High Surf in Hawaii, March 17 to 23, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Wed Mar 17 2021

.SYNOPSIS...
Unstable clouds and showers are moving into windward and mountain
areas on the breezy trade winds of all islands this morning. A
cloud band associated with a dissipated cold front will drift
into the islands from the north on Thursday, enhancing shower
activity for all islands into Friday. Moderate to breezy trade
winds will strengthen to windier conditions as strong high
pressure system passes far north of the state from Thursday on
into the weekend. Passing showers will favor windward and mountain
areas through Sunday. Unsettled weather may continue into early
next week.

&&

.DISCUSSION...
The satellite imagery this morning shows a high pressure system
far northwest of the Hawaiian Islands with a deep trough far
northeast of the state. In the smaller scale, unstable clouds
northeast of the state are advecting clouds and showers into
windward and mountain areas of each island this morning. Another
cloud band associated with a dissipated cold front, roughly 150
miles north of Kauai, will drift into all islands from the north
on Thursday and Friday. A subtropical jet stream passing over the
Big Island will produce some high level cirrus clouds over the
island into the evening hours.

Periods of passing showers are forecast for all islands into
Saturday, due to breezy to windy trade winds, a weak upper level
trough, and upstream cloud bands enhancing showers over the state.
The weak upper level trough will help to keep our low level cloud
heights in the 8,000 to 12,000 foot range into Saturday. At the
surface, a convergent cloud band associated with a dissipated cold
front will move into the islands from the north on Thursday and
Friday, further enhancing shower activity. A strong 1030 MB high
pressure system passing from west to east far north of the
islands this week will strengthen trade winds into the breezy to
windy range from Thursday onward. Wind advisory thresholds may be
reached in some of the windier locations from Friday into the
weekend. Forecast model guidance remains in good agreement on this
wet and windy trade wind weather pattern lasting at least through
Saturday.

The long range forecast from Sunday onward remains a bit less
certain as both the American (GFS) model and the European (ECMWF)
model show a closed upper low forming over the islands on
Saturday and Sunday. Both models continue to show this low
rapidly moving northeastward out of the region on Monday and
Tuesday, as a strong trough far north of the state moves into the
Central Pacific basin. A brief drying trend is possible as this
low is ejected northeastward as longer range guidance indicates,
however our wet weather pattern may be extended into early next
week if this upper low is slow to depart the region. Stay tuned
for updates on this evolving long range weather pattern.

&&

.AVIATION...
The air mass over the state remains very moist and moderately
unstable. MVFR ceilings and scattered to numerous showers will
persist through midday across the smaller islands and windward
portions of the Big Island. Brief IFR conditions are possible in
locally heavy showers.

Mid-level subsidence over the western half of the state is
encouraging the development of a weak inversion aloft. Shower
coverage and intensity will both taper off over Kauai, Oahu, and
Molokai later today.

Regarding Interior and Leeward Big Island, weak land breezes and
mostly clear skies in the late night and morning hours will give
way to light sea breezes and inland cloudiness in the afternoon
and early evening hours.

AIRMET Sierra for mountain obscuration is now in effect for
Kauai, Oahu, Molokai, Maui, and Windward Big Island. Conditions
are expected to improve over Kauai and Oahu later today.

AIRMET Tango remains in effect for both low level and upper level
turbulence. Low level mechanical turbulence to the lee of higher
terrain is currently impacting Kauai, Oahu, and Molokai, and will
spread to Maui by late afternoon. Meanwhile, moderate upper level
turbulence between FL250-FL400 is currently impacting all areas
west of Kauai, but is expected to end over Oahu and Maui County by
late afternoon. Upper level turbulence will likely persist over
the Big Island through the evening hours.

&&

.MARINE...
A strong high northwest of the area is producing fresh to strong trade
winds. A Small Craft Advisory (SCA) is in effect for all coastal waters
due to winds of 25 knots or greater, seas 10 feet or greater, or both.
Winds will strengthen further over the next few days as the high moves
east. Gales are possible in the waters around the Big Island and Maui
by the weekend.

A High Surf Advisory remains in effect for the east-facing shores
through this evening due to exposure to a lingering northeast swell
and strengthening trade winds. The advisory will likely be extended
through the week. A small to moderate northwest swell will build
today and hold through the end of the week. Surf will remain small
along south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for exposed east
facing shores.

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
coastal waters.

&&

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Donaldson