High Surf in Hawaii, December 5 to 8, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
544 AM HST Tue Dec 5 2017

.UPDATE...
The morning package has been updated to include a high wind
warning for the Haleakala Summit and a wind advisory for the Big
Island Summits based on the latest observations and trends.

&&

.SYNOPSIS...
Gusty north-northeast winds and very dry air will overspread the
state today as a cold front continues eastward across Maui County
and the Big Island this morning. A few showers along and ahead of
this boundary will be possible early, mainly over the northern
slopes of the Big Island and Maui. Dry and stable conditions are
expected Wednesday through Thursday as high pressure builds over the
area. Winds will quickly trend down and become light beginning
Wednesday. Weak frontal boundaries will approach and move through
the islands on Friday and Saturday resulting in an modest increase
in showers. Dry and stable conditions will return by Sunday. Another
front will be possible early next week.

&&

.PREV DISCUSSION.../ISSUED /

A pattern shift over the northern Pacific will result in more of a
progressive scenario across the islands through the upcoming
weekend. Recently animated water vapor loop showed northwest flow
aloft over the state between an upper ridge to the west and an upper
trough to the northeast, which should hold through the week. This
upper pattern will feature a series of shortwave troughs diving
southeastward across the region, pushing cold fronts toward and
through the area.

For today, gusty north-northeast winds and very dry air will
overspread the state as the first cold front in a series continues
eastward across Maui County and the Big Island this morning. Model
guidance indicates 850 mb winds climbing into the 40 to 50 mph
range. Surface winds will reach the 25 to 30 mph range for exposed
areas, with potentially stronger winds developing due to terrain
accelerations that occur with northerly flow. A Wind Advisory is in
effect for the state (except for Haleakala Summit and Big Island
Summits) through 6 pm today. This may need to be extended into
this evening for portions of Maui County and the Big Island.

Dry and stable conditions are expected Wednesday through Thursday as
high pressure noses over the area behind the front. Precipitable
water (PW) values are forecast to plummet to around half of an inch
(dewpoints dropping into the mid 50s). Winds will rapidly drop off
beginning tonight, likely giving way to land and sea breeze
conditions from Wednesday into Friday. The dry air, however, will
likely preclude any afternoon shower potential each day through this
time.

A weak cold front will drift southward onto Kauai and Oahu Friday
before dissipating, with a stronger cold front quickly following
behind it on Saturday. The fronts may bring an increase in clouds
and showers going into the upcoming weekend. Guidance indicates the
second front will usher in another shot of very dry air and gusty
northerly flow following its passage Saturday and Saturday night.
Winds are expected to rapidly drop off Sunday morning with another
ridge filling in behind. Another front may approach the state from
the north around Monday night or Tuesday next week.

AVIATION...
A shallow cold front currently crossing the state has brought a
modest uptick in showers. However, limited moisture depth and
instability has impeded shower intensity. The greatest chance for
MVFR conditions in showers will be over mauka areas, and across
north and windward facing sections. Otherwise, VFR will hold sway.

The more noticeable change following the front has been an increase
in wind speeds as a new high moves in northwest of the state.
Forecast soundings have shown winds as high as 35 knots below a
stable/very low inversion by later this morning. With this setup,
an AIRMET for moderate low-level turbulence to the lee of the
islands has been issued. With the next issuance at 0600 HST, the
AIRMET will include a remark to indicate the possibility of
isolated areas of severe turbulence as well. This will likely be a
short-lived event, with low-level winds starting to ease by
tonight.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 6 PM HST this evening for Haleakala
Summit.

Wind Advisory through to 6 PM HST this evening for the rest of the
state.

High Surf Advisory until 6 PM HST this evening for north, west
and east facing shores of Niihau, Kauai, Oahu, Molokai, north and
east facing shores of Maui, and east facing shores of the Big
Island.

Gale Warning through 6 PM HST this evening for Maui County
Leeward Waters, Maalaea Bay and Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Wednesday for all remaining
coastal waters.

&&

AVIATION...Kinel
MARINE...Stall
REST OF DISCUSSION...Gibbs