High Surf Advisory 48, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
340 PM HST Sat Nov 18 2017

.SYNOPSIS...
A cool northerly flow will increase tonight as a weak front
currently over Kauai passes over Oahu this evening, Maui County
overnight, then across the Big Island early Sunday. Showers along
the weak front will be focused mainly across northern facing
slopes. The cool northerly flow will persist through Tuesday as a
stalled cutoff low sits far northeast of the state. Trade winds
will gradually return on Wednesday and will likely become breezy
and gusty on Thanksgiving as the strong high pressure settles
north of the state.

&&

.DISCUSSION...
A weak front is currently moving across Kauai, though a rather
cool northerly flow has already developed across the state. The
front dropped generally between one quarter to one inch of
rainfall along north facing slopes of Kauai today, and while a few
showers popped up along the terrain of Oahu, sea breeze driven
interior clouds produced little to no rainfall on Maui County and
the Big Island. Temperatures were running a few degrees below
normal today in the northerly flow.

A cooler northerly flow will settle over the islands tonight and
Sunday as the front moves through. The front should be clear of
Kauai by sundown, and as the weak feature passes over Oahu then
Maui County overnight, showers will be focused across north
facing terrain, though a few showers will likely spread leeward.
The cloud band will be dissipating as it reaches the Big Island
early Sunday morning. A cool airmass will move in behind the
front, with dew points dropping into the upper 50s. As a result,
low and high temps will be running around 5 degrees below normal,
causing daytime highs to struggle to top 80. On Sunday, the GFS
and ECMWF show precipitable water plummeting to around three
quarters of an inch, which is about 60 percent of normal. Thus,
expect minimal showers along north facing slopes.

In addition, strong winds will continue across the high summits of
the Big Island through the weekend. Westerly winds peak tonight
then trend down late Sunday, and a High Wind Warning is in effect
for the summits through Sunday afternoon. We will also monitor the
summit of Haleakala for possible advisory level winds later
tonight.

Minor changes are expect heading into the work week. A cool
northerly flow will persist as the front's parent low, currently
sitting about 700 miles to the north-northeast of the state is
replaced by another surface low dropping into the established
longwave trough over the eastern North Pacific. We will likely see
a couple of weak and shallow moisture bands dropping over the
state and delivering light rainfall to north facing slopes, while
leeward areas remain rather dry. Temps will remain around 3 to 5
degrees below normal near sea level, meaning overnight lows in
the low to mid 60s and daytime highs around 80.

Winds will gradually shift toward a trade wind direction on
Wednesday then build on Thanksgiving. The above mentioned surface
low to the northeast of the state will lift away from the region
on Wednesday, allowing an area of high pressure move north of the
state. This will result in building northeasterly trade winds. The
high will settle closer to the islands on Thanksgiving, likely
leading to breezy and gusty northeasterly trades into next
weekend.

&&

.AVIATION...
A weak front will continue to move south over the island chain,
reaching Maui County late tonight. Most of the showers associated
with the front are along the southeast shore of Kauai and through
the Kauai Channel as of 0130Z. Improving conditions are expected
over Kauai within the next few hours, and AIRMET SIERRA for
mountain obscuration will likely be able to dropped over Kauai by
the routine issuance at 0400Z.

For Oahu, expect clouds and showers to start focusing along the
mountains with northern exposure in the next few hours,
introducing periods of MVFR CIGS/VIS. AIRMET SIERRA for mountain
obscuration is likely to become necessary over Oahu at that time,
and for Maui County later tonight as the front continues to moves
south.

A cooler, drier airmass and a moderate north flow will continue
to fill in behind the front. Most of the smaller islands should
see predominately VFR conditions tomorrow. Northern portions on
the Big Island could see periods of MVFR CIGS/VIS tomorrow from
any clouds and showers associated with the weak front.

&&

.MARINE...
A low pressure system and its associated cold front will be the
primary weather feature impacting the islands over the next couple
of days. Northerly winds with the system will remain below Small
Craft Advisory (SCA) levels, but will result in different areas of
terrain acceleration, such as through the Kaulakahi Channel today
and tonight and the Hamakua Coast and near South Point on Sunday.
Moderate trades are expected to return Tuesday and Wednesday, and
may approach SCA levels at that time in the windier locations.

The fetch from the low will bring a rising north swell to the
north shores beginning tonight, peaking Sunday into Monday, and
then begin to trend down Tuesday and Wednesday. Advisory level
surf is expected for most areas exposed to the north swell, with
warning level surf likely for Kauai. A High Surf Warning has been
posted for Kauai and Niihau, and a High Surf Advisory for Oahu,
Molokai, Maui and the Big Island, beginning Sunday morning. This
swell will bring seas in excess of 10 ft to exposed waters, so an
SCA has also been posted for waters.

Another low in the Gulf of Alaska will provide the source for the
next northerly swell which is expected to reach the islands as
early as Wednesday night with similar heights as the swell
expected tomorrow. This swell could bring near warning level surf
for Thanksgiving before easing into the weekend.

South facing shores will continue at small levels through the weekend
in response to recent activity across the southern Pacific.

See the latest collaborative nearshore and swell forecast for Oahu
for more details on the surf at: www.prh.noaa.gov/hnl/pages/SRF.php

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM Sunday to 6 PM HST Monday for north
facing shores of Oahu, Molokai, Maui and the Big Island.

High Surf Warning from 6 AM Sunday to 6 PM HST Monday for north
facing shores of Niihau and Kauai.

High Wind Warning until 6 PM HST Sunday for Big Island Summits.

Small Craft Advisory from 6 AM Sunday to 6 PM HST Monday for
all waters except Maalaea Bay and areas south and west of the Big
Island.

&&

Public...Wroe
Aviation...Eaton
Marine...M Ballard