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Forecast Discussion for Honolulu, HI

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000
FXHW60 PHFO 230134
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
334 PM HST Wed Jan 22 2020

.SYNOPSIS...
Variable winds and a stable land and sea breeze pattern will
prevail over most of the state through Friday afternoon. Expect
clouds and isolated light interior showers each afternoon
followed by overnight clearing across most islands, though
southeast slopes of the Big Island will experience a slight
increase in showers Thursday. A weak front will stall near Kauai
late Friday, bringing an increase in clouds and showers for the
northern end of the island chain into Saturday. Moderate trade
winds will return for the weekend, then decrease and veer
southeasterly early next week.

&&

.DISCUSSION...
Stable and mainly dry conditions persist. The subtropical ridge
has been pushed southward to just north of Kauai, putting most of
the island chain under a southeasterly background flow and south
winds near Kauai. The atmosphere is stable and rather dry, with a
strong inversion based between 4,000 and 7,000 ft and precipitable
water values running about 20 percent below January normal. As a
result, day time sea breezes have produced clouds over interior
sections, but only a few gages have measured any rainfall this
afternoon.

Little change is expected over the next 48 hours. The subtropical
ridge will be pushed over the islands by an approaching front. A
stable land and sea pattern will hold over most areas with minimal
shower activity favoring afternoon hours. An area of moisture
will move up over the Big Island from the southeast on Thursday,
providing a slight boost in shower activity over mainly the Kau,
Puna, and Hilo Districts through Friday. We will also watch the
small pocket of moisture passing just west of the state, as it may
provide a small increase in shower chances over interior Kauai
tomorrow afternoon.

The weakening front will drift southward and stall near Kauai
late Friday and Saturday. Kauai will likely experience an
increase in clouds and showers, while surface high pressure
building to the northwest drives increasing trade winds state-
wide. A persistent mid level ridge will maintain stable
conditions, with modest showers confined to windward slopes.

Trades will decrease and will likely become disrupted again early
next week. Another approaching front will push the subtropical
ridge southward, leading to southeasterly winds on Monday and
possibly another round of light and variable winds by Tuesday.
Stable conditions should prevail.

&&

.AVIATION...
Sea breezes have delivered on their promise of afternoon interior
cloud build ups across the island chain. The Big Island is a bit
of an exception, as southeastern interior areas have remained
basically cloud-free. The region from Kona to Upolu Point to
Hilo, however, is experiencing quite the opposite with visible
satellite showing a fair amount of cloud cover. As land breezes
take over tonight, expect clearing conditions across all islands.
A repeat of these conditions is on tap for Thursday.

No AIRMETS are in effect and none are anticipated.

&&

.MARINE...
A rather active period for high surf is expected for the remainder
to the week and on through the weekend as strong low pressure
systems pass from west to east across the North Pacific. These
low pressure systems will be rather robust swell producers. A
large and very long period northwest swell as been gradually
rising today as indicated by both the nearshore and offshore
buoys. This swell will peak tonight, then lower gradually Thursday
through Friday. A High Surf Warning is currently posted for most
north and west facing shores. Another large northwest swell is
expected to build late Friday and Friday night, peak on Saturday,
then lower gradually Saturday night through Sunday. Warning level
surf is once again expected during the peak of this swell. A
slightly smaller advisory level northwest swell is expected early
next week.

Only small background south swells are expected through the week
and on through the weekend. Light winds will keep surf small along
east facing shores with a slight increase in short period choppy
later this weekend as the trade winds return. However some south
and east facing shores may see some localized higher surf over the
next several days from the large northwest swells refracting
around islands and reefs.

High pressure is centered far to the east-northeast of the area
with a ridge extending westward to a position just north of Kauai.
This is resulting in light southeast winds across the area. A
front will be approaching the area from the northwest Thursday and
Friday with south to southwest winds developing over the west end
of the state. The front will stall and dissipate over the area
Saturday and Sunday. Winds will remain below Small Craft Advisory
(SCA) levels through Friday but will likely increase over the
weekend as a new high build in to our north. Winds may reach SCA
levels over the typically windy waters around Maui County and the
Big Island. However, the large northwest swells mentioned above
will likely cause seas to reach SCA levels over exposed waters at
times.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai, for north
facing shores of Maui, and for west facing shores of Big Island.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Dye
MARINE...Burke

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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