High Surf in Hawaii, February 23 to 28, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
317 AM HST Tue Feb 23 2021

.SYNOPSIS...
High pressure north and northeast of the islands will keep breezy
to windy trades in place through the weekend. An old frontal band
will bring an increase in showers to the state today, targeting
mainly windward areas. Drier trade wind weather is expected
tonight through Wednesday night. Upper level troughing along with
an influx of moisture from the east and southeast, will likely
increase trade wind shower coverage and intensity Thursday
through the weekend.

&&

.DISCUSSION...
Currently at the surface, a broad 1038 mb high is centered around
1500 miles northeast of Honolulu, while a cold front is located
around 650 miles west-northwest of Kauai. The resulting gradient
is producing moderate to breezy trade winds across the island
chain early this morning, with winds lighter in the more
sheltered leeward areas. Infrared satellite imagery shows clear to
partly cloudy skies in most leeward areas, with mostly cloudy
conditions in most windward locales. Radar imagery shows
scattered showers moving into windward areas, with a few showers
spilling over into leeward communities. Main short term concern
revolves around trade wind trends during the next few days.

High pressure will hold in place northeast of the state today and
tonight, as the cold front shifts eastward and weakens into a
trough around 400 miles west-northwest of the islands by daybreak
Wednesday. The trough will begin to retrograde westward early
Wednesday as the high northeast of the islands begins to
strengthen and shift closer to the state. The high will then merge
with another high far north of the islands Wednesday night, and
strengthen to a very impressive 1047-1048 mb Thursday into
Friday. The high will then weaken as it shifts southward Friday
night and Saturday, before merging with a new strong 1042-1044 mb
high far to the north of the islands on Sunday.

Needless to say, robust high pressure north and northeast of the
state will keep the gradient tight during the next 7 days. The one
factor that will keep the trades from realizing their full
potential will be the lack of a strong low level inversion during
this time. Nevertheless, breezy to windy conditions can be
expected statewide, with the trades peaking Wednesday through
Friday. Wind Advisories will likely be required for portions of
the state as the trades peak.

As for the remaining weather details, we will likely see shower
activity continue to increase across windward Maui and the Big
Island early this morning as a band of low clouds and showers
associated with an old front moves in. This band of moisture
should continue to march westward through the morning hours,
bringing an increase in showers to the remaining smaller islands.
Showery weather will likely continue across Kauai as well as
windward Maui and Big Island this afternoon, with some improvement
expected over Oahu and Molokai. Drier and more typical trade wind
conditions should then develop statewide tonight through Wednesday
night as precipitable water values drop to an inch or less in
most areas. Showers will favor windward and mauka areas during
this time, particularly at night and during the early morning
hours, with a stray shower reaching leeward communities at times
due to the strength of the trades.

Thursday through the weekend, model solutions are in decent
agreement showing troughing aloft developing over the island
chain, with moisture moving in from the east and southeast. This
should bring an increase in trade wind shower coverage and
intensity during this time, which could linger into early next
week.

&&

.AVIATION...
Occasional MVFR has been noted over Windward Big Island this
morning as better moisture and more organized showers approach
from the east. Conditions are still expected to deteriorate to
more widespread MVFR with greater shower coverage during the
remainder of the morning. Windward Maui will likely follow suit
within the next few hours. The approaching band of showers will
traverse the remaining islands during midday resulting in an
eventual increase in clouds and showers over the windward slopes
and coasts.

AIRMET Sierra is in effect for tempo mountain obscuration for
Windward Big Island and may be expanded to include portions of
Maui later this morning.

AIRMET TANGO is in effect for lee turbulence below 9kft for the
entire state.

&&

.MARINE...
A strong high far northeast of the state will continue to drive
fresh to strong easterly trades across most Hawaiian Island
coastal and offshore waters. The mid week strengthening of this
high will further tighten the pressure gradient back across the
central Pacific. This will ultimately increase local trade wind
speeds from Wednesday into the weekend. A Small Craft Advisory is
in effect for the majority of the coastal waters, with the
exception of those waters southwest of Kauai, to account for the
strengthened trades and responding rough seas. The potential of
winds within the Alenuihaha Channel to frequently gust to gale
force has prompted a Gale Watch that will be in effect from
this evening through Wednesday afternoon.

Recent ASCAT satellite passes confirm a large region of strong
trades upstream of the state generating a long fetch that will be
the impetus to the arrival of higher easterly swells. Strong trade
winds will also build large wind-driven easterly seas. The
combination of both heightened trade wind waves and east swells
will translate to rough sea conditions and higher east-facing
shoreline surf. Surf heights along eastern-facing shores will
exceed surf advisory levels this week. A High Surf Advisory (HSA)
will remain in effect for most island's eastern exposures for
the remainder of the week.

A couple of small, overlapping relatively longer period west to
northwest swells will travel around the islands the next few days.
The first of these swells has arrived and will level out through
the day. The second set of similar low west to northwest swells will
move through during mid to late week. Surf along north and west-facing
shores will remain well below HSA levels through Friday. Surf along
south-facing shores will remain small into the weekend. A small bump
in southern swell arriving later in the week will provide a slight
increase to south-facing shoreline surf by Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.

Gale Watch from this evening through Wednesday afternoon for
Alenuihaha Channel.

Small Craft Advisory until 6 PM HST this evening for Alenuihaha
Channel.

&&

DISCUSSION...Jelsema
AVIATION...JVC
MARINE...Blood