Heavy Rain in Hawaii, December 16 to 17, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
814 PM HST Thu Dec 16 2021

.SYNOPSIS...
Trade winds are expected to gradually strengthen Friday through
Saturday, as a strong surface high sets up north of the islands.
Clouds and showers will favor the typical windward areas into the
weekend. A wet trade wind pattern is expected early next week
aided by an upper level low southwest of the islands and a front.

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.DISCUSSION...
The forecast trend remains the same in the coming days, that is
toward a very wind, wet and unsettled regime. Showers have ramped
up across Oahu this afternoon and evening, mainly the Koolau's
but also the Waianae coast. These showers are from the band
mentioned in the early morning discussion. The band avoided the
Big Island by staying north and instead targeted Maui and Oahu.
Accentuating these showers is an upper level trough now over the
central islands. It has moved southeast from Kauai 24 hours ago,
where it kicked of a burst of heavy showers over the Garden Isle.
Current satellite and radar shows a rather active area of showers
and possibly a thunderstorm or two some 50 miles off the Hamakua
coast. A cumulonimbus (CB) clouds is also note about 40 miles
northeast of Kauai. These features are just an indication of a
unstable air mass over the islands. The presence of these CB's
warrant including isolated thunderstorms for the entire stretch of
our windward waters for the rest of tonight and Friday. The upper
trough is slated to reach the Big Island Friday, therefore,
thunderstorms will be noted in the Friday's forecast for the Big
Island as well. The upper trough clears but stalls just east of
the Big Island Friday night. An upper ridge replaces the trough
over the islands over the weekend.

Latest GFS and ECMWF models have the upper low to be a tad further
west of the previous model runs for Sunday, but drifts to a
position southwest of the islands Monday. The surface low that has
been mentioned in previous discussion drops south, stalls south of
Midway Atoll and fills Sunday into Monday.

With this said, the heavy weather appears further away from
Kauai. For the islands, more or less, expect a typical trade wind
shower patter Sunday and Sunday night. A front drops down on the
islands Monday afternoon at the earliest, and with the upper low
now to the southwest of the islands, we are looking at a wet and
unsettled Monday night through Tuesday time frame.

As for the winds, trade winds were in the light to moderate range
today, with the stronger winds favoring Maui, the Big Island, and
their adjacent windward waters. Friday's wind pattern is similar
to today's pattern. From Friday night on, the trade winds will be
on the upswing, becoming breezy by Saturday evening. Another
boost in the trades is slated for Sunday afternoon and evening,
where it becomes pretty windy, which may warrant a low level wind
advisory. There is the potential for this event to last into
Tuesday.

&&

.AVIATION...
Moderate east to southeast winds will continue through the second
half of the week as a front passes far to the north. An upper
trough over the region combined with a sufficient amount of
moisture moving in from the east has led to increasing shower
coverage over portions of the state this evening, which prompted
AIRMET SIERRA for windward Oahu, Molokai, and Maui for MTN
obscuration. This activity will become more likely over windward
sections of the Big Island overnight into Friday. Expect this
trend to persist through Friday, with some leeward and interior
areas becoming more active where sea breezes form. Additionally, a
few thunderstorms can't be ruled out over the windward waters
from Oahu to the Big Island overnight and again over the Big
Island slopes Friday afternoon.

AIRMET TANGO for moderate turbulence between 28000 and 48000 feet
remains in effect this evening. Will likely drop this overnight as
the turbulence diminishes.

&&

.MARINE...
A cold front approaching the region from the northwest will
continue to weaken the ridge north of the islands and keep light
to moderate trade winds with land and sea breezes over terrain
sheltered areas into Friday. Trade winds will slowly increase from
Friday afternoon onward as high pressure builds back in north of
the state. Fresh to strong trade winds are forecast over Hawaiian
coastal waters from Saturday through Wednesday, with periods of
near gale strength trades in the windier waters around Maui and
the Big Island. An upper level disturbance will bring increasing
shower trends along with isolated thunderstorms into the weekend.

Moderate surf along north facing shores and small surf along west
facing shores will hold through tonight. Two overlapping northwest
swells, primarily from a 310 direction, will increase surf heights
along north and west shores from Friday through Sunday. These
surf heights may approach High Surf Advisory levels this weekend
and then decline on Monday. A small medium period north-
northeast swell arriving late Monday should keep surf heights
elevated along north facing shores with some wrap into eastern
shores through Wednesday. East facing surf heights will briefly
subside through Friday as the trade wind swell eases. However,
increasing trade winds this weekend will boost surf heights along
E facing shores into early next week. S facing shores will remain
flat through the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...H Lau
AVIATION...Gibbs
MARINE...Bohlin