High Surf Advisory 9, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
318 AM HST Sun Feb 4 2018

.SYNOPSIS...
Showers and thunderstorms associated with an approaching cold front
and upper trough will remain in the forecast through the first half
of the week. The front is forecast to slowly advance down the
island chain beginning late tonight into Monday over the northwest
islands, then the southeastern end of the state late Monday
through Wednesday before clearing to the east Thursday. Drier air
and north to northeast winds will fill in across the state behind
this front Wednesday through Friday as a ridge of high pressure
builds north of the state.

&&

.DISCUSSION...
Water vapor loop showed a broad upper trough that has been
positioned over the central Pacific for the past several days
beginning to shift eastward over the state, exhibiting more of a
negative tilt as impulses move through it. At the surface, low
pressure has developed around 1000 miles northwest of Kauai in
response to a shortwave trough that moved east of Midway over the
past 24 hrs. A cold front extending south then southwest from
this surface reflection to an area a couple of hundred miles
northwest of Kauai was analyzed. Infrared satellite imagery showed
a line of thunderstorms developing along and ahead of this front
moving over buoy 51101 early this morning, moving eastward around
35 mph. Farther east, pre-frontal convergent bands within a plume
to deep tropical moisture being drawn northward across the state
will continue to support bands of heavy showers developing and
lifting northward over the smaller northwest islands today as the
front approaches.

Guidance remains in good agreement and supports the cold front
approaching and advancing down the island chain beginning late
tonight into Monday over the northwest islands, then the
southeastern end of the state Monday through Wednesday. Falling
upper heights associated with the upper trough shifting eastward,
a sufficient amount of deep layer shear (0-6 km; 30-40 kt),
increasing instability and forcing for ascent combined with deep
tropical moisture will support heavy showers and thunderstorms.

Impacts associated with this activity include the potential for
flash flooding and damaging winds associated with strong
thunderstorms. Provided the shear profiles, a few rotating cells
associated with the heavy showers and storms can't be ruled out,
especially as the main line of storms near the front moves
through. The flash flood watch has been expanded across Maui
County and continues through Monday. Over the summits of the Big
Island, deep moisture and cold temperatures aloft will support a
winter storm warning Monday night through Wednesday.

Drier air and northerly winds will slowly fill in across the state
behind this front Tuesday through midweek as a ridge of high
pressure builds north of the state. A trend back toward normal with
a return of moderate to breezy trades will be possible Thursday
through Friday.

&&

.AVIATION...
An unseasonably moist air mass prevails statewide due to southerly
low-level flow, leading to relatively low cloud bases. As a front
approaches from the northwest, increasing instability over the
western end of the island chain will fuel the threat of heavy
showers and isolated thunderstorms in pre-frontal bands over
Kauai and adjacent waters this morning, spreading slowly eastward
this afternoon and tonight. Some of the storms could produce
strong and gusty winds, with areas of MVFR conditions expected and
local IFR conditions possible.

AIRMET Sierra for mountain obscuration is currently posted for
Kauai and Oahu. Maui County and the Big Island will have
prevailing VFR conditions through today due to a nearby surface
ridge. Showers and thunderstorms are forecast to spread eastward
across Maui County tonight.

&&

.MARINE...
The current northwest swell has trended down overnight and early
this morning, causing surf heights to decrease below warning
levels along north and west facing shores of the smaller islands.
Surf heights remain at warning levels along west facing shores of
the Big Island. Surf heights will continue to decrease through
Thursday as the swell declines.

A Small Craft Advisory remains in effect for most coastal waters
primarily due to the large swell. A moderate southeast to south
wind will also contribute to the combined seas but remain below
small craft levels. Winds will diminish late today into Monday.
The next potential for small craft conditions will be during the
second half of the new week.

A front approaching from the northwest will bring a chance for
thunderstorms to the coastal waters, beginning near Kauai today
and spreading to the Big Island by Monday night. The front is
expected to stall and weaken over the central waters during the
coming week, contributing a slight chance for thunderstorms
through Wednesday. Thunderstorms that develop may contain gusty
winds and heavy rain.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Monday afternoon for Niihau-Kauai-Oahu.

Flash Flood Watch from this evening through Monday afternoon for
Molokai-Lanai-Kahoolawe-Maui.

High Surf Warning until 6 PM HST this evening for Kona-Kohala.

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters except Maalaea Bay-

&&

DISCUSSION...Gibbs
AVIATION...Jacobson
MARINE...Powell

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