366 
FXHW60 PHFO 201952
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Sun Nov 20 2022

.SYNOPSIS...
Strengthening high pressure to the north will generate breezy to 
locally strong trade winds into Tuesday. Low clouds and passing 
showers will primarily focus over windward coasts and slopes, but 
some showers will drift to leeward areas, and a few of them may be 
briefly heavy over the Big Island. A cold front is expected to sweep 
down the island chain on Thanksgiving, delivering a brief period of 
showers to most areas, followed by very strong and gusty northeast 
winds that will ease by next weekend. 

&&

.DISCUSSION...
Strengthening high pressure N of the islands will support breezy to 
locally strong ENE trade winds into Tuesday, with some easing and 
backing of the winds Tuesday night and Wednesday as an approaching 
front weakens the high. A very strong high (~1045 mb central 
pressure) building NW of the islands will ensure that this front 
gets ushered down the island chain Wednesday night and Thanksgiving 
Day, followed by very strong NE winds. Global model guidance has 
been fairly consistent with this general solution for several days 
now, which leads to increased confidence in the forecast, but there 
remains some uncertainty as to the timing of the frontal passage, 
and the strength of the NE winds following the front. The potential 
does exist for very strong and potentially damaging wind gusts. If 
deemed necessary, a Special Weather Statement may be issued to 
increase awareness of this potentially impactful weather event. 

In the meantime, a Wind Advisory is in effect for portions of Maui 
County and the Big Island, including the Summits of Mauna Kea and 
Mauna Loa, as a deep-layer ridge builds N of the islands, and a mid-
level trough persists just E of the Big Island. Although incoming 
moisture is not especially abundant (PWAT 1.2" - 1.5"), the 
atmosphere remains somewhat unstable, especially over the E end of 
the chain, with the 12Z PHTO sounding indicating a lack of a 
subsidence inversion. Radar and satellite show scattered unstable 
open-celled cumulus elements riding in on the trade wind flow, with 
some small heavy rain cores leading to briefly heavy downpours in a 
few windward locales. 

The forecast continues to anticipate that the mid-level ridge 
building N of the islands will lead to warming mid-level 
temperatures and increased stability statewide, with low clouds 
delivering just a few passing trade showers into Wednesday, 
especially night and morning. The exception may be the Big Island, 
as the mid-level trough persists just a few hundred miles to the E, 
potentially leading to increased trade wind shower coverage. 
Guidance also indicates an area of increased moisture arriving on 
the Big Island Monday night/Tuesday that will likely fuel 
increased windward showers. 

A high pressure cell currently just NE of Japan will track E and 
strengthen the next couple of days as a low rapidly develops E of 
the high. A front extending southward from the low will be forced 
over the islands Wednesday night and Thanksgiving Day as the high 
moves into a position N of the of area. While a relatively brief 
period of rain is expected as the front moves quickly from N to S 
through the island chain, the wind behind the front will be the 
weather feature of greater interest. Guidance has been consistently 
indicating sustained 35 mph NE winds moving over the islands, 
periodically indicating sustained winds near 40 mph. Winds are 
expected to ease somewhat Thursday night into the weekend as a cut-
off low forms on the front to the NE of the islands, and the high 
rapidly weakens. It's these developments that leads to some 
reduction in forecast confidence for Thanksgiving Day and beyond.  

&&

.AVIATION...
High pressure building north of the main Hawaiian Islands will 
produce locally strong east-northeast trade winds today through 
Monday. These winds are likely to remain elevated throughout the 
forecast period, including during the overnight hours. Brief passing 
showers will favor north through east sections of the islands, 
especially during the overnight to early morning periods.

AIRMET Sierra remains in effect this morning for mountain 
obscuration above 2,000 feet for north through east sections of 
Kauai, Oahu, Molokai, Maui, and the Big Island. However, latest 
trends in surface observations as well as latest satellite and radar 
imagery suggest that coverage of low clouds and showers is 
diminishing. Therefore, this AIRMET will likely be allowed to expire 
later this morning.

Additionally, AIRMET Tango remains in effect for moderate low level 
mechanical turbulence over and to the lee of island mountains. This 
AIRMET will likely continue at least through Monday as locally 
strong surface winds remain in place.

&&

.MARINE...
A tight pressure gradient across Hawaii will result in strong to 
gale force trade winds across the coastal waters over the next day 
or so. A Gale Warning is in effect for Pailolo and Alenuihaha 
channel through tonight. A Small Craft Advisory (SCA) is also in 
effect for all Hawaiian waters through tonight, with the likelihood 
that at least some of the waters will be extended into Monday and 
beyond. A vigorous cold front could impact the Hawaiian waters 
around Thanksgiving, which may produce Gale force winds and high 
seas.

A new long-period northwest swell (310 degrees) has begun to show a 
rise on the offshore buoys. It will fill in throughout the day today 
and peak late tonight into Monday. At the peak of the swell, surf 
heights may reach near the advisory level for north facing shores. A 
much larger north-northwest to north swell is expected overnight 
Wednesday night into Thanksgiving Day. The peak of this swell could 
reach warning levels and it could coincide with the spring high tide 
during the early morning hours Thursday. Adding the potential for 
gale force winds along the cold front, there could be significant 
overwash issues and large wave run up along north facing shorelines 
both on the mornings of Thanksgiving and Friday. Latest wave run up 
forecast from PACIOOS is indicating wave run-up heights exceeding 8 
feet and approaching 10 feet. Above 8 feet, overtopping of lower 
roadways closer to the shore should be expected.

Surf along east facing shores will see a steady rise during the next 
few days as the trades strengthen. Areas exposed to north swells 
should see a significant rise during the second half of next week as 
a very large north-northwest to north swell fills in. Other than 
some wind swell wrap, south facing shores will see minimal energy 
for the foreseeable future.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Monday for Lanai Mauka-Kahoolawe-
Maui Windward West-Maui Leeward West-Kohala-Big Island Interior-
Molokai North-Molokai West-Molokai Leeward South-Lanai Windward-
Lanai Leeward-Lanai South-Maui Central Valley South-Windward 
Haleakala-South Maui/Upcountry-South Haleakala-Big Island South-
Big Island Southeast-Big Island North.

Wind Advisory until 6 PM HST Monday for Big Island Summits.

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

Gale Warning until 6 AM HST Monday for Pailolo Channel-
Alenuihaha Channel.

&&

$$

DISCUSSION...Birchard
AVIATION...Vaughan
MARINE...Foster