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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220709

Area Forecast Discussion
National Weather Service Honolulu HI
909 PM HST Sun Jan 21 2018

A long band of unstable cumulus clouds and showers near the Big
Island this evening will slowly drift north by tomorrow increasing
clouds and showers over Maui county. Elsewhere moderate trade
winds will focus clouds and showers over windward and mountain
areas of each island. A weak upper level disturbance will help to
enhance shower activity through Wednesday. Lighter winds with land
and sea breezes are forecast from Thursday onward as the ridge
north of the islands weakens.


Drier weather conditions were observed over the northern Hawaiian
Islands today mainly due to weaker than expected upper level
forcing. Higher showers will remain in the forecast for the Big
Island and Maui tomorrow as the stream of unstable clouds drifts up
over both islands. The evening forecast was adjusted slightly to
decrease chances for precipitation tomorrow over Kauai, Oahu and
Molokai with scattered showers mainly over windward and mountain
areas and isolated showers over leeward sections. Moisture trends
in the latest guidance continues to be on track with unsettled
weather spreading to the northern islands on Tuesday and

The satellite analysis this evening shows the East Pacific high
centered off the southern California coast and a long 2000 mile
band of unstable clouds and showers along 19N Latitude drifting
into the Big Island. A weak cold frontal system has stalled just
northwest of Kauai. The cloud band near the Big Island contains
scattered to numerous showers and will slowly drift northward over
the next 48 hours. Model guidance continues to show higher
Precipitable Water (PW) values in the 1 inch to 1.6 inch range
advecting northward likely increasing showers over Maui tomorrow
and the northern islands by early Tuesday morning. An upper level
disturbance will also pass over the islands helping to lift the
boundary layer and increase both windward and leeward shower

A stronger frontal system will move into the Central Pacific by
the middle of the week. This system will help break down the
ridge north of the islands allowing on-shore sea breezes and off-
shore land breezes to develop each day Thursday through Saturday.
Land surface heating during the daytime hours will produce clouds
and a few showers over interior sections with overnight clearing.


Moderate trade winds will continue through Monday as a ridge of
high pressure persists to the north of the islands. Clouds and
showers will favor windward areas through the period, with
showers also reaching leeward areas from time to time. The
majority of the shower activity will remain over the eastern end
of the state however, where prevailing MVFR conditions are
expected tonight and Monday.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of the Big Island and Maui. These conditions are
expected to persist through the night and possibly through the
day on Monday.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. The AIRMET is
expected to remain in place through Monday.


Guidance remains in good agreement and depicts fresh to strong trade
winds through the first half of the week as the ridge of high
pressure strengthens in the wake of a weak front passing to the
north. Advisory-level winds will likely setup over the windier
channel waters east of Molokai and south of the Big Island,
beginning as early as tonight. A combination of winds and seas will
support small craft advisory conditions over most waters through as
late as Wednesday.

ECMWF and GFS both support a downward trend for the local winds
through the second half of the week as a potent gale-force low
passes north of Midway Monday night into Tuesday, then tracks slowly
northeastward to an area well north of Hawaii Wednesday through
Friday. Light and variable (land/sea breeze conditions) winds can't
be ruled out for the northwest islands as early as Thursday, then
for the entire state by Friday, which could linger into the weekend.

The high surf advisory in place for east facing shores continues
through 6 AM HST Monday and will likely be extended due to a
persistent large area of strong trades south of high pressure
between Hawaii and the west coast. Will continue to monitor buoy
trends and observations overnight and make final decision in the
late night package for which islands will be covered in this.

For northern shores, surf will steadily rise overnight and through
the day Monday as a new, long-period, west-northwest (300-310 deg)
swell fills in. Forerunners associated with this source have already
hit the offshore buoy northwest of Kauai earlier today within the 18-
22 sec energy band, and are now showing up at the nearshore PacIOOS
Hanalei and Waimea buoys. This source will slowly fill in and likely
reach advisory levels for north and west facing shores of the
smaller islands by the end of the day Monday (potentially earlier
in the day for Kauai). Surf will remain below advisory levels
along north and west facing shores of the Big Island from this

For southern shores, surf will come up overnight into Monday as a
new, long-period, south (170-180 deg) swell fills in from recent
activity across the southern Pacific. The nearshore PacIOOS Pearl
Harbor buoy has already begun to register an uptick in energy
within the lower frequency bands (16 sec) this evening. Will
likely see this source peak through the night Monday night, then
trend down into midweek. Current forecast is likely slightly
underdone and will be adjusted upwards in the overnight package.


High Surf Advisory from midnight tonight to 6 PM HST Tuesday for
Niihau-Kauai Leeward-Waianae Coast-Oahu North Shore-Molokai

High Surf Advisory until 6 PM HST Tuesday for Kauai Windward-
Oahu Koolau-Molokai Windward-Maui Windward West-Windward

High Surf Advisory until 6 AM HST Monday for Olomana-South Big
Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman