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Hawaiian Islands Synoptic Discussion and Guidance

762
FXHW60 PHFO 172006
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1006 AM HST Sun Jan 17 2021

.SYNOPSIS...
A wet pattern is on tap for the islands during the first half of
the coming week. This comes about as a front moves down the island
chain this afternoon and tonight, then stalling around Maui and
the Big Island between Monday and Tuesday. The front, or trough,
is then slated to drift back to Oahu and Kauai between Tuesday
afternoon and Wednesday. The front will have good support aloft
from an upper level disturbance, resulting in heavy showers and
isolated thunderstorms. On top of the rain, strong northerly
winds will follow in behind the front. Moderate to strong easterly
trade winds return to the region during the second half of the
week with a bit of a wet flavor.

&&

.DISCUSSION...
The leading edge of a cold front is located about 160 miles
northwest of Kauai this morning. This front is moving toward the
state at around 20 mph. For rest of the day today, we will see
light to moderate southerly winds ahead of the front which will
bring clouds and scattered showers across the state. As the front
nears Kauai this afternoon into evening, increasing chances of
heavy rain and isolated thunderstorms are expected. A brief burst
of heavy rain is likely as the front quickly makes its way
through Kauai late this afternoon into the evening.

As the front makes its way through Oahu and Maui County tonight,
we should see the front slowing and eventually stalling near Maui
and the Big Island on Monday. With the forward speed of the front
slowing, this will bring a greater threat for flooding concerns,
thus a Flash Flood Watch is in effect for Oahu and Maui County
starting tonight and continuing through Tuesday. Behind the front,
we will see strong (15 to 30 mph) northerly winds fill in and
will likely lead to an issuance of a Wind Advisory by Monday for
the smaller islands.

Although the front looks to dissipate on Tuesday, moisture
associated with the front will continue to linger near Oahu and
Maui County. The strong northerly winds should taper off by then,
and breezy trades should return. The trade winds should focus the
showers across windward and mountain areas. A trade wind pattern
is expected for the second half of the week but due to lingering
moisture, we will likely see a more of a wet trade wind pattern.

&&

.AVIATION...
A front will reach Kauai and move down the island chain later
today through tonight. While clouds and showers are associated
with the frontal cloud band, showery prefrontal cloud areas are
affecting the islands now. The bulk of this activity is confined
to Kauai and adjacent waters but will gradually shift east.
Prevailing VFR is therefore forecast for Oahu through Big Island
for the balance of the daylight hours.

For Kauai, the probability of MVFR will begin increasing over
Kauai by this afternoon as low-level moisture increases. Issuance
of AIRMET Sierra for tempo mountain obscuration is likely by this
evening. The arrival of the cold front is expected for Kauai
around 02z/4pm HST and around 06z/8pm HST for Oahu and will be
marked by a greater coverage of showers, some locally heavy,
followed by abrupt shift to gusty northerly winds.

&&

.MARINE...
An approaching cold front has pressed a ridge axis over the state.
This has created weakly convergent, light to moderate south to
southwesterly winds across the majority of the waters. The front
will enter the nearshore Kauai waters late this afternoon and make
it into Oahu's waters later this evening. The front is forecast
to stall out over the central island chain going into early Monday.
The approach and passage of the front will introduce locally
heavy showers and the potential for isolated thunderstorms. The
movement of the upper trough over the western islands through
Tuesday will maintain this shower and thunderstorm potential.

A High Surf Warning (HSA) is in effect for exposed north and
west-facing shores through Monday night. Despite a brief lowering
of surf below warning levels on Kauai this morning, along with
surf falling below warning levels along the remaining smaller
isles by tonight, the HSA will remain in effect to account for the
next large northwest swell's arrival tonight. This powerful
northwest swell will build tonight through Monday and push surf
back up to warning levels along north and west-facing smaller
island shorelines through Monday morning and north-facing Big
Island shores Monday afternoon. Peak surf heights late Monday into
early Tuesday are expected to be slightly lower than they were
during this most recent weekend XL surf event. Strong north to
northeast winds in association with an early week cold front will
result in lesser quality wave shapes during the passage of the
highest swell. While these most recent swell directions of around
330 degrees are not traditionally favorable for significant
impacts within both Kahului and Hilo Bay Harbors (as with a more
north swell direction), there is a Marine Weather Statement in
effect to address the potential for some harbor surge impacts.

A Small Craft Advisory (SCA) remains in effect for all waters
except Maalaea Bay through the day as passing northwest swells
continue to produce greater than 10 foot seas. Seas will diminish
today with a slowly subsiding swell. Seas will again be on the
rise tonight and Monday with a newly arriving northwest swell. A
Gale Watch will be in effect beginning tonight for the Alenuihaha
Channel and westward waters as strong post-frontal north to
northeast winds may frequently gust to greater than gale force.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 PM HST this evening through Tuesday
afternoon for Oahu-Molokai-Lanai-Kahoolawe-Maui.

High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Surf Advisory until 6 AM HST Tuesday for Kona-Big Island
North and East-Kohala.

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

Gale Watch from 6 PM HST this evening through late Monday night
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-Pailolo Channel-Alenuihaha Channel.

&&

$$

DISCUSSION...Kino
AVIATION...Powell
MARINE...Blood

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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