Strong Wind in Hawaii, February 29, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Sat Feb 29 2020

.SYNOPSIS...
Windy and occasionally very wet trade wind weather will continue
into Monday, with brief heavy downpours expected, and a slight
chance of thunderstorms. Lighter winds and fewer showers are
expected from Tuesday into Thursday. Winds and showers may increase
again heading into next weekend.

&&

.DISCUSSION...
Strong and gusty NE to E trade winds are being supported by a 1040
mb high centered far NNE of the islands. A potent mid-level low
centered just NE of the islands has destabilized the island
atmosphere, thereby turning our typical trade wind showers into
heavy downpours. Additionally, a few thunderstorms are seen near the
core of the low over waters E of the islands, and in the convergent
plume W of the Big Island. A forecast update was issued this morning
to expand the ongoing Wind Advisory to all areas of the state,
except the summits of Maui and the Big Island, where criteria are
higher. Also added RW+ to the forecast through tomorrow - to
highlight the intense showers that are riding in on the trade winds -
but also noteworthy that these showers are not expected to lead to
significant flooding as they are moving WSW at 30-40 mph. As some
icing (mainly due to freezing rain) was observed on the summits of
Haleakala and the Big Island this morning (and more is expected), a
Winter Weather Advisory replaced the Winter Storm Watch.

Latest infrared satellite images show an abundance of cold cloud
tops near and E of the islands, with water vapor imagery
highlighting the cause; a closed low aloft centered just E of the
islands. This low has very cold temperatures at it's core, which has
destabilized the atmosphere. Despite relatively low PWAT (near 1
inch) and surface dew points in the upper 50s to lower 60s, heavy
shower cores are noted on radar, with many of these cores growing in
size and strength when they interact with the island terrain.
Latest radar VAD wind profile from Molokai shows that NE to E winds
extend to at least 30 thousand feet, allowing occasional windward
showers to roll right over the terrain into leeward areas (except
leeward Big Island), which will continue to receive their share of
brief soakings through Sunday. Temperatures over the Big
Island/Haleakala summits are about as cold as we typically observe,
and this will continue tonight, with gradual warming thereafter.
Biggest threat of impactful winter weather with this event is
expected to be freezing rain, with a few snow showers possible with
the deepest convection.

Latest analysis and forecast indicates a subtle N-S gradient in
trade wind speeds, with winds stronger over the N half of the area,
as opposed to the S half. This is expected to continue over the next
48 hours or so, as a weak and subtle surface trough is expected to
develop E/SE of the islands, in response to the low aloft. Although
the statewide Wind Advisory extends through Sunday night, there is
higher confidence in it verifying over Kauai and Oahu where winds
are expected to increase slightly as the trough develops. Peak wind
speeds are expected from tonight through Sunday night, with winds
diminishing thereafter (and veering to the SE) as the surface trough
moves from E to W across the islands Monday and Tuesday. The now
slow-moving mid-level low is expected to linger near the islands
through Sunday night before lifting out to the N Monday/Tuesday. The
lighter winds and warming temps aloft will lead to a much more
settled weather pattern, with fewer showers.

Diminished trade winds will prevail until the end of the week as the
nearby surface trough keeps the gradient relaxed, with a ridge of
high pressure persisting to the N. As the trough dissipates and a
new strong high builds N islands Friday into the weekend, trade
winds will likely increase. A weak upper level trough could increase
shower coverage, but not to the extent currently observed.

&&

.AVIATION...
High pressure to the north of the islands will help produce moderate
to strong northeasterly trade winds today. A low pressure system
approaching the islands from the northeast will bring an increase in
shower activity to all islands, mainly affecting the windward
sections, though a few showers could spill over the terrain and into
leeward areas. In addition, isolated thunderstorms may occur due to
the unstable air aloft associated with this feature. Periods of MVFR
conditions will be present along the windward sides of the islands
with isolated MVFR conditions on the leeward sides due to mountain
obscuration from low clouds and and showers.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai, Maui, and the Big Island.
These conditions should improve later this morning.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. These conditions will likely
persist through the weekend. AIRMET Tango also remains in effect for
upper level turbulence over Kauai and Oahu. These conditions will
likely persist through the day.

&&

.MARINE...
Strong east-northeast trades associated with 1040 mb high pressure
northeast of the state will persist through Monday. Pailolo and
Alenuihaha Channels, Maalaea Bay and waters around South Point of
the Big Island will remain near gale-force levels through Sunday.
WAVEWATCH III and ECMWF-Wave guidance depict seas building into the
12-18 ft range by tonight across much of the area within a couple of
hundred nautical miles to the northeast of the windward waters. The
Small Craft Advisory (SCA) remains in effect for all waters due to a
combination of winds and seas through Monday.

In addition to the rough water conditions, shower coverage will
increase through the weekend as an upper low moves into the area. A
surface trough is forecast to move through the waters from east to
west through early next week, possibly with a brief period when the
local winds trend down into the light to moderate range for some
locations (low confidence this far out).

Surf along east facing shores will steadily rise through the weekend
due to the aforementioned scenario evolving over and upstream of the
islands. Heights will hold around the advisory level (8 ft) today,
then climb to near warning levels Sunday through Monday. Will
monitor buoys for potential need for upgrade to High Surf Warning.

Surf along north and west facing shores will gradually ease over the
weekend as the northwest swell that peaked Friday eases. This will
be the beginning a quiet stretch for north and west facing shores
through much of next week. Some of the trade wind energy, however,
may wrap into exposed north facing shores each day. Surf along south
facing shores will remain small with mainly background south and
south-southwest swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Monday for portions of Kauai, Oahu,
Molokai, Lanai, Kahoolawe, Maui and the Big Island.

High Surf Advisory until 6 PM HST Monday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Winter Weather Advisory through Sunday afternoon for Haleakala
Summit and Big Island Summits.

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

&&

DISCUSSION...TB
AVIATION...JT
MARINE...TS