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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 092004

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Sun Dec 9 2018

Breezy trade winds will prevail through the week. Showers will
mainly be focused over windward and mountain areas, with some
reaching the lee areas of the smaller islands at times.


Water vapor imagery shows an upper level high northwest of the
state and a trough to the east, with the tail end of the trough
extending back toward the Big Island. At the surface large high
pressure is located northwest of the state. The trailing end of
the old front that moved through the state has weakened, leaving a
showery cloud band near the Big Island. 12z soundings show a
strong inversion around 6kft at Lihue and a weaker inversion
around 10kft at Hilo. Early morning MIMIC total precipitable water
imagery shows values around 1.0-1.1 inches across much of the
area, with pockets of drir air embedded within the moisture bands.

Overnight rainfall has been greatest across windward sections of
Maui and the Big Island with showers associated with a lingering
cloud band. A new moisture band embedded in the trades has also
reached Kauai, enhancing showers there. Not much change is
expected for today. Breezy trade winds will focus showers across
windward section. A strong pressure gradient is in place across
the island. As the high passes to our north, winds may even bump
up a notch on Monday. So far today we have received a few reports
of sustained winds 30 mph or greater (at the Lanai and Kohala
airports), indicating that the existing wind advisory for portions
of Maui and Hawaii Counties is on track. This advisory will
likely need to be extended through Monday as similar if not
stronger conditions are expected.

The remnant moisture band near the Big Island will thin out by
Monday, while the band near Kauai drifts toward Oahu. Beyond
Monday we get into a regular trade wind pattern with areas of
moisture embedded in the low-level flow. A front passing by well
north of the state on Wednesday will cause wind speeds to diminish
slightly, as the subtropical high is pushed farther east. However
a new high builds in right behind the front, tightening the
gradient across the islands and bringing us back to breezier
conditions for Thursday/Friday. The trade winds will focus most
rainfall across windward sections. However, the strength of the
winds will spread some showers to leeward areas as well.


There is a cloud band between Maui and the Big Island draped over
the Alenuihaha Channel. This will cause the windward side of Maui
and the Big Island to experience MVFR conditions through this
afternoon into tonight. As a result, AIRMET Sierra is in effect
for mountain obscuration for Maui and the Big Island and is
expected to stay in effect until the band dissipates and/or
pushes south probably early on Sunday. All other islands can
expect passing MVFR conditions in windward mauka areas.

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


High pressure north of the state will maintain strong trade winds
through the week. A slight increase in trade winds is expected on
Monday as the high strengthens and gradually shifts to the east.
With the overnight ASCAT pass already showing 30 knot winds
around the Big Island, a Gale Warning is certainly possible for
the typical windy areas around the Big Island Monday. Meanwhile, a
Small Craft Advisory (SCA) is in effect for all Hawaiian waters
due to strong tradewinds and combined seas over 10 feet. As the
high gradually shifts east over the next few days, a new high will
build over the state from the northwest Wednesday into Thursday.

Surf and combined seas will be on the increase today, as a new
large NNW swell peaks today along most N and W facing shores, and
strong trade winds drive surf up along E facing shores. A High
Surf Warning is posted for exposed north and west facing shores,
with a High Surf Advisory (HSA) posted for the west facing shores
of Oahu, where greater swell shadowing is expected. A HSA is also
posted for E facing shores of most islands due to strong trades
driving an increase in short-period wind waves, with surf
expected to remain elevated for most of the week. Lastly, a Marine
Weather Statement (MWS) highlights the potential for harbor
surges due to the NNW swell, especially in Hilo and Kahului

Today's large NNW swell will gradually diminish Monday, with
another long-period NW swell arriving Tuesday expected to produce
advisory-level surf along exposed N and W facing shores on
Wednesday. A moderate-sized NW swell is possible toward the end
of the week. With a prolonged period of elevated trade winds
expected, the HSA for E facing shores is likely to continue
through the week. See the Collaborative Surf Discussion (SRDHFO)
for details on the sources of the swells that are expected to
affect the islands.


High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

High Surf Advisory until 6 AM HST Monday for Waianae Coast-South
Big Island.

Wind Advisory until 6 PM HST this evening for Lanai-Kahoolawe-
Maui Central Valley-South Big Island-Big Island North and East-

Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian




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