to upper-level trough will continue moving away from the
region on Thursday, leaving the islands in a less favorable
position for organized convection. A mid to upper-level ridge will
try to build over the Northeast Caribbean Friday into the
weekend, which may further decrease local instability, potentially
bringing relief from the current weather conditions.

At the surface, GFS Total Precipitable Water guidance suggests
values returning to the typical values for this time of year from
Thursday onward. However, patches of increased moisture may arrive
occasionally. Regardless of whether we observe a mixture of
sunshine and clouds each day, strong afternoon convection could
develop due to local effects and sea breeze variations each day.
Additionally, due to the atypical high sea surface temperature,
the advection of a cooler air mass across the warmer waters may
result in frequent nighttime showers, reaching the windward
sections each night.

Temperature-wise, 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.

AVIATION...
(18Z TAFS)

The potential for VCTS and showers will continue across most sites,
promoting MVFR to brief IFR conditions, especially across PR
terminals through 06/23Z. VCTS/VCSH are likely across all sites
through the rest of the period. E to NE surface winds at 10-15 kts,
and then becoming lighter and more variable after 05/23Z

MARINE...

A surface high-pressure building across the Western Atlantic will
push a surface trough further east away from the Northeast
Caribbean, promoting light to moderate trade winds. From late
Tuesday night onward, pulses of northeasterly swell will spread
across the regional waters. Afternoon convection may result in
strong thunderstorms across the coastal sections of northern and
western PR each day. Increasing shower and thunderstorm activity is
likely on Tuesday and Wednesday.

&&

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

$$

UPDATE...DSR