High Surf Advisory 8, NWS JSJ

Area Forecast Discussion
National Weather Service San Juan PR
135 PM AST Fri Nov 29 2019

.SYNOPSIS...

Generally fair weather conditions are expected to prevail across the
local islands for today. However, there is the potential for
isolated to scattered showers pushing onshore in northwestern
Puerto Rico and spreading eastward later this afternoon. A weak
frontal boundary is anticipated to make its way into the region
for the weekend, bringing a somewhat increased shower activity.
Early next week, an increasingly unstable weather pattern is
forecast due to the combination of deeper tropical moisture and
another frontal boundary pushing into the area.

&&

.SHORT TERM...Rest of today through Sunday...

The surface ridge of high pressure extending across the area from
the southwest Atlantic will continue to weaken today, as a broad
area of low pressure, and its associated cold front, slowly lifts
northward into the Northwest Atlantic. This pattern will sustain a
COL and weak troughiness across the forecast area into the weekend.
As a result, the surface winds will be light and variable, with an
overall northerly component to persist all through the low levels of
the atmosphere. As the ridge re-establishes itself and gradually
strengthens north of the forecast area by Sunday, a more north to
northeast wind flow is forecast. Moisture availability will remain
near normal, as occasional patches of low level moisture from a weak
frontal boundary will sink southwards and move across the region
from time to time. In the upper levels, a weak ridge will dominate
through Saturday, causing northwesterly winds and stable conditions
aloft.

By Sunday, the ridge is to erode and flatten, promoting a
more zonal, westerly wind flow. At this time, the forecast calls for
overall pleasant weather conditions across most of the islands.
However, isolated to scattered shower activity can be expected each
day especially during the early morning and afternoon hours. Some
shower activity may be a bit more significant late in the afternoon
and during the early evening, as occasional bands of cloudiness with
embedded showers are forecast to move through the region. On Sunday,
weak low-level moisture convergence, combined with local effects and
diurnal heating, will bring periods of moderate to locally heavy
showers across portions of the central interior and western PR.
Recent model guidance continues to suggest that the best chance for
the shower development and good rainfall accumulations will be on
Sunday afternoon.

.LONG TERM...Monday through Saturday...From Prev Discussion...

As the surface high pressure moves eastward into the east-central
Atlantic and a frontal boundary moves into the western Atlantic,
the low-level flow will become southeasterly by Monday. This will
advect deeper moisture into the area with precipitable water
values expected to increase to up to 1.8 inches. Therefore, an
increase in afternoon convection is expected, especially across
northwestern Puerto Rico. More widespread rainfall activity is
possible by Tuesday and Wednesday as the frontal boundary
approaches the area and a polar trough amplifies across the
western Atlantic. These features will serve to increase the
vertical motion. Thunderstorms are also more likely given that
the 500 mb temperatures are expected to cool to around -7 degrees
celsius. At this point, it is still too early to pinpoint how
much rainfall and which locations have the best potential to
receive the heaviest activity.

Weather conditions are expected to improve by the latter half of
next week as the polar trough lifts to the north and a mid-level
ridge builds overhead. This will result an erosion of the moisture
content.

&&

.AVIATION...

VFR conds at all terminals. Isold-Sct SHRA ovr regional
waters, and vcty of the local islands til 29/23z. SCT-BKN lyrs nr
FL025...FL050 durg prd with Isold SHRA en route btw islnds. Sfc wnds
lgt/vrb bcmg calm aft 29/23z. L/lvl wnd fm W-NW 5-15kts blo FL100,
then backing bcmg fm N abv. No sig operational wx impacts attm.

&&

.MARINE...

Tranquil marine conditions continue through tonight, with seas less
than 4 feet anticipated. Tomorrow, however, will bring an end to the
tranquility, as a northerly swell makes its way into the local
waters. Deteriorating conditions, with seas becoming hazardous, are
expected by the afternoon tomorrow in outer Atlantic waters, and by
tomorrow night for the rest of the local Atlantic waters and
passages. A small craft advisory has been issued for the outer
Atlantic waters, and additional advisories for other zones will
likely be issued later today. Hazardous marine conditions are
expected to persist into early next week.

There is a low risk of rip current at all local beaches for today.
However, the swell arriving tomorrow will also lead to deteriorating
conditions at the beaches. The rip current risk will be moderate to
high for north-facing beaches of Puerto Rico tomorrow morning, and
for most north-facing beaches across the local islands by tomorrow
night. Additionally, high surf conditions are expected to be
associated with this swell event, with breaking wave height
exceeding 10 feet at many of the north-facing beaches.

&&

.PRELIMINARY POINT TEMPS/POPS...
SJU  75  86  75  87 /  50  30  30  40
STT  75  88  76  86 /  40  40  40  40

&&

.SJU WATCHES/WARNINGS/ADVISORIES...
PR...None.
VI...None.
AM...Small Craft Advisory from 2 PM Saturday to 8 PM AST Monday for
     Atlantic Waters of Puerto Rico and USVI from 10 NM to 19.5N.

&&

SHORT TERM...RAM
LONG TERM....CRS
PUBLIC DESK...FRG

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