Winter Weather Advisory 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Mon Feb 8 2021

.SYNOPSIS...
A cold front will sweep down the island chain this afternoon through
Tuesday, bringing a period of clouds and showers, and a slight
chance of a thunderstorm. Cool and breezy northerly winds will
strengthen after the front passes, shifting to moderate trade winds
Wednesday through Friday, with the trade winds bringing a few
windward showers. Winds may shift to the southeast and bring
increased moisture starting next weekend.

&&

.DISCUSSION...
The leading edge of a ragged-looking cold front is seen on visible
imagery to be about 100 miles NW of Kauai, moving SE near 15 mph. A
ridge of high pressure is nosing in from the NE over the Big Island
while a surface trough lies to the E and SE of the Big Island. Light
winds and clear skies prevail over the Big Island and parts of Maui
County, while leeward portions of Kauai and Oahu have scattered to
low clouds developing due to the light to moderate SW flow in place.

The leading edge of the cold front is expected to reach Kauai by
this afternoon, progress to Oahu and Maui County tonight, reaching
the Big Island early Tuesday, and then clearing the Big Island
Tuesday night. Associated moisture is expected to stall and pool
several hundred miles E of the islands. While moisture associated
with the front is expected to remain fairly limited/shallow, it will
bring a period of clouds and showers to most island areas as it
passes. Another pocket of very cold air will move over the islands
in association with the front, dramatically destabilizing the island
atmosphere as 500 mb temperatures drop from -11C to -18C. This will
bring a slight chance of thunderstorms, while also dropping the
freezing level below 10000'. As we saw last week, winter weather can
occur on the Haleakala Summit in this regime, in addition to the Big
Island Summits. A Winter Weather Advisory may be issued for these
areas later today, with the potential for impacts to occur early
Tuesday, possibly lingering into Tuesday night on the Big Island if
moisture is slow to clear.

Cool and breezy northerly winds will strengthen after the front
passes, driving dew points into the lower 50's (F), and bringing a
renewed chill to the islands. Trade winds and a gradually warming
air mass are expected Wednesday through Friday, as surface high
pressure tracks steadily E along 30N. The post-frontal N winds may
deliver a few low clouds and light showers to N facing slopes and
coasts, with the focus for clouds and showers shifting back to the
typical windward areas as winds veer to the ENE, allowing low-level
dew points increase. As the low aloft exits the area to the E, a
dramatic warming is expected at 500 mb, supporting a very stable
island atmosphere, with limited shower coverage and intensity.

A gradual increase in low-level moisture is expected to continue
through the weekend as the flow turns to the SE, with some of the
pooled frontal moisture sliding back toward the islands. Guidance
shows moisture increasing further in about a week as another mid-
level low induces a surface trough over the islands, potentially
fueling an increase in showers.

&&

.AVIATION...
A cold front north of Kauai will approach the island during the day
today before making its way further southeast to other islands
tonight and the Big Island tomorrow. Ahead of the front, light winds
and VFR conditions will prevail with vjust a few isolated showers.
However, as the front moves further southeast, expect increasing
clouds with heavier showers and isolated thunderstorms, which could
result in mountain obscurations, brief MVFR ceilings/visibility.
Post-frontal northeasterly winds will be breezy to locally strong
during the day tomorrow.

There are no AIRMETs currently in effect.

&&

.MARINE...
Winds have shifted out of the southwest, and will veer out of the
west today as a front approaches from the northwest and pushes the
high off to the east of the area. The leading edge of the front is
expected to reach Kauai waters this afternoon, progress to Oahu and
Maui waters tonight, then pass over the Big Island on Tuesday. The
front will bring a slight chance of thunderstorms and showers.
Locally strong north winds behind the front will shift to northeast
trade winds by Tuesday night.

The current northwest and north swells will continue to slowly
subside today. A new, large northwest swell is expected to build
late tonight and peak Tuesday, with surf likely reaching advisory
levels along north and west facing shores. Another, even larger
northwest swell is expected Friday night, with a still larger
northwest swell possible early Sunday. East and south shore surf
will remain small through the forecast period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Birchard
AVIATION...Vaughn/Foster
MARINE...Almanza