Flash Flooding and Winter Storm in Hawaii, December 3 to 7, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
327 AM HST Fri Dec 3 2021

.SYNOPSIS...
Unsettled weather will persist through the weekend and likely
through the first half of next week. A cold front will be moving
into Maui County in the coming hours, and on to the Big Island
this afternoon, where it is slated to stall tonight and Saturday.
A low pressure system will form just to the northwest of Kauai on
Saturday and Saturday night, causing the winds to turn southerly
on Sunday and persist into early next week. Aided by a nearly
stationary upper level trough, unsettled conditions are likely
through Wednesday across most if not all of the main Hawaiian
Islands.

&&

.DISCUSSION...
Through this afternoon.

Not a lot of changes made to the forecast package. The front is
pushing through Oahu as of 2 am HST, and heading into Maui County.
North to northwest winds will spreading to Oahu in the next
couple of hours. Kauai County in under a strengthening north to
northwest wind flow. Barking Sands, on the west end of Kauai has
north wind at 20 mph this hour. Lihue, which is blocked by the
mountains, has northwest winds of 12 mph. Some stronger downslope
winds are likely to come into play over on the southeast side of
the island as the day progresses. This surge of very strong winds
is likely to start diminishing this afternoon. With the showers,
although there aren't much clouds on satellite imagery to the
north of Kauai, there are plenty of small showers coming in with
this northerly wind flow.

The initial surge of showers that pushed through Oahu and now
Maui County, without much noise. There were a few brief heavy
showers, including one threshold rain alarm at Lahainaluna Maui a
few minutes ago. These clouds and showers with the initial surge,
appears to be struggling to reach westernmost Big Island, from
Upolu Point to Kailua-Kona Airport. Perhaps due to a offshore land
breeze. There is a second surge of showers, now pushing through Oahu,
which is likely the front. So more showers will be moving into
Maui County in the next couple of hours.

While Kauai and Oahu calms down in the way of heavy showers, the
focus shifts to Maui County where the front is expected to
struggle through the county for a good part of the day. A strong
upper level disturbance rotating around the upper trough has to
potential to set off a good chance of a few thunderstorms to
Maui County and the Big Island mainly this afternoon.

The front will usher in a cool air mass affecting primarily Kauai
and Oahu. The day time for today and Saturday will be around 80
degrees F. Nighttime minimums tonight will be in the 60s. Dew
points are tab to be in the upper 40's in interior Kauai and 50's
across Oahu tonight. Dew point temps will be on the rise Saturday
as the winds turn southerly.

The islands are under a complicated weather scenario, so there will
be refinement to the forecast.

.PREVIOUS DISCUSSION...
An ill-defined cold front will be ushering in a period of unsettled
weather for the main Hawaiian Islands that will last through the
weekend and into early next week. The front is likely over Kauai
with northwest winds behind the boundary. It is advancing towards
Oahu. Complicating the weather map is the north to south oriented
trough that had stalled in the Kauai Channel earlier in the day,
but is now in the process of being absorb by the front. Not to be
confused, the actual front is noted in the satellite imagery as
the leading of a cloud field, some 240 miles NW of Kauai.
Northerly winds are noted blowing north to south across this
front, thus making it as a dissipating front. So, from now on, we
will be focusing the new cold front currently advancing toward
Oahu. We this said, the strong northerly winds are lagging a few
hours behind the front, but should reaching Kauai just before
sunrise Friday.

The timing has the cold front reaching Oahu around midnight
tonight, then it is on to Maui County after midnight. The front is
slated to stall over or in the vicinity of the Big Island Friday
night and Saturday. A strong upper level disturbance will be
moving into the islands' area Friday that will accentuate the
moisture in place especially, especially Maui and Big Island
counties. On Saturday and Saturday night, another but stronger
upper level disturbance will produce a surface low northwest of
Kauai. This low will bring on a moist southerly wind flow for all
islands by Saturday night. The front that had stalled near the
Big Island is slated to move west as a trough and dissipates. A
Flash Flood Watch is in effect for the entire main Hawaiian
Islands, good through the weekend.

Under the northerly wind flow, is a cooler air mass that will
usher in cooler air mass with daytime highs around 80 and dew
points in the upper 50's and lower 60's. The southerly winds will
change all this, boosting the dew points into the mid 60's. It
should be noted that the post frontal winds are expected to be
rather strong over Kauai, warranting to a Wind Advisory covering
the Friday time period.

An approaching upper level disturbance will bring on very strong
winds to the Big Island summits on Friday night, and continuing
into early next week. A High Wind Warning has been issued that
will become effective from 6 pm HST Friday through late Sunday
night. Additionally, a Wind Advisory has been issued for the
summits of Haleakala, effective from 6 pm HST Friday through 6 am
HST Monday as well for lesser but still very strong winds.

A lot of weather is on tap for the Big Island summits in the days
ahead. Snow is expected over the Big Island summits as the front
stalls near the Big Island Friday night into Saturday. And with
the expected strong winds, a Blizzard Warning is now in effect
from Friday night through Saturday night for the Big Island
summits.

The surface low will be supported by a strong upper level low
above it Saturday night. These two features will linger west of
Kauai through at least Tuesday. Instability, along with low level
convergence, will increase rainfall chances for a good portion of
the state Saturday night through Tuesday. This type of weather
scenario, is very difficult to pin down impact details. Heavy
rainfall will be a threat and shower bands may line up over
portions of the state for extended periods of time, increasing the
threat for flooding. The southerly winds may strengthen to where
locally gusty winds may affect the so-call windward side of the
islands, such as the Kailua and Kaneohe areas of Oahu.

&&

.AVIATION...
A band of showers along a frontal boundary will continue moving
through the island chain today. Land breezes cleared out Big
island skies temporarily, but clouds and showers will increase
there as the frontal cloud band approaches. AIRMET Sierra for
mountain obscuration is in effect for Kauai, Oahu and Molokai,
but will likely be expanded to include Maui and the Big Island
later today. Post-frontal clouds and showers later in the forecast
period could contain IFR ceilings and visibilities, resulting in
more extensive mountain obscuration. With strong winds moving in
behind the front, AIRMET Tango may also be needed for mechanical
turbulence and sustained wind speeds in excess of 30kts.

&&

.MARINE...
A vigorous cold front has begun moving through the state from the
northwest this morning. Winds have been turning toward the north
and are increasing in speed as the front begins to move down the
island chain. The front is expected to stall near the Big Island
tonight and Saturday as a strong low aloft develops near Kauai.
The low aloft will spawn a surface low just west of Kauai Saturday
night, with this kona low lingering near the islands for at least
several days. A convergence band of showers east of the low will
develop over the islands, with a strong potential for unsettled
weather.

As the low develops; today's strong northwest and north winds
will shift to the west and weaken Saturday, then shift to the
south and southwest and strengthen Sunday. Although confidence is
not all that high, generally expecting low pressure to linger west
of Kauai through the middle of next week, keeping an unstable
southerly flow in place over the area.

As the front initially moves down the island chain, strong and
gusty winds are expected, especially in and near any heavy showers
or thunderstorms that may develop, with the potential for strong
thunderstorms. A Gale Warning is posted for waters between Kauai
and Niihau as winds surge today, with winds and seas ramping up
to the point that a Small Craft Advisory (SCA) has been posted for
many zones. Although winds will ease Saturday, combined seas will
be well above the 10 foot threshold for SCA through the weekend.

Locally generated wind waves, plus a building north-northwest
swell, are expected today. This swell will shift to the north and
continue to build through the weekend as the wave period
increases. This fairly long-lived large north swell will likely
generate surf heights close to 30 feet along north facing shores
Saturday and Sunday, requiring a High Surf Warning. High tide
early Saturday and Sunday could lead to some wave runup in some
coastal areas. This swell will also lead to significant harbor
surges in both Kahului and Hilo harbors, and will likely bring
Advisory-level surf to west facing shores.

Expect minimal surf along east facing shores through the day,
with some increase this weekend as the large north-northwest swell
turns to the north, allowing some swell to wrap around the
islands. Surf along south facing shores will rise slightly over
the weekend and early next week due to locally generated wind
waves. A small, long-period south swell is also expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Monday afternoon for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central
Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island
Interior-Kauai North-Kauai East-Kauai South-East Honolulu-
Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley
North-Maui Central Valley South-Windward Haleakala-Kipahulu-
South Maui/Upcountry-South Haleakala-Big Island South-Big Island
Southeast-Big Island East-Big Island North.

Wind Advisory until 6 PM HST this evening for Niihau-Kauai-

High Wind Warning from 6 PM this evening to 6 AM HST Monday for
Big Island Summits.

Blizzard Warning from 6 PM this evening to 6 AM HST Sunday for
Big Island Summits.

Wind Advisory from 6 PM this evening to 6 AM HST Monday for
Haleakala Summit.

Small Craft Advisory from 6 AM this morning to 6 PM HST Sunday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Pailolo Channel-Alenuihaha Channel-Big
Island Windward Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Kauai Leeward Waters.

Gale Warning from 6 AM this morning to 6 PM HST this evening for
Kauai Leeward Waters.

&&

H Lau/Powell/Kinel