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Hawaiian Islands Synoptic Discussion and Guidance

929
FXHW60 PHFO 200630
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Thu Jan 19 2017

.SYNOPSIS...
Winds will become locally light and variable overnight into Friday
as a front approaches from the northwest. This front will advance
across the smaller islands Friday night and Saturday, then stall
near the Big Island from Saturday night into early next week.
Expect increasing showers across the state with the front. Behind
the front, very strong trade winds are expected for the state this
weekend, with winds easing early next week.

&&

.DISCUSSION...
Dry conditions prevail across the state on this Thursday evening.
Radar shows just a few small showers north of Molokai and
northeast of Oahu, while rain gauges report only isolated light
rainfall amounts during the past 12 hours, with most locations
remaining dry. Surface analysis depicts a weakening high pressure
area northeast of the state, with a cold front located several
hundred miles northwest of Kauai. Aloft, water vapor imagery shows
a strong ridge centered to our east, and fast zonal flow over the
North Pacific with an embedded shortwave trough moving east of the
international dateline. The 00Z soundings show a dry and stable
airmass in place over the islands, with PW values below one inch
and very strong inversions based near 6K feet. MIMIC-TPW satellite
imagery shows a dry airmass surrounding the state, and only a
slight increase in moisture along the frontal boundary to our
northwest.

Overnight, expect mainly dry conditions to continue, as the very
stable airmass with strong inversions in place prevents most
shower development. Have nudged rain chances down overnight and
included mention of only light showers for those places that do
still have a chance of rain. Updated gridded/text forecasts have
been sent.

On Friday, the shortwave trough currently moving east from the
dateline will amplify as it translates further east, kicking the
surface cold front into motion toward the state. A period of light
winds with local seabreezes is likely through midday, but high-
resolution models show northeast winds already beginning to
increase across western parts of the state during the afternoon as
strong high pressure behind the front builds eastward. A dry and
stable airmass will remain in place over the islands in advance of
the front, so expect only isolated showers at most, with rainfall
amounts remaining light to nil.

Friday night and Saturday, the front is forecast to advance
southeastward across the smaller islands, nearing the Big Island
by late Saturday. An increase in shower activity, especially for
windward areas but with some spillover leeward is expected along
and behind the front. However the main weather impact will be the
very strong northeast winds developing late Friday night and
Saturday, as strong high pressure moves north of the state.
These winds will be strongest at higher terrain, through mountain
gaps, and downslope of mountains. Winds will likely be strong
enough to reach advisory levels across many areas. A Special
Weather Statement has been issued to highlight the expected windy
conditions this weekend.

The front is forecast to stall near the Big Island Saturday night
and Sunday, with windy conditions continuing statewide, along
with frequent showers windward and scattered showers leeward.

Heading into the early part of next week, longer range models
show winds veering to the east and southeast and easing as the
old front dissipates and another front moves quickly eastward
across the north Pacific. A somewhat wetter pattern than we have
observed recently is likely to continue through the early week
period as models keep some added moisture lingering around the
islands, with a weak trough aloft providing a weakened inversion
as well. The next front seems likely to pass mainly north of the
state around Tuesday, then a developing deep mid level trough to
our east pushes a drier airmass southward across the islands
Wednesday and Thursday. Moderate to fresh and rather dry trade
winds should prevail during that time.

&&

.AVIATION...
Areas of low clouds over windward coastal waters this evening
will reach the islands under decreasing trade winds, bringing some
showers. Still expect mainly VFR conditions for the area, though
brief MVFR ceiling may affect the windward sites through early
Friday morning.

Trade winds will continue to weaken overnight as high pressure ridge
stays just north of the state, with localized land breezes
developing through early Friday morning.

No airmet is expected through early Friday morning.

&&

.MARINE...
As the current large west-northwest swell gradually lowers, a
reinforcing northwest swell is expected to arrive later tonight
and Friday with advisory level surf expected. Another large
northwest swell is expected to arrive Monday night and Tuesday.
Current guidance indicates that this swell will reach High Surf
Warning levels.

Strengthening winds behind a cold front passing across the area
this weekend will cause for a significant increase in short period
choppy surf along north- and east-facing shorelines, likely
bringing advisory level surf. There will also be a series of
small, mainly background south swells throughout the forecast
period.

A Small Craft Advisory remains posted for waters exposed to large
northwest swells. Strengthening winds behind the above mentioned
front will cause Small Craft Advisory conditions across the entire
area and possibly gale force conditions over the typically windy
waters around Maui County and The Big Island. Thus a Gale Watch
has been issued for those waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for north and west
facing shores of Niihau Kauai Oahu and Molokai, and for north
facing shores of Maui.

High Surf Warning until 6 AM HST Friday for west facing shores of
the Big Island.

Gale Watch from Saturday afternoon through Sunday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-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.

&&

$$

DISCUSSION...Jacobson
AVIATION...Hui
MARINE...Kinel

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



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