High Surf in Hawaii, March 24 to 28, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
342 AM HST Tue Mar 24 2020

.SYNOPSIS...
Strong trade wind weather remains in the forecast through Thursday
as a strong high pressure system moves in from the north. Drier
and more stable air moving into the region will limit rainfall
chances through Thursday afternoon with the highest chances for
showers forecast along windward mountain slopes in the overnight
to early morning time periods. An upper level disturbance
developing north of the islands will bring increasing shower
activity to the region with locally heavy rainfall possible from
Thursday night through Sunday.

&&

.DISCUSSION...
A strong 1040+ high pressure system north of the state will drift
south over the next few days increasing surface pressure
gradients and wind speeds across the Hawaiian Islands. These
strong winds will exceed wind advisory thresholds across the
state with gale force winds in some of the windier eastern
waters. The Wind Advisory was expanded this morning to cover
additional areas on Kauai, Oahu, Molokai, and Maui for 25 to 35
MPH sustained winds and up to 50 MPH gusts. Drier and more stable
air moving in from the north and a weak upper level ridge will
limit shower activity statewide through Thursday.

The weather picture this morning remains fairly dry with stable
cumulus and stratocumulus clouds upstream shown on satellite
imagery, along with isolated to scattered showers on radar moving
into northeastern slopes of each island. Scattered high clouds are
moving in from the west over Kauai and Oahu this morning, expect
periods of high cirrus clouds to continue drifting over the
islands through the week leading to colorful sunrise and sunsets.
This mornings upper air balloon soundings from Lihue and Hilo
continue to show a strong subsidence inversion between 6000 and
8000 feet respectively. Short range guidance indicates this
inversion height will remain around the 7000 foot elevation mark
through Thursday allowing clouds to deepen enough for scattered
occasionally numerous windward and mauka showers, with isolated
elsewhere, favoring the overnight and early morning hours.

The weather picture changes starting on Thursday as the high
pressure system north of the islands weakens and several upper
level lows drift into the Hawaii region from the north. These
unstable disturbances aloft will lift the subsidence inversion
height, allowing deeper clouds to form over the islands, and
produce increasing rainfall trends from Thursday night into the
weekend. Heavy showers may work their way into the forecast from
Friday through Sunday. Long range guidance from both the American
(GFS) and the European (ECMWF) models are struggling with the
location and strength of these upper low centers this far out in
the extended range forecast. Impacts for each island will depend
heavily on the final position and intensity of these multiple upper
low centers. Overall the confidence is fair in wet weather trends
for all islands from Thursday night into Sunday, while confidence
in island by island impacts such as precipitation amounts, heavy
rain potential, and thunderstorms are poor at this time. Expect
changes to the Friday through Sunday forecast impacts through the
week as model guidance settles on a better solution. Stay tuned.

&&

.AVIATION...
Strong high pressure to the north and a stable island atmosphere
will combine to bring increasingly strong and gusty ENE trade
winds into Wednesday. The primary hazard to aviation will be low-
level turbulence over and downwind of island terrain. AIRMET TANGO
is posted for tempo moderate low-level turbulence below 8000',
but may need to include isolated severe turbulence later today or
tonight as winds increase. With a dry air mass moving in from the
E, low clouds riding in on the trades will likely only produce
isolated MVFR VIS/CIG in -SHRA, primarily along windward slopes.

&&

.MARINE...
High pressure north of the state will keep strong trade winds
blowing into the weekend. The trades will be particularly strong
today through Wednesday night, with Gales expected in the windier
waters around Maui and the Big Island, where a Gale Warning is
currently in effect. A Small Craft Advisory (SCA) remains in
effect for all other marine zones through Wednesday night.
Although winds are expected to trend lower Thursday through the
weekend, an SCA will likely be required for much of the marine
area through Thursday night and possibly into Friday.

The current west-northwest swell will continue to lower today.
Very small background north and northwest swells are then expected
tonight through early next week.

Rough surf will build along east facing shores during the next
couple days due to a combination of strong trade winds over and
upstream of the islands. Advisory level surf is expected along
east facing shores of most islands through Friday and possibly
into the weekend. A High Surf Advisory (HSA) remains in effect
east facing shores of most islands through Tuesday night, and
this will likely need to be extended in time.

A long-period south swell will peak at low-end advisory levels
today and tonight, then gradually lower Wednesday through
Thursday. A HSA remains in effect for south facing shores of all
islands through 6 AM Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Wednesday for south and east
facing shores of all islands.

Wind Advisory until 6 PM HST Wednesday for portions of Kauai Oahu
Molokai Lanai Kahoolawe Maui and the Big Island.

Gale Warning until 6 AM HST Thursday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Thursday 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-Big Island
Windward Waters.

&&

DISCUSSION...Bohlin
AVIATION...Birchard
MARINE...Jelsema