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Hawaiian Islands Synoptic Discussion and Guidance

162
FXHW60 PHFO 040701
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
901 PM HST Thu Dec 3 2020

.SYNOPSIS...
Trade winds will strengthen through the rest of tonight, becoming
rather breezy Friday before dropping off again Saturday. Light and
variable winds this weekend over the smaller islands will give way
to increasing trade winds Monday. Expect a boost in trade showers
with these trades.

&&

.DISCUSSION...
An eastward moving 1022 mb surface high will be passing some 580 miles
north of the main Hawaiian Islands Friday, moving at a speed of 45 mph.
In doing so, it will bring on an 8 to 10 hour burst of breezy trade
winds to the islands. By early Saturday morning, the high will be some
1000 miles northeast Oahu, and continuing east at a slightly slower
speed. By Saturday afternoon, mainly moderate trade winds will cover
the Big Island, Maui, and Molokai, while the remaining islands will
be under a light southeast wind flow. This flow will be weak enough
to bring on sea breezes, leading to some late afternoon clouds
over interior and lee areas. These clouds will clear out Saturday
night for these islands. Elsewhere, light to moderate trades will
settle in around the Big Island and Maui. A few showers can be
expected for the windward areas of Molokai, Maui, and the Big Island.

Before hand, a band of showers will be moving through the islands from
east to west, with the trades. Latest satellite imagery shows the north
to south oriented, 60 mile wide band of clouds and embedded showers
just reaching Laupahoehoe Big Island. The band cuts through the Puna
district and into the waters off the Kau coast. This band is expected
to reach eastern Maui by sunrise Friday, then onto Molokai and Oahu
Friday afternoon. The band weakens upon leaving Oahu early Friday
evening, but it will still give a boost of trade showers for Kauai
late Friday night.

On Sunday, the smaller islands, such as Lanai, Oahu, and Kauai,
will still be in the wind shadow of the Big Island and Maui,
leading to light and variable winds for these islands. Windward
Maui, Molokai, and the Big Island, stays in a moderate trade wind
flow. Several hundred miles northwest of Kauai, a weakening front
will be approaching Kauai. Light trades are expected to fill in
across Kauai and Oahu Sunday night, then gradually strengthen to
mainly moderate speed on Monday. At this time, the remnants of
this front is expected stop short of reaching Kauai.

Trades will continue to build to locally breezy by Monday night.
This comes as a new surface high, 1025 mb, eases north of Kauai
Monday night and bridge with the high that had passed north of the
islands 3 days earlier. Locally breezy trades will persist through
most if not all of next week.

During the stretch of light winds over Kauai and Oahu, the air mass
is expected to be stable so that showers will be limited from the
daytime heating and sea breezes.

&&

.AVIATION...
A narrow band of low clouds continues to move into the Big Island
from the southeast direction. This cloud band will bring a few
showers and low cloud ceilings with temporary mountain
obscurations to the northeast and southeast slopes. These low
clouds may reach east Maui by Friday morning.

A migratory high pressure system moving eastward, far north of
the Hawaiian Islands, will produce increasing trade winds into the
moderate to breezy range across the region on Friday. Passing
showers are forecast mainly over mountains and along north and
east facing slopes of each island. Higher shower trends are
expected during the overnight to early morning hours.

AIRMET Sierra in effect for Tempo Mountain Obscuration over the
northeast and southeast slopes of the Big Island. This AIRMET will
likely continue through the early morning hours.

&&

.MARINE...
The large northwest swell that peaked Wednesday will continue
trending down overnight into Friday - likely enough to dip below
the High Surf Advisory threshold for exposed north and west facing
shores of the smaller islands. Despite the downward trend, the
Small Craft Advisory will hold over the windier channel waters and
Maalaea Bay as fresh to strong trades return briefly Friday and
Friday night. Trades will drop off once again over the weekend,
especially over the western end of the state where land and sea
breeze conditions could return Saturday afternoon through Sunday.
Easterly trades will return statewide next week and hold in the
moderate to fresh category each day.

The active and progressive pattern across the North Pacific
will continue over the weekend into next week, featuring a series
of fronts passing the state to the north. Guidance takes a
gale, currently east of Japan, quickly eastward toward the Date
Line Friday, then northeastward into the Gulf of Alaska over the
weekend. A compact, hurricane-force low is expected by the time
it reaches the Date Line Friday (42-MB drop in 24-hrs as it
rapidly develops).

ECMWF-Wave and WAVEWATCH III guidance reflect this and depict
seas climbing into the 35-45 ft range focused at the island waters
around or just over 1000 NM away Friday. Long-period forerunners
should arrive early Sunday, then peak Sunday night into Monday
driving surf heights along exposed north and west facing shores
well above warning levels (XL category) and seas above the 10 ft
advisory threshold. Surf associated with this episode should dip
below the advisory levels by the midweek period as the swell
eases. No significant swells are shown through the second half of
next week.

This long stretch of advisory- to warning-level surf discussed
above will only exacerbate the ongoing coastal erosion issues -
especially with water levels running higher than predicted each
day. The best chance for water to reach vulnerable coastal
properties and roadways will be around and after daybreak Monday
when the peak daily high tide will potentially coincide with the
peak of the swell.

Surf along east facing shores will remain small over the weekend,
then trend up slowly next week as the trades fill in locally and
upstream of the islands.

Surf along south facing shores will remain small with mainly a
mix of small, short-period southeast and background south-
southwest energy moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Friday 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 from 6 AM Friday to 6 AM HST Saturday for
Kauai Northwest Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Leeward Waters-Kaiwi Channel-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.


&&

$$


H Lau/Bohlin/Gibbs

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