High Surf in Hawaii, December 5 to 8, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Wed Dec 5 2018

.SYNOPSIS...
Moderate to breezy trades will slightly weaken tonight through
Thursday night, then strengthen once again late Friday through
the weekend as high pressure builds north of the state in the wake
of a passing front. A combination of an upper trough moving over
the region and residual moisture associated with the tail-end of
the front passing to the north will support increasing shower
coverage over portions of the state through the weekend. Strong
trades will likely hold through early next week.

&&

.DISCUSSION...
Latest surface analysis and satellite imagery showed high pressure
centered northeast of the islands and a lingering band of
clouds/moisture lurking south of the Big Island. Recent radar
imagery showed scattered showers associated with this cloud band
tracking from east to west between 50 and 100 miles south of the
Big Island. Elsewhere across the state, quiet conditions prevailed
with minimal rainfall accumulations measured in the past several
hours.

Short-term (through Friday) guidance remains in good agreement and
shows a weakness within the ridge developing as a front passes north
of the state later today through Thursday. Trades will respond
and slightly weaken as this boundary passes. Mostly dry conditions
will prevail with precipitable water (PW) values holding around
an inch. Any showers that do develop will favor the typical
windward and mountain locations.

For the extended period (weekend through early next week), guidance
remains in decent agreement and supports a slight increase in
rainfall chances over the weekend. This will be in response to a
combination of an upper trough and residual moisture associated with
the tail-end of the aforementioned frontal boundary dropping south
over the islands. PWs are forecast to increase from around an inch
to 1.1-1.3 inches as this boundary drops south over the northwest
islands Saturday, then over Maui County and the Big Island
Saturday night through Sunday. In addition to the rain chances
this weekend, trades are expected to increase and remain gusty
through this period as strong high pressure builds to the north
behind the front (may near or reach wind advisory thresholds for
some areas).

&&

.AVIATION...
A 1024 MB high pressure system north of the Hawaiian Islands will
continue to drift eastward today moving slowly away from the
state. Moderate trade winds will weaken slightly as the high
pressure ridge weakens and the next frontal trough approaches the
islands from the northwest. Expect a dry trade wind weather
pattern to continue through Thursday with clouds and isolated to
scattered showers favoring windward and mountain slopes. VFR
conditions will prevail with only brief periods of MVFR ceilings
possible over windward and mountain slopes.

AIRMET Tango remains in effect over mountains and south through
west sections of all islands below 9000 ft MSL due to tempo
moderate turbulence. No additional AIRMETs expected over the next
24 hours.

&&

.MARINE...
A 1025 mb surface high located 600 nm NNE of Oahu this morning,
will be moving ENE at 10 to 15 kt in the next 18 hours. A ridge
will trail behind the high and dissipated as a front, currently
located 630 nm NW of Kauai, pushes slowly eastward. This front is
slated to move into and dissipate over the northern offshore
waters on late tonight through Thursday. In response, the trades
will ease off slightly today and tonight.

A strong 1030 to 1033 mb high follows in behind this dissipating
front. Strong and gusty NE trade winds are expected to spread
across the local waters between Thursday and Friday. The trades
will ramp up to near gale force over the Alenuihaha and Pailolo
channels Friday and persist through the weekend. It is likely this
windy condition will last through the first half of next week.

Buoy 51101 had a peaked swell height of 15 feet and a period of
15 to 17 seconds at 11 pm HST. It is a couple of feet lower as of
2 am HST. The near-shore Waimea Bay buoy had a reading of 12 feet
and 18 seconds as of 2 am HST. So there is still some room to
grow as the morning progresses, but it should peak later this
morning on Kauai and Oahu's north and west facing shores. The High
Surf Warning has been updated and now goes out to 6 pm HST
Thursday. By Thursday night, the surf should be in the advisory
range, and this condition is likely to persist into Saturday.

A moderate to large north- northwest to north pulse will arrive
Saturday evening, and max out Sunday. This swell may reach low end
warning level surf for the north facing shores, and advisory
level surf on the west side on Sunday.

For the east facing shoreline, surf will be small to moderate in size
today through Thursday, then build up to advisory level surf over
the weekend. This is in response to the expected strengthening of
the trade winds to strong and blustery. Surf along south facing
shores will remain small through the forecast period.

A Small Craft Advisory (SCA) is currently in effect for all
Hawaiian Waters. Part of the SCA cover some of the near-shore
waters due to the breezy trades of 25 kt or higher. The remainder
of the near- shore waters, especially around Kauai and Niihau,
pertain to the rising NW swell, with seas reaching or going above
the 10 feet threshold. SCA conditions, whether it is wind or seas,
or both, will hold through the weekend, and likely through the
first half of next week. This is due to the strong and gusty NE
trades building in over the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until 6 PM HST Thursday for Kona-Big Island
North and East.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters-

&&

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Lau