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000
FXCA62 TJSJ 160852
AFDSJU

Area Forecast Discussion
National Weather Service San Juan PR
452 AM AST Tue Apr 16 2024

.SYNOPSIS...

Wet and unstable conditions continue through at least tomorrow
with favorable conditions for heavy shower and thunderstorm
development. Quick river rises and water surges with flash
flooding, and landslides are possible. Conditions should improve
by the latter half of the week. Small Craft Advisories are in
effect through late tonight. A wet pattern is forecast for next
week.


&&

.SHORT TERM...Today through Thursday...

An unstable and very moist pattern continued across the region
overnight and will persist today through Wednesday. A surge of
tropical moisture trailing a surface trough with axis now crossing
Hispaniola and the Mona passage will merge with the moisture and
instability associated with the deep layered trough and frontal
boundary approaching from the northwest. Recent satellite imagery
and doppler weather radar showed strong thunderstorms developing
over the offshore Atlantic waters, while shower activity continued
to increase across the Caribbean waters and areas between eastern
Puerto Rico and the U.S. Virgin Islands. These showers and
thunderstorms continued to produced areas of locally heavy rains and
frequent lightning mainly over the coastal waters. Showers are
however expected to reach the U.S. Virgin islands and portions of
east and southeastern Puerto Rico during the rest of the morning
hours, then spread and redevelop across the remainder of the islands
throughout the day. Early morning low temperatures were in mid to
upper 70s along the coastal areas and in the upper 60s to low 70s in
higher elevations. Winds were generally light and variable but
occasionally higher with the passing showers.

Through the rest of the period, especially until Wednesday afternoon,
active and unstable weather conditions are forecast to continue due
to the meandering upper trough and good moisture convergence. Recent
model guidance as well as recent satelite derived Precipitable water
products and analyses all suggest precipitable water (PWAT) values
to range between 1.70-2.00 inches across the region today into
Wednesday then diminishing by late Wednesday and on Thursday. This
pattern will favor good potential for continued scattered to
numerous showers and isolated thunderstorm development across
portions of the islands and coastal waters at least through early
Wednesday. By Wednesday afternoon and on Thursday, with still
lingering moisture availability along with diurnal and local effects
expect some additional convective development particularly during
the early morning and late afternoon hours. By then prevailing winds
will become more east northeast with drier air filtering in by the
latter part of the period. Due to the unsettled weather pattern and
saturated soils, additional heavy rains will lead to quick river
rises and water surges with flash flooding, and landslides,
especially through early Wednesday. Consequently, today through
Wednesday there will be moderate to high potential for showers and
thunderstorm development across most of the islands, with periods of
excessive rainfall impacts and urban flooding along with mudslides
in areas of steep terrain likely. Thereafter, by late Wednesday and
on Thursday, improving conditions expected so far, but localized
showers and isolated thunderstorms cannot be ruled out with periods
of locally heavy rains still possible as well as the potential for
additional minor urban and small stream flooding in isolated areas
along with mudslides and rockfall in areas of steep terrain.

&&

.LONG TERM...Friday through Tuesday...

Areas of low pressure spread across the western to central Atlantic
basin will maintain moderate NE winds through the weekend along with
enough water content in a lingering airmass to allow passing showers
and afternoon convection to fire up during the afternoon due to
local effects. By Sunday, the location of a surface high in the
western Atlantic will assist the lifting of a moist airmass from the
eastern Caribbean. This in combination with a mid to upper-level
trough, with the jet max over the region by Saturday through late
Sunday, will allow an increase of passing showers and afternoon
convection mainly across western Puerto Rico. Winds begin to
strengthen and veer, becoming more easterly to southeasterly Monday
to Tuesday, respectively, as the aforementioned surface high moves
into the central Atlantic. Elevated Precipitable water values and
breezy conditions will allow a wet pattern to prevail.


&&

.AVIATION...
(06z)

SHRA/Isold TSRA will persist across the local flying area due to
proximity of a surface trough interacting with an amplifying upper
trough and cold frontal boundary quickly approaching from the
northwest. Brief MVFR conds psbl due to SHRA and LOW CIG with
SHRA/Isold TSRA en route to TJSJ and USVI terminals til 16/14Z.
Isold-SCT TSRA ovr offshore Atl waters N of the islands ovr Anegada
passage. MTN TOP OBSCR ovr Ern PR til 16/14Z and over ovr ctrl and
west interior of PR fm 16/16z-23Z with VCSH/VCTS psbl at TJBQ/TJSJ
and USVI terminals fm 16/15z-16/23Z. SFC Winds lgt/vrb bcmg fm SE 12-
15 kts and ocnly hir gusts aft 16/14Z.


&&

.MARINE...
A surface ridge over the north-central Atlantic and broad surface
trough across the eastern Caribbean will generate moderate to locally
fresh east-southeasterly winds. A fading northerly swell and choppy
wind driven seas will continue to promote choppy seas across the
Atlantic waters. An upper level trough and induced surface trough
will continue to promote inclement weather across the local waters
for the next few days, with squally weather and thunderstorms
possible at times. Another northeasterly swell is forecast to arrive
by Thursday.


&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory until 6 AM AST Wednesday for AMZ711.

&&

$$

SHORT TERM...RAM
LONG TERM....RC