High Surf in Hawaii, November 22 to 24, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
326 AM HST Sun Nov 22 2020

.SYNOPSIS...
Wet and gusty trade winds will continue today as a strong high
pressure system building north of the region will combine forces
with unstable clouds moving in from the northeast. Trade winds
will weaken next week with periods of showers continuing through
Wednesday. The highest shower coverage will favor the overnight to
early morning hours. Drier trends and light to moderate trade
winds are forecast for most areas on Thanksgiving Day.

&&

.DISCUSSION...
This morning's satellite picture shows a large band of unstable
clouds and showers lingering east to northeast of the Hawaiian
Islands. This long band of showers is riding in on the trade winds
bringing wet weather conditions to all islands this morning.
Radar imagery shows scattered to numerous showers from the
unstable cloud band moving into the windward slopes of each
island. Some of the stronger showers will drift over leeward
sections through the morning hours with lighter coverage in the
afternoon. The highest rainfall amounts will continue to favor
windward and mountain slopes. A subtropical jet south of the state
will produce some high cirrus clouds over the Big Island, these
cirrus clouds will thicken and spread northward across the state
later this morning and afternoon.

Short range forecast guidance continues to show a fairly wet
weather pattern continuing into the first half of next week.
Today's rainfall will be driven by strong trade winds and the
unstable cloud band, remnants of an old cold front. A Wind
Advisory remains in effect through 6 PM HST this evening for the
windier portions of Molokai, Kahoolawe, Maui and the Big Island of
Hawaii.

On Monday, as the trade winds decrease into the moderate
to breezy range, an upper level trough deepens just northwest of
the island chain. This trough will help keep the subsidence
inversion on the weak side, allowing deeper and more unstable
clouds to form. Rainfall from Monday through Wednesday will still
favor the overnight to early morning hours, however some enhanced
afternoon showers will develop under the influence of this upper
level disturbance. Tuesday into Wednesday morning will likely be
the wettest time period this week as the trough axis passes from
west to east down the island chain. Isolated thunderstorms are
possible on Tuesday near Kauai, spreading into Oahu from Tuesday
night into Wednesday morning, and then moving into Maui County on
Wednesday morning.

On Thanksgiving Day, the upper level trough axis will drift
eastward through the Big Island by the early morning hours. Some
lingering showers are possible along windward slopes from Hilo
to Hawi mainly in the early morning hours. However more stable
conditions will develop across the state after the upper trough
passes through each island. Generally we see drying trends for
most areas for Thanksgiving day with light to moderate trade
wind flow. The good news is the global weather models, American
(GFS) and European (ECMWF) are showing more consistency with this
solution and confidence is fair in these drying trends for the
holiday five days out into the future. Stay Tuned.

By Friday another weak cold frontal band will move in from the
north. Cooler and drier air moving in behind this cold front will
bring cooler temperatures and lower humidity trends for the next
several days. A high pressure system will build back in far north
of the state producing a return to moderate to breezy trade
winds. Time to dust off those jackets with fair chances for low
temperatures dropping into the mid 60's along with moderate to breezy
trade winds by next weekend.

&&

.AVIATION...
A strong 1032 mb surface high lies 970 nm NNE of Oahu early
this morning. It will be moving E 10 to 15 kt for the next 48
hours only to be taken over by a new high on Tuesday. But for the
next 12 hours or so, the trade winds will be at its strongest,
followed by a slight weakening trend into Monday. AIRMET Tango for
low level mechanical turbulence below 9k feet, will continue
through today, and likely beyond.

The leading edge of a east to west oriented band of clouds and
showers that was over Molokai at 7 pm HST last night, has advanced
westward to Kauai. But the band has loosen up somewhat to where
there are breaks in the cloud cover such as off windward Big
Island. Low ceiling and showers continue from Upolu Point to Hilo
to Volcano this hour where there is a AIRMET Sierra. Should
improving conditions hold, AIRMET Sierra for windward Big Island
will be lifted.

Due to the low frequency of showers, AIRMET Sierra for windward
Maui and Molokai will be lowered by the next update at 6 am HST.
Tops of the clouds and showers are mainly around 10k feet with
higher tops to 15k feet especially windward Big Island.

High cirrus clouds will be spreading across the island skies this
morning, from the W and SW, becoming thick after 22z. The dense
cirrus will not begin to thin out until Monday.

&&

.MARINE...
Trade winds remain strong and a Small Craft Advisory (SCA)
has been extended through tonight for all waters except for the
Alenuihaha Channel, which has an extended Gale Warning through
tonight. Seas have also increased and are expected to remain high
through tonight. Winds are expected to start decreasing Monday,
so the Gale Warning for the Alenuihaha Channel will be replaced by
an SCA at that time. Expect the area covered by the SCA to scale
back on Monday as the winds subside.

Trade winds are bringing choppy, elevated surf to east facing
shores, and a High Surf Advisory remains in effect through
tonight.

A north-northwest swell will keep surf elevated, but below
advisory levels, along north and west facing shores. Surf is
expected to subside through the week. Another north swell may
arrive at the end of the week, providing a boost to surf. Surf
along south-facing shores will remain small over the same time
period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST this evening for Molokai Windward-
Kahoolawe-Maui Leeward West-Maui Central Valley-Windward
Haleakala-Leeward Haleakala-South Big Island-Kohala-Big Island
Interior.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

Gale Warning until 6 AM HST Monday for Alenuihaha Channel.

&&

DISCUSSION...Bohlin
AVIATION...Lau
MARINE...Powell