High Surf in Hawaii, December 18 to 19, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Fri Dec 18 2020

.SYNOPSIS...
Strong trade winds will continue into Saturday, gradually diminish
Sunday and Monday, then become light and variable Tuesday and
Wednesday. The trade winds will deliver clouds and brief showers to
windward areas of the smaller islands, with widespread showers over
windward Big Island occasionally spreading to windward Maui through
the weekend. A dry weather regime is expected statewide when the
trade winds diminish early next week. A front may bring showers and
increased northeast trade winds next Thursday and Friday.

&&

.DISCUSSION...
An area of increased moisture (associated with a surface trough and
low aloft) near the Big Island will continue to stream moisture
toward windward Big Island through the weekend, with showers
occasionally spreading to windward Maui. While fewer windward
showers are expected over Kauai and Oahu, brief showers will remain
likely.

A pool of cold air in the mid-levels is expected to linger near and
E of the Big Island through the weekend, acting to destabilize the
increased low-level moisture that has been pooling in the area for
the last couple of days. This will keep windward Big Island fairly
wet, but with the trough axis in the upper-levels shifting E of the
area, rainfall coverage and intensity is not expected to become
sufficiently extreme to warrant a Flash Flood Watch. The associated
instabilty may allow an isolated thunderstorm to develop, but the
probabilites will be diminishing after today.

As the low-level trough near the Big Island passes S of the islands
and gradually weakens over the next 24 hours, the high to the
distant NE will remain nearly stationary, supporting locally strong
trade winds. A Wind Advisory remains posted for windier portions of
Maui County and the Big Island, where terrain most accelerates the
flow. The high will gradually weaken over the weekend, and trade
wind speeds will trend down in response. As a front approaches from
the NW early next week, a ridge associated with the high will move
over the islands, and winds will become light and variable. This
will allow island-scale night and morning land breezes and afternoon
sea breezes to drive sensible weather over the islands Tuesday and
Wednesday.

A mid-level ridge building over the area from the W this weekend
will bring increasing stability over the smaller islands, but the
trough lingering to the E will keep moderate instability in place
over the E end of the state. Guidance indicates even warmer mid-
level temperatures developing statewide Tuesday and Wednesday,
indicating a very stable island atmosphere, thus the aforementioned
sea breezes are not expected to lead to any significant
precipitation.

A significant trough aloft may pass over the chain in about a week.
Latest model guidance is remarkably consistent indicating that the
moisture associated with the approaching front will be pushed over
the islands by high pressure building NW of the area, with the
trough potentially acting to enhance the moisture. The high will
support increasing NE trade winds as well.

A Winter Weather Advisory for the Big Island Summits remains in
place today. Web cams indicate a dusting of snow occurred overnight,
while ranger reports indicated icy roadways this morning. Still the
potential for some wintry weather through tonight possibly, and the
Advisory may need to be extended.

&&

.AVIATION...
A reservoir of tropical moisture has been drawn northward
encompassing the Big Island and, to a lesser extent, Maui in
response to an upper trough now moving east of the state.
Widespread and MVFR has been observed over Big Island and will
continue through today and tonight, including the potential for
isolated thunderstorms. AIRMET Sierra remains in effect there. For
the remainder of the area, prevailing VFR and breezy to locally
strong trades can be expected through the daylight hours.

By tonight, this pool of moisture will be drawn over the remaining
islands coincident with a weakening trade inversion. As a result,
increasing increasing windward showers are forecast to exhibit some
vertical growth and increased leeward drift through the overnight
hours. AIRMET Sierra may eventually be needed.

AIRMET Tango remains in effect for both lee & upper-level turbulence.

AIRMET Sierra remains in effect for Windward Big Island.

&&

.MARINE...
High pressure far off to the northeast will maintain a tight
pressure gradient over the islands, producing moderate to locally
strong trade winds through early Sunday. A Small Craft Advisory
(SCA) is in effect for all nearshore waters through 6 PM HST
Saturday, except the Alenuihaha Channel that is under a Gale Warning
through this afternoon. The SCA will also account for rough sea
conditions due to the combo of winds and a northwest swell, with sea
heights around 10 to 14 feet likely persisting through Saturday
evening. A downtrend in winds and seas is expected Sunday and Monday
as the high weakens and the associated ridge is shunted southward by
an approaching cold front.

A High Surf Advisory (HSA) remains in effect until noon today for
exposed north and west-facing shores due to a long-period northwest
swell. This swell is beginning to diminish at the nearshore buoys
this morning and will continue to fade today. The HSA will likely be
allowed to expire as scheduled. A new long-period west to northwest
swell is expected to arrive Saturday night into Sunday, with peak
surf heights remaining just below advisory levels. A larger,
slightly longer-period west-northwest swell is possible around the
middle of next week.

Strong trade winds will create rough surf along exposed eastern
shorelines for the next couple of days, so a HSA has been issued for
those areas through 6 PM HST Saturday. Out-of-season small southerly
swells are expected to give south shore surf a small bump this
weekend through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 12 PM HST today for North and West facing
shores of Niihau Kauai Oahu Molokai and for North facing shores of
Maui.

High Surf Advisory until 6 PM HST Saturday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Wind Advisory until 6 PM HST this evening for Molokai Windward-
Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui
Central Valley-South Big Island-Big Island North and East-Kohala.

Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Small Craft Advisory until 6 PM HST Saturday 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.

Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday
for Alenuihaha Channel.

Gale Warning until 6 PM HST this evening for Alenuihaha Channel.

&&

TB/JC/TS