High Surf in Hawaii, December 27 to 28, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Sun Dec 27 2020

.SYNOPSIS...
An upper level low moving through the Big Island will produce
enhanced showers with isolated thunderstorms today. Drying trends
are forecast for most islands through the day as the upper
disturbance drifts south through the Big Island, and the low level
cloud band lifts north of the state. Trade winds will decrease in
strength and will become more east to southeasterly through
Wednesday. These lighter winds from a more southerly direction
will bring up a plume of VOG from Kilauea on the Big Island into the
smaller islands during the first half of next week. Moderate to
breezy northeasterly trade winds will return by New Years Eve just
in time for the New Years celebrations.

&&

.DISCUSSION...
Satellite imagery this morning shows an upper level low drifting
southward through the Big Island. A few thunderstorms are
developing over Hawaii County under this upper low. Enough
instability and forcing will keep isolated thunderstorms in the
forecast over the eastern end of the state through the afternoon
hours. The low level convergent cloud band associated with an old
frontal boundary over the northwestern islands will slowly drift
northward today. Expect decreasing shower trends across the state
brief passing showers mainly affecting windward and mountain
areas.

For today, Trade Winds will decrease in strength and veer towards
the east southeast direction. The Wind Advisory was cancelled this
morning as wind speeds are falling below advisory thresholds. Most
islands will see a transition back into a more stable pattern
with drier trends as the low level convergence band drifts north
of the state and an upper level low slowly moves south of the Big
Island. The one exception to these drying trends will occur over
the eastern and southeastern slopes of the Big Island, as a low
level trough moves into the island from the east and the upper low
drops in from the north. The conjunction of both these systems
along with easterly to southeasterly winds will enhance showers
along southeastern and eastern slopes of the Big Island through
the afternoon and evening hours.

Monday through Wednesday, a fairly stable weather pattern develops
with an upper level ridge in place over the islands keeping a
strong trade wind temperature inversion in place at 6000 to 8000
foot range. However, trade winds will become light and veer
towards a more east to southeast direction. These large scale
southeasterly winds tend to keep the smaller islands, northwest of
the Big Island, in a wind and rain shadow area. This means local
scale land and sea breezes will develop with brief passing
showers are possible, trending towards island interior sections
through Wednesday. East and southeast slopes of the Big Island
will continue to see periods of shower activity in this pattern.
These southeasterly winds may bring a light Volcanic smOG, also
known as VOG, back to the smaller islands for the first half of
next week.

Thursday and Friday, a migratory high moving from west to east
across the Central Pacific will bring moderate to breezy trade
winds back into the state from New Years Eve to New Years Day.
These stronger trade winds will arrive just in time to blow out
the VOG, and the firework smoke from our New Years celebrations.

&&

.AVIATION...
Thunderstorms that developed over Big Island early this morning
in advance of the passing upper trough have begun to dissipate,
giving way to more a more widespread coverage of stratiform
showers. Some redevelopment of showers and localized MVFR will be
possible over Big Island again today before the upper trough
exits to the southeast. Breezy and drier trades supporting
occasional windward showers and predominantly VFR conditions
prevail elsewhere.

AIRMET Tango remains in effect for all islands for moderate
turbulence between FL250-FL350 as well as mechanical lee
turbulence.

&&

.MARINE...
An transiting high far northeast of the area will produce strong
trade winds through the rest of the weekend. Because of a
combination of winds and seas, a Small Craft Advisory is now in
effect for most coastal waters east of Oahu through early evening.
The Gale Warning that had been in effect has been cancelled. A
High Surf Advisory is also in effect for exposed east facing
shores through this afternoon because of strong trades and short-
period choppy surf. The trade winds will weaken and veer east
southeast by early this week.

The current northwest swell continues to trend downward, with a
small reinforcement from the same direction expected late today.
This will be followed by a more significant, advisory-level
northwest swell Monday night. More larger, reinforcing northwest
swells are forecast to arrive Tuesday and Thursday. Surf from
these larger swells will likely produce warning level surf along
most north and west facing shores, and will only gradually fade
through next weekend. No other significant swells are expected.

&&

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

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

DISCUSSION...Bohlin
AVIATION...JVC
MARINE...Kinel