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fxhw60 phfo 221406 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
406 am HST Mon Jan 22 2018

Synopsis...
a long band of unstable cumulus clouds and showers continues to
drift across The Big Island this morning. Expect increasing shower
coverage across the windward slopes of The Big Island this morning
with clouds and showers expanding to the island of Maui later
this afternoon and evening. 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 through Saturday as the ridge
north of the islands weakens.

&&

Discussion...
the satellite imagery this morning shows an enhanced area with
deeper clouds approaching the windward slopes of The Big Island
expect increasing showers from South Point to Hilo to Upolu
Point with isolated thunderstorms in the forecast. Clouds and
showers will reach the summits of maunakea and Maunaloa
later this morning with snow showers in the forecast for the
peaks. A Winter Weather Advisory was issued this morning for
Summit elevations above 12000 feet with 3 to 5 inches of total
snowfall expected through 6 PM HST.

Model guidance continues to show the band of clouds and showers
near The Big Island expanding northward later today into Maui
with higher precipitable water (pw) values in the 1 inch to 1.6
inch range advecting northward into the northern islands of Oahu
and Kauai by 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 coverage.

Tuesday and Wednesday, moderate trade winds will continue with
clouds and showers focused mainly along windward slopes of each
island. The combination of higher precipitable water amounts and low level
instability moving northward from The Big Island and an upper
level disturbance will help allow some of these windward showers
to drift over leeward sections of each Island.

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 land
breeze clearing.

&&

Aviation...
moderate trade winds will continue through tonight 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 Big Island this morning,
with showers then spreading northwestward into windward sections
of the smaller islands this afternoon and tonight. Prevailing MVFR
conditions are expected across windward sections of The Big
Island through the period, with MVFR conditions spreading
northwestward into windward sections of the smaller islands this
afternoon and tonight.

Airmet Sierra is in effect for mountain obscuration across
windward sections of The Big Island. These conditions are
expected to persist through the day and possibly through the
overnight hours as well. The mountain obscuration airmet may be
needed this afternoon for leeward sections of The Big Island as
well. Additionally, the airmet may need to be expanded to include
windward sections of the smaller islands this afternoon and
tonight.

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 through tonight.



&&

Marine...
guidance remains in good agreement and depicts fresh to strong trade
winds holding 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. The overnight ascat pass reflected this and showed up
to 25 kt in the Alenuihaha Channel and south of The Big Island. A
combination of winds and seas will support Small Craft Advisory
conditions over most waters through as late as Wednesday.

European model (ecmwf) and GFS both support a downward trend for the local winds
through the second half of the week as a 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 has been
extended through Tuesday due to a persistent large area of strong
trades south of high pressure between Hawaii and the West Coast.
This background easterly swell combined with strengthening trades
locally will support rough surf continuing through this time.
Guidance indicates this easterly source holding through the week,
despite the trades trending down locally Thursday into the
weekend.

For northern shores, surf will steadily rise through the day
today as a new, long-period, west-northwest (300-310 deg) swell
continues to fill in from a hurricane-force low late last week
across the far northwest Pacific. This swell has been slowly
building (slower than predicted) at the offshore buoy northwest
of Kauai through the night, which will be the trend at the exposed
beaches through the day as this source builds down the island
chain. Surf should reach advisory levels for exposed north and
west facing shores of the smaller islands by this afternoon (could
be late afternoon for Oahu and Maui county), then hold into
Tuesday before trending down into midweek.

For Southern Shores, surf will come up today 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 been slowly notching up overnight within the lower
frequency bands (14-16 sec). Will likely see this source peak
through the night tonight, then trend down into midweek.

&&

Hfo watches/warnings/advisories...
high surf advisory until 6 PM HST Tuesday for Niihau-Kauai
leeward-Waianae coast-Oahu North Shore-Molokai leeward-Maui
Central Valley.

High surf advisory until 6 PM HST Tuesday for Kauai windward-
Oahu koolau-Olomana-Molokai windward-Maui windward west-windward
Haleakala-south Big Island-Big Island north and east.

Winter Weather Advisory until 6 PM HST this evening for Big
Island summits.

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.



&&

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