301
FXCA62 TJSJ 081604 AAA
AFDSJU

Area Forecast Discussion...UPDATED
National Weather Service San Juan PR
1204 PM AST Fri Nov 8 2024

.UPDATE...After a fairly tranquil overnight period, showers and
thunderstorms developed over the Caribbean south of Puerto Rico in
a zone of convergence set up by the south southeasterly flow and
moved inland. Most of the island except the far northwest portion
received at least a measurable amount and a wide swath of the
island from Cabo Rojo to Caguas to Naguabo and Rio Grande received
over one half inch. A few spots had more than two inches including
spots in Aibonito and Salinas and offshore from Naguabo.
Temperatures remained on the cool side in eastern Puerto Rico
holding in a range from the mid 70s to the lower 80s. Areas in the
west and south and also in Saint Croix reached the mid to upper
80s as of 11 AM AST.

As the upper and lower level lows move west the entire wind
profile over Puerto Rico and the U.S. Virgin Islands has become
south southeast to south. This has brought the promised moisture,
but will also bring in drier air now approaching from the
southwest into southwest Puerto Rico. A tropical wave will enter
the forecast area midday on Saturday and begin to spread
considerable rainfall across the area: first in the U.S. Virgin
Islands and then in eastern Puerto Rico later in the day.

Marine conditions are improving and it is expected that the high
risk will moderate on Saturday and small craft advisory conditions
will fade overnight tonight.

&&

.AVIATION...
(12Z TAFs)

SHRA and TSRA have dissipated over PR and the USVI, but remain
active in isolated areas ovr the Mona Channel and over wrn
Vieques and offshore of ern PR. So altho VFR conds will prevail
SHRA and TSRA will have MVFR/IFR conds and mtn obscurations. Winds
are southerly up thru FL500, but LLVL winds will become SSE and
then E aft 09/06Z. Sfc winds are southerly or variable at less
than 11 knots. Max winds SSE 35-45 kts btwn FL365-430.

&&

.HYDROLOGY...

Rain showers and thunderstorms will continue intermittently
overnight and redevelop on Sunday for another round of heavy
rains. We expect between 2 and 5 inches in eastern Puerto Rico and
the U.S. Virgin Islands during that period, although amounts will
likely be less for northwest Puerto Rico. This will be sufficient
for some areas to see flash flooding from bankfull rivers and
above capacity drainage systems. In addition soils are over 90
percent saturated in many areas of eastern and interior Puerto
Rico such that landslides will become much more likely. Therefore
a flash flood watch has been issued for Puerto Rico and the U.S.
Virgin Islands from noon on Saturday to 8 PM on Sunday. Be aware,
however, that this will not be the end of wet conditions. The
synoptic pattern will be favorable to the continuation of periodic
heavy rains through at least Friday of next week.

&&

.PREV DISCUSSION... /issued 504 AM AST Fri Nov 8 2024/

SYNOPSIS...
Unsettled weather is returning later today and will
likely stick tonight, bringing more showers and thunderstorms. With
saturated soils, flooding impacts are possible. Are you planning to
head out on the water? You might want to rethink thathazardous seas
and life-threatening rip currents will continue today. The good
news? Marine and coastal conditions should improve during the
weekend, although stormy weather may disrupt some outdoor plans.
Sunday, however, is looking more promising. Stay safe and informed
by following us on our social media channels! For the latest updates
on excessive rainfall, rip currents, and other hazards, visit the
Experimental Graphical Hazard Weather Outlook:
https://www.weather.gov/erh/ghwo?wfo=sju.

SHORT TERM...Today through Sunday...

Overnight radar and satellite analysis indicate generally fair
weather conditions, except for the US Virgin Islands, where showers
and a few isolated thunderstorms generated isolated rainfall totals
of up to three quarters of an inch since 8 PM AST. Overnight
temperatures ranged from the lower 60s in higher elevations to
around 80 degrees across Vieques, Culebra, and the U.S. Virgin
Islands. Winds were generally light to calm and variable.

Despite relatively fair weather observed across the region in the
last 6 to 12 hours, unsettled weather will likely return. Before
drifting further to the southwest and away from the region, the
upper-level trough will maintain favorable conditions for
thunderstorm development today, with 500 mb temperatures at typical
levels, around -6 degrees Celsius. At the surface, the dominant
feature, a trough of low pressure, with the axis extending across
the forecast area, will continue its westward movement today.
Nonetheless, its induced weak southerly flow will continue to
support an influx of abundant tropical moisture. This feature will
be quickly followed by a robust tropical wave tonight into Saturday,
with its associated moisture lingering over the region through
Sunday. In agreement with model guidance, satellite-derived
precipitable water values across the region suggest above-normal
November levels holding through most of the period, fluctuating
between 2.1 and 2.3 inches. Consequently, this synoptic setup should
support frequent and strong thunderstorms over the next 24-48 hours,
with high rain chances (70-90%) across all land areas.

Since a weaker steering flow will increase the likelihood of higher
rainfall accumulations from slow-moving showers, an elevated risk of
excessive rainfall exists today, raising concerns for urban and
small stream flooding, with localized flash floods possible. On
Saturday, the risk was elevated to significant levels, raising even
higher concerns for flash and river flooding and mudslides across
the region. Additional hazards include frequent lightning and gusty
winds generated by thunderstorms. Despite the increased chance of
rain and cloudiness, southerly winds today will result in another
warm day, as indicated by warmer-than-normal 925 mb temperatures.

According to current model guidance, Sunday`s weather pattern will
likely be less stormy, with more localized showers and isolated
thunderstorms, primarily in the afternoon. Flooding impacts are
likely to occur due to periods of moderate to locally heavy rainfall
over soils that are already saturated. For updates on excessive
rainfall, lightning, and any other hazard risk in the coming days,
please visit the Experimental Graphical Hazard Weather Outlook at
https://www.weather.gov/erh/ghwo?wfo=sju.

LONG TERM...Monday through Friday...

No major changes are expected during the long-term period. The mid
to upper levels will maintain stability, providing mostly dry
conditions and warmer temperatures at 500 MB, which will limit the
development of deep convection. At the surface, the islands will
be mostly dominated by a broad area of high pressure extending
from the western Atlantic into the central Atlantic, gradually
shifting eastward, driven by an upper-level low reflected across
all levels. By Monday, this pattern will result in moderate to
strong easterly wind flow across northern sections. Driven by this
wind flow, abundant moisture from the previous tropical wave will
spread across the islands, enhancing the potential for early
morning showers due to an advective pattern, as well as afternoon
convection triggered by diurnal heating across the western
interior.

From Tuesday through Friday, a surface low pressure system,
reflected at the mid-levels, will move southeastward in response
to an existing surface high pressure over the CONUS. This will
create a trough just northwest of the region, resulting in veering
windsshifting to east-southeast by Tuesday, southeast by
Thursday, and light to variable by Friday. As the week progresses,
columnar moisture is expected to decrease from Thursday to Friday
with PWAT as the global models suggest of 1.4 to 1.5 inches.

AVIATION...

(06Z TAFS) VFR conditions will prevail through 08/12-14Z. SHRA/TSRA
later this morning, bringing brief MVFR/IFR conditions at TJPS and
USVI terminals after 08/14Z, and at TJSJ/TJBQ after 08/17Z. Periodic
SHRA/TSRA pulses, associated with an approaching tropical wave, may
affect operations at all TAF sites after 09/00Z. Winds will remain
light to calm and variable, increasing to 5-10 knots from the south-
southeast, with higher gusts possible near TSRA activity.

MARINE...

Hazardous marine conditions will persist for at least the rest of
the day, improving during the night hours as energy from the
previous northerly swell and winds diminish across the local
islands. Starting tomorrow, seas will remain at 5 feet or less due
to a weakening in the pressure gradient resulting from a trough
associated with a low-pressure system moving across the region.
Additionally, local waters will experience an increase in
thunderstorm activity, leading to hazardous localized marine
conditions. A tropical wave will move northwestward into the area
by Saturday into Sunday, resulting in similar conditions and
increasing wind speeds by Monday.

BEACH FORECAST...

Recent data buoy at San Juan reported large breaking waves at
around 9 feet. Similar conditions will persist for the rest of the
morning hours across the northern section of Puerto Rico, where
hazardous swimming conditions will prevail. Given the actual
conditions, there is a high rip current risk across all the north
PR and Culebra. Beachgoers are urged to remain out of the northern
exposed local beaches. Improving coastal conditions are forecast
from Saturday into Sunday with a moderate risk of rip currents.

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through late tonight for PRZ001-002-005-
     008-012.

     Flood Watch from Saturday afternoon through Sunday evening for
     PRZ001>013.

VI...Flood Watch from Saturday afternoon through Sunday evening for
     VIZ001-002.

AM...Small Craft Advisory until 6 PM AST this evening for AMZ711-712-
     716-741.

&&

$$

SHORT TERM/AVIATION...WS
UPPER AIR/PUBLIC....GRS