High Surf in Hawaii, December 5 to 8, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Sat Dec 5 2020

.SYNOPSIS...
Trade winds will ease and shift out of the southeast today,
leading to a land and sea breeze pattern over much of the state
through the weekend. Showers will favor windward slopes of the
Big Island and east Maui, while daytime sea breezes produce
clouds and a few light showers over interior terrain elsewhere.
Moderate easterly trade winds and a typical shower pattern will
rebuild on Monday. Winds may briefly decrease and shift
southeasterly again Wednesday and Thursday.

&&

.DISCUSSION...
Trade winds will steadily decline and shift out of the southeast
today. The surface ridge driving the trades currently sits about
150 miles north of Kauai, and as this feature is pushed southward,
local winds will ease and shift out of the southeast. With the
western half of the island chain being put in the wind shadow of
the Big Island, sea breezes will dominate over most areas this
afternoon. A ridge aloft is building overhead from the west,
leading to stable conditions with a firm inversion between 4,000
to 6,000 ft, and the available low-level moisture in the trade
wind flow is modest. This points to clouds and only light showers
forming over interior terrain this afternoon.

The stable, southeasterly low-level winds will persist through
the weekend. Winds will be more easterly around the Big Island and
east Maui, where windward areas will experience modest rainfall.
Elsewhere, a land and sea breeze pattern will dominate, leading to
clearing overnight and clouds and a few showers over interior
terrain Sunday afternoon.

Trade winds will return Monday and Tuesday. A front that will
stall several hundred miles north of the islands this weekend will
wash out, and surface high pressure will pass about 800 miles
north of the state, driving moderate trade winds. Mid-level
ridging will maintain stable conditions, focusing showers over
typical windward slopes.

Local winds will likely ease and potentially shift out of the
southeast again on Wednesday and Thursday as another north Pacific
front develops and stalls north of the islands. A mid- to upper-
level trough will pass over the state at this time, providing
some instability that could enhance showers.

Into the long term, stable conditions should return on Friday, and
trades are expected to rebuild heading into next weekend.

&&

.AVIATION...
High pressure northeast of the state will weaken over the next 24
hours as a cold front approaches and stalls northwest of Kauai. As
a result, the trades will diminish from east to west across the
state today. Sea breezes are expected to develop in many areas this
afternoon, with land breezes taking over tonight. Rather dry weather
is expected across the island chain, with only a few isolated showers
affecting windward areas this morning and again tonight, as well
as interior sections this afternoon.

AIRMET TANGO remains in effect for moderate turbulence aloft from
Molokai through the Big Islands and could continue through this
afternoon before diminishing.

AIRMET TANGO also remains in effect for low-level turbulence over
and downwind of terrain for all islands. Conditions should improve
by late this morning for most, if not all, areas.

&&

.MARINE...
Trade winds will be on the decrease today as high pressure moves
off to the northeast of the area. The Small Craft Advisory (SCA)
that is currently posted for the waters around the Big Island will
likely be lowered later today. However, a SCA will likely be
posted once again for waters exposed to a new large northwest
swell filling in on Sunday. Also a new high will be passing by to
our north Monday and Tuesday with winds likely increasing the SCA
levels over the typically windy waters around Maui County and the
Big Island.

The current moderate size northwest swell is running a bit
smaller than model guidance. Based on buoy readings northwest of
the Kauai, surf heights will likely remain just below advisory
levels today and tonight.

Long period forerunners from a new large northwest swell, are
expected to arrive Sunday. A High Surf Warning will likely be
issued later today for the anticipated arrival of this large
swell. This swell is expected to peak late Sunday and Sunday night
before gradually lowering Monday and Monday night. Another, but
slightly smaller northwest swell is expected to fill in on
Tuesday. This swell is expected to peak late Tuesday and Tuesday
night before gradually lowering Wednesday and Thursday. Advisory
level surf is anticipated during the peak of this swell.

Trade winds over and upwind of the state, will continue to produce
small, short period choppy surf along east facing shores through
the weekend and on through most of next week. Small surf will
continue along south facing shores through Monday with a slight
uptick possible Tuesday through Thursday as a small long period
south southwest swell moves through the area.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Wroe
AVIATION...Burke
MARINE...TS