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Hawaiian Islands Synoptic Discussion and Guidance

693
FXHW60 PHFO 241416
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
416 AM HST Fri Jan 24 2020

.SYNOPSIS...
A stable land and sea breeze pattern will continue through
this afternoon for most islands under a weak subtropical ridge. A
weakening cold front will move into Kauai and Niihau later
tonight, spreading clouds and showers over both islands through
Saturday. Trade winds also return late Friday afternoon through
Saturday time period spreading windward and mountain showers to
all islands. Winds shift towards a more southeasterly direction
over the western islands on Sunday and Monday pushing the remnants
of the front northward away from the state. Land and sea breezes
are forecast in a stable light wind pattern from Tuesday into
Thursday. Another cold front may bring wet weather trends in the
extended forecast for next weekend.

&&

.DISCUSSION...
Broken clouds over the western islands of Maui and Hawaii Counties
are associated with low level wind convergence around the high
pressure ridge lingering over the Hawaiian Islands. These clouds
are fairly stable under the trade wind subsidence inversion and
are not high enough to produce more than isolated rainfall. Low
cloud ceilings will linger along the north and windward slopes of
Maui and the Big Island today with diminishing coverage elsewhere.
A weakening cold front shows up on this mornings satellite imagery
roughly 150 miles northwest of Kauai. This frontal zone will move
over the island later this evening.

Today through Sunday, a cold front will approach Kauai from the
northwest will spread clouds and showers across the Garden Isle
later this evening. The forward motion on this front will stall
out with clouds and showers lingering over Kauai and Niihau
through Saturday. Trade winds will bleed out ahead of this cold
front later this afternoon, spreading windward and mauka showers
eastward down the island chain through Saturday. Moderate trade
winds will continue to blow across the eastern half of the state
through Sunday. Wind directions across the western islands will
veer towards a more southeasterly direction on Sunday and Monday.
These more southerly winds will push the remnant cold frontal
clouds and showers northward away from the western islands with
improving weather conditions forecast over Kauai County.

Monday through Friday, a large scale trough anchored over the
Central Pacific far north of the islands will bring another round
of stable light winds with local scale land and sea breezes through
the work week. Long range guidance shows another weakening cold
front moving into the western islands on Friday. Additional clouds
and showers are possible over Kauai and Oahu by next weekend and
wet weather trends were introduced in the extended range forecast
grids from next Friday onward. However, large differences remain
between the American (GFS) and European (ECMWF) models. The GFS
model brings the front into Kauai and Oahu on Friday, while the
European model keeps the entire frontal boundary north of the
island chain. Expect these differences in frontal placement to
diminish as the forecast time period grows shorter.

&&

.AVIATION...
This morning, a dissipating cold front approaching the state from
the northwest, combined with surface high pressure over the islands,
will again allow for light and variable flow. Expect VFR conditions
with most cloud bases above 3000 feet. However, a weak moisture
push from the southeast will bring a slight chance of showers to
the eastern portions of the Big Island.

For this afternoon, sea breeze development appears likely along
most coastal regions with mainly dry conditions. As the
aforementioned front approaches this evening, weak tradewind flow
may return to Kauai and Oahu.

No AIRMETS are anticipated at this time.

&&

.MARINE...
Light and variable winds will continue this morning, before
shifting around to the north and northeast by late this
afternoon, then increasing to moderate and fresh levels tonight.
Locally strong easterly trade winds will then prevail through the
weekend. A new front will approach and stall out to the west of
the islands early next week, diminishing the winds over the
western islands while moderate to locally strong east-
southeasterly winds hang on in the unsheltered areas around Maui
and the Big Island. Another front approaching from the northwest
during the middle and latter part of next week, will diminish
winds further, with light and variable winds prevailing across
most of the island chain.

Seas have dropped below Small Craft Advisory (SCA) thresholds
early this morning, therefore the advisory has been cancelled.
Seas are expected to rise to SCA levels across a large chunk of
the marine area again tonight and Saturday as a new large
northwest swell arrives. Trade winds will also strengthen to SCA
levels across portions of the marine area this weekend. SCAs will
likely be required for most marine zones through the weekend due
to a combination of winds and seas, with some areas holding onto
SCA level winds and/or seas into early next week.

Surf will keep pumping along north and west facing shores as a
progressive weather pattern across the northern Pacific keeps a
series of large northwest swells moving through the islands. The
current swell has dropped below High Surf Advisory (HSA) levels,
therefore the advisory has been cancelled.

A new large northwest swell will fill in from northwest to
southeast down the island chain late tonight into Saturday with
surf peaking at warning levels on Saturday. This swell will be on
a downward trend Sunday, but may still hold at warning levels
during the day before dropping to advisory levels Sunday night.
As this swell eases early next week, advisory level surf may hold
through Monday before dropping below HSA threshold Monday night
and Tuesday. Over the Big Island, this swell will likely bring
advisory level surf to exposed west facing shores over the
weekend. Another large northwest swell will be possible during the
middle to latter part of next week.

Surf along exposed east facing shores will remain small through the
period due to the lack of persistent trades locally and upstream
across the eastern Pacific.

Surf along south facing shores will remain small through next week.
A couple background south swells expected over the weekend and
again on Tuesday should be enough to keep things from going flat.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Jelsema

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



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