109
FXCA62 TJSJ 081955
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
355 PM AST Wed May 8 2024

.SYNOPSIS...Showers and thunderstorms are expected to diminish
during the evening hours. Although trough aloft is expected to
move away, locally induced afternoon showers and thunderstorms
are still expected Thursday and Friday. There is a significant
flood risk across the local islands due to saturated soils and
river running well above normal values. Any rainfall will result
in sharp raises along small streams and rivers.

Model guidance still suggests a mid- to upper-level ridge trying
to build over the Northeast Caribbean from Saturday onward,
promoting stable conditions.

&&

.SHORT TERM...Tonight thru Friday...

Another active day has been observed across the local islands,
with showers and thunderstorms over portions of central Puerto
Rico. Rainfall totals so far have ranged between 2 and 4 inches
with isolated higher amounts. Several Flash Flood Warnings were
issued.

The organized convection affecting the local islands will decrease
as the upper-level trough moves east and away from the forecast
area. Although convergence aloft is expected by Thursday night, fast
moisture erosion is not expected at this time. In fact, precipitable
water values will exceed 2.0 inches on Thursday, decreasing just
below 2.0 inches on Friday. As a result, available moisture will
combine with sea breeze convergence and diurnal heating to result in
additional showers and thunderstorms on Thursday and Friday
afternoons. At lower levels, east-to-east southeast winds at 10
knots or less will prevail through the forecast cycle.

Under this evolving pattern, showers and thunderstorms are expected
to develop across the northwest quadrant and portions of the San
Juan Metro Area each afternoon. Due to the already saturated soils
and rivers running above average values, heavy rainfall will
produce sharp rises of rivers and small streams.

In addition, warmer temperatures and higher heat indices are
likely under east- southeast winds.

.LONG TERM...Sat thru Wed/issued 439 AM AST Wed May 8 2024/

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

Model guidance also suggests a return to the climatological
moisture values over the region, with the advection of occasional
patches of enhanced moisture. Additionally, we are more likely to
observe variable weather conditions with a mixture of
sunshine/clear skies and clouds daily. Also, the NASA aerosol
optical thickness (AOT) guidance one more time suggests the
arrival of an air mass with possible African dust particles by the
upcoming weekend, which may extend into next week. Meanwhile,
intense heating, local effects, and sea breeze fluctuations each
day will drive strong afternoon convection. Furthermore, the
above-normal warmer sea surface temperature will also enhance the
typical nighttime warm air convection. These frequent nighttime
showers could affect the windward sections and local waters of the
U.S. Virgin Islands and Puerto Rico.

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)

SHRA/TSRA will continue to affect the area, causing mountain
obscuration across the Cordillera Central gradually moving towards
the coast through at least 23Z. Tempo MVFR to brief IFR conds are
possible through 23z at TJBQ/TJSJ/TJPS. Winds will turn light and
variable after 22z with VCSH expected for the USVI and TJSJ
terminals. Another round of SHRA and TSRA expected tomorrow along
the Cordillera Central after 17Z. Winds are expected from the ESE at
5-10 kts.

&&

.MARINE...

A surface high-pressure north of the islands will yield easterly
winds through Friday, becoming more east-southeast during the
weekend. Pulses of northeasterly swell will spread across the
Atlantic waters and Caribbean Passages. Weather conditions across
the local waters are expected to improve Thursday onwards.

&&

.BEACH FORECAST...

Expect increasing seas of 4 to 6 feet across most of the local
beaches in the Atlantic. As a result a moderate risk of strong rip
currents is expected across the north and east-facing beaches in
PR/USVI.

&&

.HYDROLOGY...

There is a significant flood risk across the local islands due to
saturated soils and river running well above normal values. Any
rainfall will result in sharp raises along small streams and
rivers as well as mudslides, pariculalry central Puerto Rico.
Some rivers will likely overflow their banks.

&&

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

$$

SHORT TERM...OMS
AVIATION...CVS