Areal Flood Advisory 384, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
347 PM AST Wed Oct 13 2021

.SYNOPSIS...

The wet pattern continues into the end of the week. Weak
southwesterly steering flows will keep the bulk of the afternoon
activity over the northeastern part of Puerto Rico, including the
metro area, with slow-moving showers bringing locally high
rainfall amounts. There remains an enhanced risk for flooding,
especially in the San Juan metro area.

&&

.SHORT TERM...Tonight through Friday...

Persisting moisture is expected over the region into the end of the
week. Precipitable water values are forecast to remain around or in
excess of 2 inches through the short term period. Meanwhile,
troughing in the low- to mid-levels will also continue to provide
instability over the region. There is more than sufficient moisture
to combine with the instability to lead to enhanced shower activity.

Steering flow over the region will also continue out of the
southwest, becoming increasingly light. As such, the bulk of the
afternoon activity caused by sea breeze convergence will likely be
in northeastern and interior Puerto Rico, including the metro area.
As the steering flow is expected to be weak into the end of the
week, showers will be relatively slow-moving, and locally
significant rainfall amounts are likely. As such, there is an
enhanced potential for urban and small stream flooding, as well as
flash flooding.

.LONG TERM...Saturday through Thursday...

The aforementioned surface trough is to lift northeast across the
region to maintain light south to southwest wind flow and moist low
level convergent zone through at least early Sunday. The local winds
are forecast to become more east to northeast by Sunday afternoon
onwards, with the moisture convergence to focus over the eastern
and and southeastern sections of Puerto Rico as well as over the
U.S.Virgin Islands through Tuesday. Winds will again shift and
become more easterly and increase on Wednesday and Thursday when
the the easterly trades will return with accompanying periods of
passing clouds and showers across the region with a more typical
weather pattern to follow.

That said, expected high moisture content to continue with PWAT
values up to 2 inches to linger across the region through Sunday.
The upper level ridge is forecast to erode, as a short wave trough
crosses north and east of the region and will therefore maintain
marginally unstable conditions aloft. This along with local effects
and good daytime heating, expect the possibility of enhanced convection
to continue with showers and thunderstorms development likely especially
during the afternoon and evening hours across portions of the islands.
Therefore the potential for urban and small stream flooding will remain
moderate to high across the islands.

By Monday and for the latter part of the period, the weather pattern
will continue to gradually change and improve with an erosion of low
level moisture and the building of the surface high pressure ridge
north of the region. This in turn will increase the easterly trade
wind which is expected by Tuesday, followed by a wind surge so far
forecast to cross the region Wednesday through Thursday. By then a
more seasonal weather pattern can be expected, with periods of
quick passing morning cloud and showers across the regional waters,
followed by locally and diurnally induced afternoon showers and mostly
isolated thunderstorm over the interior and west sections of Puerto
Rico.

&&

.AVIATION...

SHRA/TSRA expected in and around the PR terminals thru 22z,
causing mainly tempo MVFR to brief IFR at TJSJ and from late this
afternoon through the evening hours across the USVI terminals.
Generally south-southwest winds will continue around 10 kts
through Thursday, except VRB near TSRA.

&&

.MARINE...

Lingering northeasterly swell will sustain persisting choppy
conditions for portions of the local waters, mostly in the offshore
Atlantic. Seas will be generally 6 feet or less for the Atlantic
waters, and lower elsewhere in the local waters, especially the
Caribbean. Conditions will gradually improve as the swell subsides
through tonight.

There remains a high risk of rip currents for northern beaches of
Puerto Rico and Culebra. Elsewhere, there is a low risk of rip
currents from the southeast to the southwest of Puerto Rico, with a
moderate risk for the vast majority of other local beaches.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  77  89  77  88 /  60  80  70  80
STT  79  86  79  86 /  70  70  70  60

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...High Rip Current Risk through Thursday afternoon for North
     Central-Northeast-Northwest-San Juan and Vicinity-Southeast.

     High Rip Current Risk through late tonight for Culebra.

VI...None.
AM...None.
&&

SHORT TERM...ICP
LONG TERM....CRS
PUBLIC DESK...GL