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Hawaiian Islands Synoptic Discussion and Guidance

522
FXHW60 PHFO 220657
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
857 PM HST Wed Oct 21 2020

.SYNOPSIS...
A humid and light wind situation will persist over Kauai and Oahu
for the next day or two, leading to periods of locally heavy
showers. A drier and more stable air mass will hold over the
eastern islands through Friday. Over the weekend, an area of
clouds and moisture may approach the islands from east, resulting
in a boost in windward showers for the eastern islands.

&&

.DISCUSSION...

The weather map shows what use to be a front and now a trough
just west of Kauai and Niihau. This trough is oriented northeast
to southwest. And what appears to be a trough lies northwest to
southeast across Oahu. Minor perturbations in the southwest flow
aloft, plus other factors such as daytime heating, etc., are what
helped set off the spotty heavy showers/isolated thunderstorm on
Oahu this afternoon. This latest round of heavy showers is
dissipating as it moves into the waters off Honolulu/Waikiki.
Isolated showers are present across Oahu now, but they should be
diminishing in the coming hours as a land breeze takes over.
Although the Kauai radar is rather quiet over land, there are
scattered showers advancing towards the island from the southwest.
So, we will be monitoring Kauai/Niihau closely overnight.

A new upper level low will be appearing some 600 miles north of
Kauai later tonight, with the trough axis shifting east from Kauai
to over Oahu. This shift may nudge the surface trough west of Kauai
to over Kauai on Thursday before dissipating over Kauai Channel
Thursday night and Friday. The trough over Oahu stays in place
through Thursday, then dissipates through Friday as well. The
light wind caused by these features combined with daytime heating
will lead to some spotty heavy showers for especially Oahu. Maui
County may see an isolated afternoon shower or two. Kauai may
encounter periods of showers into Friday, whether it is day or
night.

The upper low weakens Friday night. On Friday and the weekend, a
new front will stall a couple of hundred miles west of Kauai. The
dissipation of the trough will allow the surface ridge over Maui
to move just north of Kauai on by Saturday evening. This will
bring on a light southerly wind flow to Kauai, and a light to
moderate east to southeast flow elsewhere. This pattern is
expected to hold through Sunday. But an area of clouds and showers
will be nearing the Big Island early Saturday morning. As the
moisture moves up the island chain, it'll bring some windward
showers to at least Maui and the Big Island over the weekend.

Another north Pacific trough will dig toward the state early next
week, likely weakening winds again and possibly leading to an
increase in showers across portions of the state.

&&

.AVIATION...
A low pressure system far north of the Hawaiian Islands will keep
light and variable winds across the region with land and sea
breezes over each island.

Unsettled weather will continue over Kauai, Niihau and Oahu for
the next 24 to 48 hours. Overnight shower activity over the
these islands will remain in the isolated to scattered range,
this shower activity will increase again by afternoon into the
scattered to numerous range. Isolated thunderstorms are possible
in this unstable environment. Expect periods of MVFR conditions
with local IFR conditions possible in heavy showers for these
western islands.

More stable and drier conditions are forecast over Maui and
Hawaii Counties with sea breezes building cloud ceilings over the
eastern islands each afternoon, with land breeze clearing at
night.

No AIRMETs in effect. An AIRMET for Tempo Mountain Obscuration may
be issued on Thursday for low cloud ceilings over the islands of
Kauai and Oahu.

&&

.MARINE...
There appears to be a weak surface trough just northwest of Kauai
aNd a weak northeast to southwest oriented surface ridge near
Maui County early this evening. These features are keeping light
and variable winds in the vicinity of the smaller islands, while
light to moderate easterly trade winds are over the waters just
east and southeast of the Big Island. In addition, abundant low
level moisture and an upper level low will keep the atmosphere
unstable. This will allow locally heavy downpours and a chance of
thunderstorms to continue over the coastal waters in the vicinity
of Oahu and Kauai, as well as the northern offshore waters.
Looking ahead to early next week, the global forecast models show
a front moving into the northwestern offshore waters Monday night.
This will keep light winds over the smaller islands early next
week.

The current northeast swell, which has been producing moderate
surf along east facing shores, will continue to slowly subside
through Friday. A small, medium period south swell will continue
through the weekend. Surf along other shores will remain small
through Friday. A new long period north-northwest swell arriving
late Saturday may peak near the High Surf Advisory threshold
along north facing shores of some of the smaller islands Sunday.
This north-northwest swell will gradually subside from Monday
into mid-week.

Peak monthly tides and higher than normal sea levels may produce
local coastal flooding around the time of the high tide Thursday
morning. See the Special Weather Statement for more information
about these nuisance coastal flooding conditions.

&&


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

&&

$$


H Lau/Bohlin/Houston

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