Special Marine Warning 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1023 AM HST Tue Jan 19 2021

.SYNOPSIS...
Heavy showers are heading for Kauai this afternoon and will likely
remain overnight with increasing potential for flash flooding.
Some clearing is expected over the rest of the state but wet trade
wind weather is in store through the week.

&&

.DISCUSSION...
A surface trough located over the Kauai Channel is on its way
toward Kauai and showers are expected to increase midday on the
Garden Isle. Some of these showers will be heavy for prolonged
duration and there is high potential for flash flooding this
afternoon and into the night. The current flash flood watch will
likely be extended for Kauai County with the issuance of the
afternoon forecast package. The trough is also creating strong
northeasterly winds around and over Kauai and Niihau with a wind
advisory in effect through today. The surface trough is expected
to move west of Kauai by Wednesday morning and threat for flash
flooding is expected to decrease.

An upper level trough over the state is providing some atmospheric
instability that has fueled isolated thunderstorms across the
area. An increase in afternoon convection is expected this
afternoon with some clearing skies from Oahu to the Big Island
allowing for island heating to boost the chance for convective
showers and isolated thunderstorms.

Once the trough pushes west of Kauai, easterly trade winds will
spread across the state with showers focused over windward and
mountain areas. However with the upper trough remaining through
the week we expect a wet trade wind pattern with rainfall maxima
overnight and during the early morning hours.

A winter weather advisory remains for the Big Island summits above
12,000 feet elevation, where an additional 1 to 2 inches of snow
accumulations are forecast with patchy freezing fog until this
evening.

&&

.AVIATION...
A trough over the Kauai Channel is moving slowly west toward Kauai.
Areas of MVFR cigs/vis can be expected along and near the trough,
particularly for windward areas in showers. Moderate to strong
north to northeast winds will persist, mainly to the the west of
the trough axis, as robust high pressure remains far north of the
islands.

AIRMET Sierra remains in effect for mountain obscuration for
windward sections of Kauai and Oahu.

AIRMET Tango remains in effect for low-level turbulence over and
downwind of the terrain from Kauai through Molokai, but should
diminish overnight. AIRMET Tango is also posted for high-level
turbulence over the entire island chain.

AIRMET Zulu is in effect for TEMPO moderate rime icing from Kauai
eastward to Molokai.

&&

.MARINE...
A sharp NE-SW oriented surface trough near Oahu is moving
steadily W, acting as a focus for cloud and shower development,
with heavy showers and a few thunderstorms also noted. With a high
pressure cell due N of the islands, near gale-force NE winds lie
W of the trough axis, with moderate to fresh E winds to the E of
the trough. The trough axis will likely be W of Kauai by tomorrow
morning, with moderate to locally strong E trade winds prevailing
statewide through the remainder of the week, supplied by a
surface high centered to the distant NE. The high is expected to
strengthen next weekend as a surface low develops to the distant
NW, leading to strong to near gale-force ESE winds.

Large, rough and disorganized NW swell will gradually diminish
through tonight as the swell turns more northerly, and a High Surf
Advisory remains posted for most N and W facing shores. Surf is
expected to diminish below advisory levels on Wednesday, and
remain below advisory levels along all shores until a large long-
period NW swell arrives Friday/Saturday, with peak surf heights
approaching warning levels along exposed N and W facing shores.
Elsewhere, trade winds will deliver short-period wind waves
through the week that may increase this weekend, as east to
southeast winds increase near and upstream of the islands. Surf
may reach advisory levels along E facing shores this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST this evening for Niihau-Kauai-
Oahu.

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

High Surf Advisory until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala-Kona-Big Island North and East-Kohala.

Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Small Craft Advisory until 6 PM HST this evening for all
Hawaiian waters except Maalaea Bay-

&&

DISCUSSION...Morrison
AVIATION...TS
MARINE...Birchard