High Wind in Hawaii, December 30, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
422 AM HST Mon Dec 30 2019

.SYNOPSIS...
A high pressure ridge north of the Hawaiian Islands will keep
moderate to breezy easterly winds in the forecast on today. Wind
speeds will increase today compared with yesterday. Wind speeds
then decrease and veer towards the southeast direction by Tuesday
night as a cold front approaches Kauai from the west. The cold
frontal band sweeps a brief period of showers west to east through
the state from Thursday morning near Kauai to Friday morning over
the Big Island. Strong and gusty northeasterly trade winds with
cooler and drier trends are forecast to move in behind the frontal
band as it passes through each island.

&&

.DISCUSSION...
Scattered bands of clouds are drifting in with the easterly trade
winds this morning on satellite imagery. Expect isolated to
scattered showers favoring windward and mountain areas mainly in
the overnight to early morning hours through Wednesday. A cold
front roughly 700 miles northwest of Kauai continues to move
towards the state arriving over the western islands on Thursday.

The large scale weather pattern keeps a 1034 MB surface high far
northeast of the state with ridge extending southwest from the
high center towards the island chain. The ridge axis will remain
just north of the island chain keeping the state in a east to
southeast wind pattern over the next few days. Wind speeds will
increase today across the region with strong and gusty winds
forecast to start later tonight over the highest peaks of the Big
Island. A wind advisory was issued for the summits of Maunakea
and Maunaloa from 6 PM this evening through 6 PM HST Tuesday.
Windy conditions are also forecast for Haleakala tonight and
tomorrow, however wind speeds over the 'House of the Sun' should
remain just under summit wind advisory thresholds.

A cold front moving into the region on Thursday will bring
significant changes to island weather as it sweeps west to east
across the state. The latest weather model guidance brings
showers and low clouds along the front into Kauai by Thursday
morning, then the front weakens as it passes through Oahu and Maui
County by Thursday afternoon and evening. Expect a brief period
of low clouds and showers along the frontal band with strong and
gusty northeasterly trade winds moving in quickly as the frontal
trough passes through each island. Cooler and drier air behind the
front will also march down the island chain behind this system.
The American (GFS) and European (ECMWF) models are in good
agreement with the timing on cold frontal passage through the
state, improving our confidence levels on island by island
impacts.

Friday through Sunday, a strong high pressure system will move
eastward across the region, increasing local area pressure
gradients, and producing breezy to windy trade winds through
Sunday. The GFS and ECMWF models show large differences in wind
speeds associated with this building high pressure system. We
blended both these global models to reduce wind speed errors,
leaning towards the GFS on wind direction. If these high wind
speed trends continue then wind speeds may exceed wind advisory
thresholds starting on Friday especially over wind favored
mountain areas. Passing isolated to scattered showers will tend to
favor north through east slopes of all islands through this time
period.

&&

.AVIATION...
A strong surface ridge positioned to the north and northwest of
the forecast area will maintain broad easterly flow across the
state. Multiple bands of clouds and showers are visible on
satellite embedded in the easterly flow. AIRMET Sierra for
mountain obscuration across Windward Big Island will likely be
cancelled toward morning, but may need to be reissued by late
afternoon or early evening. Elsewhere, expect isolated MVFR CIG
and VIS over mountains and in showers.

&&

.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
short-period, choppy surf along the east facing shores, but below
advisory level. A small, long period south southwest swell will
decline today.

An extra large northwest (310 degree) swell will begin filling in
around mid to late 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 later in the afternoon and Maui County by
the evening. A High Surf Warning goes into effect this morning
through Tuesday night for exposed north and west facing shores of
the smaller islands. The Big Island may also see effects later Monday
night and Tuesday, but not nearly as large, as it will be partially
blocked by the smaller islands. Will likely issue a High Surf Advisory
for west facing shores of the Big Island later today or tonight.
This swell will peak Monday night into Tuesday morning and decline
through the middle of the week.

The Small Craft Advisory (SCA), already in effect for most coastal
zones, expands to all coastal waters this 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 until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Windward Haleakala.

Wind Advisory from 6 PM this evening to 6 PM HST Tuesday for Big
Island Summits.

Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian
waters-

&&

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...TS