High Surf in Hawaii, December 29 to 31, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Sun Dec 29 2019

.SYNOPSIS...Light southeasterly winds are expected to strengthen
Monday high pressure rebuilds northeast of the islands. Brief
trade showers will be focused mainly over windward and mauka areas.
A front approaching from the northwest around New Years Day will
likely bring southeast winds, and an increase in cloud cover and
showers from early Thursday into Friday.

&&

.DISCUSSION...
Made some additional tweaks to the first half of tonight's
forecast to better account for the lack of clouds over and near
the islands. Otherwise have left the forecast as is while
continuing to monitor the models for the front's impact later this
week.

The morning surface analysis continues to show a ridge to the
northwest of the islands, with a high to the northeast. The
pressure gradient shows light southeasterly flow moving over the
islands, which is in line with the satellite loops. Winds are
strongest and more easterly over/near the Big Island and Maui,
with lighter, slightly more southeasterly winds over/near Kauai
and Oahu.

The models remain in good agreement for the short term with the
high to the northeast lifting northward a bit and strengthening,
which will allow winds to increase a bit Monday night and into
Tuesday. A relatively dry airmass over the islands into Tuesday
night will keep rainfall totals on the lighter side.

Heading into the new year, we continue to see model differences.
Both the GFS and ECMWF show a front approaching from the northwest,
and both have an upper level trough accompanying the front by
Wednesday. This will bring an uptick in shower activity as the
front moves down the island chain, beginning Thursday morning. The
ECWMF is a bit faster and has a deeper upper level trough
accompanying the front. Earlier runs of the ECMWF kept northerly
winds behind the front longer, but the latest run has the winds
veering around to the northeast by Friday morning. The GFS brings
winds back to the northeast a little faster, but also increases
winds during the end of the week. The current forecast reflects
winds more in line with the GFS, but not quite as strong as the
past two runs of the GFS.

The current forecast reflects a front with increased shower
activity reaching the western islands by Thursday morning, with
showers spreading down the island chain. The inconsistencies
between the models and from run to run in the individual models
continues low confidence in the details of the forecast during
the second half of the week.

&&

.AVIATION...
MVFR conditions prevail at most TAF sites with the exception of
PHLI and PHNY where there has been brief MVFR conditions in low
clouds. Expect this general trend to continue through the
overnight hours at PHLI with improving conditions expected at
PHNY. Local MVFR conditions are possible at times tonight over
east facing slopes of all islands.

No AIRMET's are currently posted at the present time and none are
expected through tonight.

&&

.MARINE...
The ridge of high pressure north of the state will gradually
slide southeast over the next several days as a front passes far
north of the area. This is resulting in east southeasterly and
slightly weaker winds for waters around Kauai tonight. Meanwhile,
fresh to strong easterly winds will continue across the rest of
the coastal waters around the state through Tuesday night creating
locally short-period choppy surf along the east facing shores
(though surf heights will remain below advisory levels for east
shores). Elsewhere, a moderate northwest swell will continue to
impact exposed north and west facing shores tonight. A small,
long period south southwest swell will decline tonight.

An extra large northwest (310 degree) swell will begin rolling in
around mid-morning for Kauai and Niihau from a hurricane-force
low that set up east of Japan last last week. This swell is
expected to reach Oahu around afternoon and Maui County by late in
the day. A High Surf Warning goes into effect Monday morning
through Tuesday night for exposed north and west facing shores of
the smaller islands. The Big Island may also effects beginning
Monday night, but will be partially blocked by the smaller islands
so surf heights will not be as high. Will assess the situation
tonight and likely issue any highlight for the Big Island early
tomorrow morning. This swell will peak Monday night into Tuesday
morning and decline through the middle of the week.

A Small Craft Advisory (SCA) is posted for most coastal zones
(with the exception of Oahu and Maui County leeward waters)
tonight due to a combination of winds and northwest swell. All
coastal waters will fall into the SCA Monday morning due to the
expected increase in seas from the incoming northwest swell.

Beginning late Thursday into Friday, a large, mid-period north
swell will build in and continue into the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from 6 AM Monday to 6 AM HST Wednesday for
Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Molokai-Maui Windward West-Windward Haleakala.

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

Small Craft Advisory from 6 AM Monday to 6 AM HST Wednesday for
Oahu Leeward Waters-Maui County Leeward Waters.

&&

DISCUSSION...M Ballard
AVIATION...Burke
MARINE...Foster