Flash Flood and Debris Flow in Hawaii, January 10 to 12, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
327 AM HST Fri Jan 10 2020

.SYNOPSIS...
Wet and windy trade wind weather is expected to continue with
periods of heavier showers possible, especially along windward
and mauka areas today. An upper level low developing southwest of
Kauai tonight is expected to pass just west of the state over the
weekend and will continue to bring the threat of heavy rain.
Winds are expected to slightly decrease later in the weekend and
gradually shift out of the east-southeast as a trough develops
west of the state early next week. Unsettled weather will likely
continue during the first half of next week, especially over the
western half of the state.

&&

.DISCUSSION...
Strong high pressure north to northeast of the area will maintain
strong and gusty trades today. Aloft, an upper level trough to
the northeast continues to support a wet trade wind pattern early
this morning. For today, frequent showers will continue to develop
and quickly move through, especially over windward and mauka
areas. Some showers may be locally heavy due to the added
instability associated with the upper trough.

Tonight through Saturday, an upper level low developing southwest
of Kauai is expected to track north and pass to the west. This is
expected to draw deeper moisture northward into the area and
likely a greater chance for heavier showers. A Flash Flood Watch
remains in effect for Maui and the Big Island through Saturday
afternoon. We will continue to monitor the situation for Oahu and
Kauai and a Flash Flood Watch will certainly be possible as this
low develops southwest of Kauai tonight. The current Wind Advisory
has been expanded to include Kauai and is now in effect through
this afternoon. Gusty winds will be possible with passing showers
throughout the day today and a downward trend in wind speeds are
expected by Saturday. A Winter Weather Advisory is in effect for
light snow accumulations over the Big Island summits through
tonight.

Strong trades will begin to ease Saturday night. On Sunday,
models show the upper low just west of Kauai along with an
induced surface trough west through southwest of Kauai. The
surface trough will cause winds to shift slightly south of east
over the western half of island chain and pull unsettled weather
northwestward over the island chain. This unsettled weather may
last into the middle of next week.

&&

.AVIATION...
Windy and wet trade wind weather will continue during the next 24
hours, bringing widespread showers and MVFR cigs/vsbys to windward
areas, and passing showers with some reduced cigs/vsbys at times
in leeward locales. AIRMET Sierra remains in effect for mountain
obscuration across windward sections of Kauai, Oahu, Molokai, Maui
and the Big Island. AIRMET Tango remains in effect for low level
turbulence over and downwind of the terrain, as well as for strong
surface winds.

&&

.MARINE...
Strong high pressure will remain to the northeast into next week.
The pressure gradient over the islands is starting to weaken, so
there have been some changes to the marine areas under Small Craft
Advisories (SCA) and Gale Warnings with the morning forecast
package. The Gale Warning remains posted for the Pailolo and
Alenuihaha Channels today, while all other waters are under an
SCA. The current SCA goes through 6pm Sunday, but winds and seas
will likely remain above SCA levels into the middle of the week.

The elevated seas are due to the fetch generated by this high
pressure system. Winds will gradually veer to the east-southeast
early next week as a trough develops to the west of the islands.

As an upper level feature over the islands into the weekend will
bring an unstable airmass that will bring the chance for locally
heavy downpours and a risk for thunderstorms through Saturday.

The above mentioned fetch is also generating warning level surf
along the east facing shores. The High Surf Warning is in effect
through Saturday, but surf will remain elevated at advisory levels
into the middle of the new week. Over the next few days, the
combination of the large surf, strong onshore winds, high
astronomical tides, and elevated ocean water levels will produce
wave runup and localized coastal erosion along east facing shores.

A long-period northwest swell arriving today could produce surf
around the advisory level tonight into Saturday. A smaller
northwest swell is expected to fill in Sunday and gradually
decrease early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Kauai-Oahu South
Shore-Waianae Coast-Oahu Koolau-Waianae Mountains-Molokai-Lanai-
Kahoolawe-Maui Windward West-Maui Leeward West-Maui Central
Valley-Windward Haleakala-Leeward Haleakala-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

High Surf Warning until 6 PM HST Saturday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through Saturday afternoon for Maui-Kona-South
Big Island-Big Island North and East-Kohala-Big Island Interior.

Winter Weather Advisory until 6 AM HST Saturday for Big Island
Summits.

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-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 PM HST this evening for Pailolo Channel-
Alenuihaha Channel.

&&

DISCUSSION...Kino
AVIATION...Jelsema
MARINE...M Ballard