High Surf in Hawaii, December 10 to 14, 2018

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
741 AM HST Mon Dec 10 2018

.UPDATE...
Zones were updated, with adjustments made to the cloud and
POP fields. In brief, we kept Oahu, Kauai, and Molokai kind of
showery through out today, as these islands remain in the cross
hair of an east to west oriented shower band. Lowered the clouds
and POPs for windward Maui and the Big Island, as there are less
clouds and showers there.

&&

.SYNOPSIS...
A series of high pressure systems moving through the Central
Pacific this week will bring breezy to strong trade winds across
the islands through Friday. Expect showers for the next five days,
mainly over windward and mountain areas with brief periods of
leeward rainfall. Wind speeds will decrease by next weekend with
drier weather conditions as the ridge north of the islands
weakens in response to a cold front approaching the Hawaiian
Islands from the northwest.

&&

.DISCUSSION...
Satellite imagery this morning shows bands of unstable cumulus
clouds upstream of the Hawaiian Islands drifting on the trade
winds into windward slopes of each island. The most unstable of
these bands continues to develop clouds and showers upstream of
Kauai and Oahu this morning. Water vapor satellite imagery shows
an upper trough just to the north of the islands with short term
weather models moving this trough axis over the state on Tuesday
and Wednesday.

In the big picture, weather models show a persistent zonal jet
stream pattern across the North Pacific that will help produce a
progressive movement of weather systems from west to east across
the basin. For the Hawaiian Islands this means a series of high
pressure systems will track along 30 degrees north latitude
reinforcing a strong surface pressure gradient across the state
through Friday. Expect breezy to strong trade winds blowing
through the week. A wind advisory currently in effect was extended
in time this morning into Tuesday evening for portions of Lanai,
Kahoolawe, Maui and the Big Island.

A combination of good low level forcing due to strong winds mixing
through the boundary layer with convergence along mountain ranges
and upper level divergence, will help elevate the trade wind
subsidence inversion into the 7000 to 9000 foot range through much
of the work week. This higher inversion layer will allow deeper
clouds to form producing a wet weather pattern across the Hawaiian
Islands through Friday. These showers will tend to favor windward
and mountain slopes of all islands with some of the stronger
showers passing over leeward sections especially in the overnight
hours. Smaller scale changes in coverage and intensity of the
shower bands will vary from day to day, however the overall wet
trend will remain in the forecast.

The large scale weather pattern changes a bit by this weekend. The
high pressure ridge to the north of Hawaii will weaken as a cold
front dives towards the islands from the northwest. Trade winds
will diminish as the ridge breaks down and pressure gradients
weaken. By Sunday the winds at the 850 MB level will turn
southeasterly creating a blocking effect on precipitation. This
blocking effect occurs when the smaller islands fall in the lee,
or downwind, of the Big Island. The Big Island creates a bow wave
effect (imagine a large rock in a stream with water flowing
around it) pushing clouds and showers away from the islands while
enhancing stable downward motions aloft. Expect the drier period
to last through Monday as lighter winds also allow a land and sea
breeze wind regime to develop across all islands. Clouds will
trend towards interior sections of each island due to sea breeze
convergence with clearing at night due to land breeze divergence.

Global weather models are in good agreement with this weather
pattern through Sunday. Forecast solutions diverge a bit from
Monday onward due to the strength of the upcoming trough and cold
front arriving in the islands over the first half of next week.
Stay tuned as more wet weather is likely on the way beyond the
seven day forecast period.

&&

.AVIATION...
With moderate to strong trade winds in place, clouds and moisture
will continue banking against the windward slopes of all islands.
Occasional MVFR conditions are to be expected through twenty four
hours and beyond, especially for windward Kauai. AIRMET Sierra
for mountain obscuration is currently in effect for windward
slopes of Kauai and Oahu, but may be cancelled later today on
Oahu.

Moderate to strong winds will stay in place for the next few days
until the high northwest of the state shifts to the east. Until
that happens expect low-level turbulence across the state south
through west of the mountains on each island. AIRMET Tango remains
in effect for low-level turbulence below 8,000ft and strong
surface winds greater than 30 kt.

An upper level jet building over the islands from the west will
likely necessitate the issuance of an AIRMET for moderate upper
level turbulence from Kauai to the Big Island within the next few
hours.

&&

.MARINE...
A series of highs building to the north of the state will keep
strong trade winds blowing across the coastal waters through the
week. A slight increase in trade winds is expected today through
Tuesday as the first of the highs strengthens to the north of the
islands. The trades are then expected to ease slightly during the
middle of the week as a cold front passes by north of the state.
The second in the series of highs will then build in for the end
of the work week, increasing the trade winds once again. This
high is then forecast to weaken north of the state next weekend,
allowing the trade winds to ease into the moderate to locally
breezy range.

A Gale Warning in now in effect for the Alenuihaha Channel
through Tuesday. A Small Craft Advisory (SCA) remains in effect
for the rest of the coastal waters through Tuesday night. The SCA
will likely need to be extended for much of the marine area
through Friday due to winds, seas, or a combination of the two.

Buoys and satellite Altimeter data indicate that the current NNW
swell continues to slowly decline this morning, and warning level
surf is no longer occurring. As a result, the High Surf Warning
(HSW) has been cancelled for all areas. Due to slow decline in the
swell however, advisory level surf will continue at least into
the morning hours for most north and west facing shores, with the
swell gradually declining from west to east across the island
chain through the day. As a result, a High Surf Advisory (HSA) is
now in effect for north and west facing shores of Niihau, Kauai,
Molokai and north facing shores of Oahu through noon today. The
NNW direction of the swell should keep surf below HSA levels for
west facing shores of Oahu. A HSA is now in effect through 6 PM
this evening for north facing shores of Maui and the Big Island
where the swell will hold on a bit longer.

Another long-period northwest swell is due in on Tuesday and
could peak near HSA levels on Wednesday. Several smaller NW and N
swells are expected towards the end of the week. A new and larger
north-northwest swell will likely bring advisory and possibly
warning level surf to north and west facing shores late in the
upcoming weekend.

Additionally, a HSA remains in effect for east facing shores of
Kauai, Oahu, Molokai, Maui and the Big Island. This HSA has been
extended through Wednesday night and may need to be extended even
further in time due to the strong and persistent trade winds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until noon HST today for Niihau-Kauai Leeward-
Oahu North Shore-Molokai Leeward.

High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Maui
Central Valley-Windward Haleakala-South Big Island-Big Island
North and East.

Wind Advisory until 6 PM HST Tuesday for Lanai-Kahoolawe-Maui
Central Valley-South Big Island-Big Island North and East-Kohala-
Big Island Interior.

Small Craft Advisory until 6 AM HST Wednesday 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.

Gale Warning until 6 PM HST Tuesday for Alenuihaha Channel.

&&

DISCUSSION...Lau/Bohlin
AVIATION...Bedal
MARINE...Jelsema