High Surf in Hawaii, December 27 to 29, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
423 PM HST Thu Dec 27 2018

.SYNOPSIS...
Winds this evening will be light ahead of a cold front approaching
from the northwest. The front will approach Kauai late tonight
and stall out near Oahu and Maui county on Friday, bringing
showers and a few thundershowers. The weather should begin to
improve by Saturday as the front weakens to a trough and retreats
westward. Breezy trade wind weather will return for the last
weekend of 2018, and continue into the new year.

&&

.DISCUSSION...
Convection really underperformed this afternoon. The afternoon
soundings show the airmass is marginally moist and unstable, but
apparently weak mid-level ridging was enough to keep shower
activity to a minimum on all but the Big Island today. There was
just a single lightning strike on the N slope of Mauna Loa. Have
made some adjustments to PoPs for the evening in many areas to
lower them, out ahead of an approaching front that will be
affecting our weather later tonight.

Thundershowers associated with the cold front are about 260 miles
W of Kauai as of this writing and marching eastward at 20-25 mph.
The high resolution guidance continues to bring this active
weather to the Garden Isle after midnight tonight. The front is
expected to maintain enough upper support and instability to keep
locally heavy showers and even some isolated thundershowers going
as it arrives, reaching Kauai early Friday morning. The front
should gradually weaken to a trough later Friday into Friday night
near Maui County as the upper support begins to wane, bringing
diminishing wet weather to the smaller islands. Maintaining a
threat for heavy showers or thundershowers only through Friday at
this point. Out ahead of the front, some afternoon convection is
possible on the Big Island on Friday as well.

Lingering moisture in the trades as the surface trough departs to
the west on Saturday could yield some enhanced windward rainfall,
but on the whole, rain chances will continue to decrease. The
models show drier air arriving on the trades on Sun and Mon, with
rather nice weather expected for New Year's Eve and beyond, as a
nearly stationary ridge of high pressure sets up shop to the N of
the islands. This should be enough to maintain breezy to locally
windy trades through the early to middle portions of next week.
&&

.AVIATION...
While a cold front approaches from the northwest we will have
relatively light winds across the state. Expect daytime sea
breezes and nighttime land breezes for the islands. That trend
will get interrupted as the cold front gets closer, beginning with
Kauai and Oahu. Late this afternoon and early evening we may see
a few local, isolated thunderstorms on the interior potions of
the Big Island. Late tonight into early Friday morning Kauai then
Oahu will have a chance for some heavy rain and possible
thunderstorms as the front nears.

There are currently no AIRMETs in effect but expect AIRMET Sierra
for areas of mountain obscuration most likely for the areas that
see the heavy rain and/or thunderstorms on Kauai and Oahu early
Friday morning.

&&

.MARINE...
An approaching front is expected to begin impacting the northwest
waters around Kauai later tonight, then the rest of the state
Friday and Saturday, as the front stalls and weakens somewhere
near Maui County. Moderate to fresh northerly winds are expected
with the front along with isolated thunderstorms. Winds will
strengthen further as high pressure builds in this weekend.

A large, long period northwest swell is expected to build tonight,
peak on Friday, and then decline through the rest of the weekend.
Surf produced by this swell is expected to reach advisory level
heights, and a High Surf Advisory is posted for exposed north and
west facing shores. The long period energy has been observed at
the Hanalei buoy this afternoon. Another northwest swell is due
in around Tuesday next week producing surf near the advisory
levels along north and west facing shores. A Small Craft Advisory
(SCA) is also in effect through Saturday due to the incoming swell
and expected seas.

Surf is likely to stay below advisory levels along east facing
shores over the next few days. As trade winds strengthen over the
weekend, this will bump the surf along east facing shores to near
advisory levels late this weekend into early next week and bring
SCA conditions for the typical Maui County/Big Island waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for for N and W facing shores of
Niihau Kauai Oahu Molokai and N facing shores of Maui.

Small Craft Advisory for high seas until 6 PM HST Saturday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel- Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters- Alenuihaha Channel-Big Island Windward Waters.

&&

R Ballard/Chevalier/Foster