High Surf in Hawaii, February 27 to 28, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
915 PM HST Tue Feb 26 2019

.SYNOPSIS...
A slow-moving band of low level moisture over the eastern islands
along with a strong upper level trough moving overhead, will bring
unsettled weather to the eastern islands tonight into Wednesday.
Locally heavy showers and thunderstorms will be possible tonight
across Maui and through Wednesday across the Big Island,
particularly in windward areas. Elsewhere across the state,
scattered showers will bring some modest rainfall to mainly
windward areas with a stray shower spilling over into leeward
communities. Building high pressure to the northwest of the
islands will produce gusty north winds and cool conditions on
Wednesday, with the unsettled weather exiting east of the state
Wednesday afternoon. Cool northerly winds and light showers on
north facing slopes are expected Thursday into the weekend. A weak
front may move down the island chain Sunday.

&&

.DISCUSSION...
Currently at the surface, a weak convergence boundary or shear
line is oriented from west to east through the Alenuihaha
Channel and over the northern tip of the Big Island this evening.
Meanwhile, a reinforcing cold front is located around 175 miles
north-northwest of Kauai, and much further upstream a 1034 mb high
is centered around 1100 miles northwest of Kauai. North of the
shear line moderate to breezy north to northeast winds have taken
hold, while wind remain rather light and variable to the south.
Infrared satellite imagery shows a band of low clouds and showers
along and to the north of the shear line, with mainly scattered
windward showers across Kauai and Oahu. Main short term concerns
continue to revolve around the potential for flooding and winter
weather impacts at the high elevation summits on Maui and the Big
Island.

Tonight through Wednesday,
The shear line will slowly settle southward through the Big
Island overnight, with moderate to breezy north to northeast winds
overspreading the entire island by mid morning Wednesday and
continuing through Wednesday night. The band of enhanced moisture
with precipitable water (PW) values of 1.1 to 1.4 inches seen in
the UW-CIMSS total PW imagery, will remain anchored over Maui
County this evening, then slowly shift southeastward into the Big
Island after midnight, then exit east of the Big Island Wednesday
afternoon. Although moisture will be limited, strong forcing for
ascent provided by an upper level trough approaching from the west
along with entrance region forcing from a strong upper level jet,
should be enough to get a few locally heavy showers and perhaps a
rumble or two of thunder to develop mainly over windward sections
of Maui and the Big Island later tonight. The best forcing for
ascent and deepest moisture should exit Maui by daybreak
Wednesday, but is expected to hold in place over the Big Island
through around noon Wednesday, before exiting off to the east.
Although not an ideal situation for flash flooding given the low
precipitable water airmass, it can't be ruled out given the
forcing for ascent and the potential for thunderstorms, so as a
result a Flash Flood Watch remains in place for Maui through
tonight and for the Big Island through Wednesday.

Across Kauai and Oahu the airmass will be drier with PW values
generally at or below an inch. As a result, shower coverage and
intensity should remain lower here. Due to the upper level trough
moving through tonight and reinforcing cold front moving through
Wednesday, we should still see scattered showers continue to roll
into windward areas with some leeward spill over tonight and
Wednesday.

Wintry weather remains possible for the Haleakala summit tonight
and the Big Island summits through Wednesday, and as a result, the
winter weather advisory remains in place. The High Wind Warning
is also in effect for the Big Island summits through Wednesday.

Wednesday night through next Tuesday,
Model solutions are in fairly good agreement through the extended
periods. Both the GFS and ECMWF show a deep closed low northeast
of the islands Wednesday night lifting northeastward through the
end of the work week, while high pressure northwest of the state
weakens as it shifts southward and closer to the island chain.
Locally breezy northerly winds are expected to continue Wednesday
night, with the winds then slowly easing through the remainder of
the work week. The models show another strong upper trough digging
north of the islands over the weekend, leading to another closed
low developing northeast of the state Sunday through early next
week. The gradient will remain fairly loose across the islands
Friday through Saturday, with moderate to breezy northeasterly
winds then filling in Saturday night or Sunday as a reinforcing
cold front swings through the island chain. Breezy trade winds
are then expected early next week, as a new high builds northwest
of the state.

As for weather details, given the dry low level airmass building
in over the island chain with surface dewpoints falling well down
into the 50s and perhaps into the upper 40s, along with precipitable
water values at or below 1 inch, showers should remain light and
confined mainly to windward slopes and coasts through the period.
It will remain quite cool as well, with highs generally in the
70s and lows in the 50s and 60s.

&&

.AVIATION...
A band of enhanced moisture between Oahu to the Big Island will
move slowly eastward tonight, bringing an increase in the shower
activity. However, light to moderate northeast trades will
concentrate the greatest coverage along the windward coast and
mountain slopes. A trough aloft moving over the islands will act
to destabilize the atmosphere and bring a slight chance of
thunderstorms to Maui and the Big Island. Expect MVFR conditions
at times due to the lowered vis and ceilings.

AIRMET SIERRA remains in effect for Molokai, Maui, and the Big
Island below 2500 ft due to mountain obscuration from clouds and
showers.

&&

.MARINE...
As a front, currently northwest of Kauai, moves down the island
chain from late tonight through Wednesday, winds will shift out
of the north and strengthen to fresh to strong speeds through
Wednesday night. Winds will mainly stay out of the north or
north-northeast through the weekend and will begin to gradually
decrease on Thursday as a low pressure system to our northeast
begins to move east and away from the state. Winds will briefly
become gentle on Friday before increasing to moderate to fresh
speeds over the weekend.

Surf will continue to build tonight and through Wednesday as a
new large long-period northwest swell continues to fill in. This
swell is expected to peak Wednesday night near warning levels.
Besides the large northwest swell, a developing gale force low to
our northeast will bring a large short-period north swell late
Wednesday into Thursday. Due to the combination of large short-
period and long-period swells and strong northerly winds, seas
will be rough and disorganized over the next several days.

Currently, we have a High Surf Advisory (HSA) for north and west
facing shores of Niihau, Kauai, Oahu, Molokai, and north facing
shores of Maui through Thursday morning. A Small Craft Advisory
(SCA) has also been issued due to a combination of increasing
winds and seas for all Hawaiian channels and windward waters
through Thursday morning. The SCA may need to be extended through
the day Thursday but favorable winds and seas are expected to
return by Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from midnight tonight to 6 AM HST Thursday
for Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward.

Flash Flood Watch through Wednesday afternoon for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Flash Flood Watch until 6 AM HST Wednesday for Maui.

High Wind Warning until 6 PM HST Wednesday for Big Island
Summits.

Winter Weather Advisory from midnight tonight to 6 PM HST
Wednesday for Big Island Summits.

Winter Weather Advisory until 6 AM HST Wednesday for Haleakala
Summit.

Small Craft Advisory from noon Wednesday to midnight HST
Wednesday night for Alenuihaha Channel-Big Island Windward
Waters.

Small Craft Advisory until midnight HST Wednesday night for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters.

Small Craft Advisory from midnight tonight to midnight HST
Wednesday night for Kauai Channel-Oahu Windward Waters-Kaiwi
Channel-Maui County Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Thomas
MARINE...Kino