High Surf in Hawaii, December 20 to 26, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
427 AM HST Fri Dec 20 2019

.SYNOPSIS...
Strong high pressure passing north of the islands will produce
very windy conditions today through Saturday, and possibly into
Sunday. An old front will also move through the state today
through early Saturday, bringing some wet weather to the islands.
An area of low pressure west of the state will cause mostly cloudy
conditions across the area through the middle of next week. A wet
weather pattern could develop around the Christmas holiday, but
confidence in the details remains low at this time.

&&

.DISCUSSION...
Currently at the surface, a 1033 mb high is centered around 1300
miles north-northwest of Honolulu, while an old front is located
around 50 miles north of Kauai. Meanwhile, a developing area of
low pressure is located around 850 miles west of the state. The
gradient continues to tighten up across the island chain early
this morning, with a few places already observing winds near
advisory levels. Infrared satellite imagery shows a combination of
low clouds moving in with the trades and convective debris cirrus
streaming overhead, resulting in partly to mostly cloudy
conditions. Radar imagery shows scattered showers moving into
windward areas, with some of these showers reaching leeward areas
as well. Most of the shower activity is confined to the western
end of the state, particularly Kauai and Oahu in closer proximity
to the old frontal remnants. Main short term concern revolves
around the very strong trade winds expected during the next few
days.

A strong 1033-1035 mb high will shift eastward well to the north
of the islands during the next few days. Meanwhile, the remnants
of an old front will push steadily southward through the island
chain this afternoon through Saturday morning. At the same time,
an intensifying low will lift northward well to the west of the
state. All of these features will result in a tightening pressure
gradient across the island chain today, with winds expected to
really ramp up along and behind the old remnant front as it pushes
through the islands. Very windy conditions are expected to
overspread the state tonight through Saturday and possibly linger
into Sunday. As a result, a Wind Advisory is now in effect
through 6 PM this evening, and a High Wind Warning is in effect
from 6 PM this evening through 6 PM Saturday.

More uncertainty in the wind forecast begins next week. The GFS
drags a front into the island chain during the middle to latter
part of the week, while the ECMWF keeps the front west of the
state. Given the continued uncertainty in the forecast, will
follow a middle of the road solution which results in breezy to
windy trades through early next week, with winds shifting around
to the southeast and easing during the middle and latter part of
the week.

As for weather details, trade wind weather will continue through
early next week, with some fluctuation in shower coverage and
intensity. Typical trade wind shower coverage can be expected
in most areas this morning. We'll see a notable increase in the
coverage and intensity of the trade showers this afternoon
through Saturday as moisture associated with an old front pushes
southward through the island chain. The old frontal remnants are
expected to push through Kauai this afternoon and this evening,
through Oahu and Maui County tonight, and through the Big Island
after midnight tonight through Saturday morning. Behind the
frontal remnants, it appears that conditions will dry out a bit,
with a decrease in shower coverage expected Saturday night. We
should see the airmass gradually moisten up by Sunday, with a more
typical trade wind shower pattern then expected through the first
half of next week. Much uncertainty continues in long range
portion of the forecast, with the GFS indicating some heavy
rainfall and thunderstorms could be in store for the area
Christmas Day through Thursday, while the ECMWF keeps the
heaviest rain and thunderstorms west of the state.

A considerable amount of mid and high cloud cover is expected
today through much of next week. As a result, mostly cloudy
conditions will be common statewide. Some thinning in the clouds
can be expected at times, but timing the days with more sunshine
filtering through will be difficult to pin down until the models
come into better agreement.

&&

.AVIATION...
Strong high pressure centered north-northwest of the islands will
increase trade winds significantly statewide over the next 24
hours. Trade winds will increase through the day today with
surface winds reaching the 20 to 30 MPH range with higher gusts,
particularly this afternoon. Stronger wind speeds are possible
over some wind favored locations. AIRMET Tango remains in effect
for moderate low level turbulence south through west of mountains
below 080. This AIRMET will likely continue through Saturday.

Expect brief periods of low cloud ceilings and showers over Kauai
and Oahu through early morning hours as bands of unstable clouds
drift across the northwestern islands. A weakening cold front will
move into the islands from the north spreading periods of MVFR
cloud ceilings and showers across the region from the afternoon
through Saturday morning time periods. AIRMET Sierra in effect
for Tempo Mountain Obscurations above 020 for north through east
sections of Kauai, Oahu and Molokai.

&&

.MARINE...
Increasing winds and seas are expected today as a strong high
pressure cell builds N of the islands. A Small Craft Advisory
(SCA) is now in effect for all waters, and will be upgraded to a
Gale Warning through the day as the strong trade winds build from
N to S over the area. The high is pushing a dissipating frontal
boundary over the islands from the N, which is expected to bring
an increase in showers. Additionally, forecast models indicate E
trade winds will surge as this boundary passes today and tonight,
with sustained open-ocean winds reaching 30 kt or more. While
sustained wind speeds may not reach gale force in all marine
zones, frequent gusts to gale force warrant the issuance of the
warning. Supporting this is the fact that buoy 51001 NW of Kauai
has reported gusts to 35-37 knots in nearly every observation
since 7 pm last night, while an overnight ASCAT pass showed an
area of gale force winds associated with the high just NW of the
Hawaiian Offshore Waters.

Combined seas are expected to build to 12-14 feet by tonight,
with a period of 9 seconds making it rather rough for boating.
Seas will diminish slightly later in the weekend, as winds ease
(and potentially veer) slightly Sunday-Tuesday. However, seas are
expected to remain 10 feet or above into Christmas, with SCA-level
E-SE winds expected in most zones.

Surf along N and W facing shores looks to remain below advisory
levels through Christmas. However, surf along E facing shores will
rise as trade winds increase, and a High Surf Advisory (HSA) has
been issued in anticipation of the increasing rough and short-
period surf. A small NNW swell will shift to the N as it gradually
diminishes over the weekend, with a relatively small NW swell
possible around Tuesday and persisting into Thursday. A larger
NW swell is possible by next Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 6 PM this evening to 6 PM HST Saturday
for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui Windward West-
Maui Leeward West-Maui Central Valley-Windward Haleakala-Leeward
Haleakala-Kona-South Big Island-Big Island North and East-Kohala-
Big Island Interior.

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

High Surf Advisory until 6 PM HST Sunday 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 Saturday for Haleakala Summit.

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

Gale Warning from 6 PM this evening to 6 PM HST Saturday for
Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-
Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST this evening for Oahu
Leeward Waters-Maui County Leeward Waters-Big Island Windward
Waters.

Small Craft Advisory until noon HST today for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters.

Gale Warning from noon today to 6 PM HST Saturday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Birchard