Winter Storm Warning 1, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Sat Jan 28 2023

.SYNOPSIS...
A surface trough near Maui County will support wet weather across
portions of the state, especially from Oahu to Maui, through the
weekend. Enhanced moisture may expand to Kauai as well by Sunday
afternoon. An upper-level disturbance will destabilize the
atmosphere, keeping locally heavy rainfall and a few
thunderstorms in the forecast through Monday, especially over
windward areas. Gusty winds are expected from Kauai to Oahu
through early Monday. A generally wet trade wind pattern is
expected to prevail from Tuesday through the remainder of next
week.

&&

.DISCUSSION...
Early this morning, a surface trough near Maui is serving as a
focus for showers and isolated thunderstorms that are streaming
from east to west into the island. Latest radar imagery shows an
area of moderate to heavy rain with occasional lightning is
persisting across the windward slopes of Haleakala over an area
where significant rainfall has already occurred over the past 12
hours. Stream gauges from east Maui show that water levels have
fluctuated throughout the night, but a number of them remain
elevated as showers continue. Impacts from flooding that has
occurred across the eastern portion of Maui since yesterday
afternoon will linger into today. Meanwhile, the moderate to heavy
showers that impacted windward portions of the Big Island during
the evening have largely diminished early this morning. Drier
conditions have been observed elsewhere with only a few windward
showers throughout the night, though a notable increase in shower
activity across Molokai has been observed over the past couple of
hours.

The surface trough will continue to slowly drift westward over the
central islands today as a deep upper trough sharpens west of the
island chain. Wet conditions are expected to continue across Maui
County, but moisture is also expected to spread as far west as
Oahu during the day with showers primarily affecting windward
areas. Therefore, the Flash Flood Watch that includes Maui County
will expand in area to also include the island of Oahu this
morning. The deepening upper trough will likely generate
widespread upper clouds and instability that will continue
thunderstorm chances across the wetter windward areas from Oahu to
the Big Island. Kauai, which will remain west of the trough, will
remain much drier.

By Sunday, the upper trough to the west-southwest of the state is
expected to transition into a compact low aloft. The surface
trough will sharpen as enhanced moisture persists over Maui
County, Oahu, and perhaps to Kauai by Sunday afternoon. Widespread
heavy rainfall and isolated to scattered thunderstorms appear
likely within the moist, unstable airmass over the islands.
Shower coverage will also likely increase over the Big Island
late Saturday into Sunday, likely bringing snow to Mauna Kea and
Mauna Loa summits. In addition, as the trough sharpens, northeast
trade winds over Kauai and Oahu will increase, potentially to Wind
Advisory levels tonight through Sunday.

Wet conditions will likely linger into Monday and possibly Tuesday
as the upper-level low weakens somewhat, suggesting that chances
for heavy rainfall will decrease as high clouds thin. Breezy
northeast to east winds will diminish and shift to the east-
southeast as a surface low and the low aloft move a little further
west of the islands. Throughout this period, the bulk of the
showers will continue to focus across windward areas. By Tuesday,
models favor the return of moderate to locally breezy trade winds.
Troughing aloft may linger west of the islands throughout the
week, however, which suggests that a generally wet trade wind
pattern may continue throughout much of next week.

&&

.AVIATION...
A surface trough over the central islands is helping to bring
clouds, showers and thunderstorms to Maui County early this
morning, which are expected to continue today. Additional showers
and thunderstorms are expected to spread to Oahu today. This
trough will remain over the area through the weekend. MVFR, and
pockets of IFR, conditions remain over the islands from Oahu to
the Big Island. Most of this is focused over the windward sides of
the islands. As such, AIRMET Sierra remains in effect for these
areas. This AIRMET is expected to remain in place through
Saturday, and likely beyond.

AIRMET Zulu is in place for possible rime icing from Molokai to
the Big Island. This too is expected to remain today, and may
need to be expanded to Oahu.

&&

.MARINE...
A surface high pressure northwest of the state will drift south
today and tomorrow. A trough near Maui and Big Island waters will
slowly drift westward today, sharpening the pressure gradient over
western waters. Moderate to locally fresh northeast winds will
strengthen to moderate to locally near gale range by Sunday. The
Small Craft Advisory has been expanded to included the Kaiwi
Channel and waters westward but may need to be expanded as winds
and seas spread.

The current northwest swell will continue to lower today. A
series of moderate, medium to short period north swells will move
through the local waters today into the first half of next week,
helping to keep surf elevated along north facing shores. A long-
period, moderate northwest swell (310 degrees) is expected to
boost surf heights to advisory levels along north and west facing
shores Sunday into Monday, before subsiding Tuesday. Choppy, east
shore surf will become increasingly rough as the wind swell picks
over the next couple of days and some wrap from the north swells
mixes in. Background south swells will keep surf small along south
facing shores today and tomorrow, then will gradually pick up
early next week as a small, long- period swell fills in.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Sunday afternoon for Oahu-

Flood Watch through Sunday afternoon for Lanai Mauka-Kahoolawe-
Maui Windward West-Maui Leeward West-Haleakala Summit-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley
North-Maui Central Valley South-Windward Haleakala-Kipahulu-
South Maui/Upcountry-South Haleakala.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel.

&&

DISCUSSION...Vaughan
AVIATION...M Ballard
MARINE...Almanza