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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 151330

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Fri Jan 15 2021

Generally light winds and mostly dry weather is expected today as
high pressure moves off far to the northeast of the area. A front
is expected to approach and gradually dissipate just northwest of
Kauai tonight and Saturday. Some increase in showers is possible
over the western end of the state with this front. A much
stronger front is then forecast to move into the area Sunday
through Monday, possibly stalling out over the eastern end of the
state through the middle of next week. Rainfall chances increase
statewide with the passage of this front, with thunderstorms and
locally heavy rainfall possible. Cool and breezy northerly flow
is expected following the frontal passage. Trade winds may return
late next week.


Currently at the surface, a 1030 mb high is centered around 2000
miles northeast of Honolulu, with ridging extending southwestward
and over the western islands. Meanwhile, a cold front is located
around 475 miles west-northwest of Kauai. The resulting gradient
is producing light to moderate east-southeasterly winds around the
Big Island, while winds remain light and variable over the rest of
the state. Infrared satellite imagery shows clear to partly
cloudy skies across most of the state, although mostly cloudy
skies conditions are in place over Kauai. Radar imagery shows a
few showers moving through Kauai and into windward Big Island,
with rain free weather over Oahu and Maui County. Main forecast
concern continues to revolve around the potential for unsettled
weather late in the weekend into early next week.

Today through Saturday...
The front far to the northwest of the state will track steadily
southeastward toward the island chain today. Light to moderate
trades will hold in place over the eastern islands during this
time, while light and variable winds will allow for land breezes
and sea breezes to develop over the western islands. The front
will stall out and gradually dissipate around 100 miles northwest
of Kauai tonight and Saturday, while a weak high builds eastward
well to the north of the island chain. This should bring a brief
return of trade winds to the area late tonight, with trades
strengthening to moderate and locally fresh levels on Saturday.
Relatively dry conditions will prevail through the period as deep
layer ridging holds in place over the islands.

Saturday night through next Thursday,
The next cold front will approach from the west Saturday night,
then move through Kauai late Sunday. The global models have come
into better agreement with the 00z model cycle, showing the front
pushing eastward through Oahu and Maui County Sunday night, then
stalling out over or just east of the Big Island Monday through
Wednesday. Cool and breezy conditions are expected following the
frontal passage, at least over the western end of the state. A new
strong high then appears to build northeast of the islands late
next week, swinging the old frontal remnants back westward across
the island chain, and bringing a return of trade winds.

Confidence has increased on the potential for some locally heavy
rainfall in association with this stronger front beginning late
Sunday and continuing through at least Wednesday and possibly
right through Thursday. At the moment, the models appear to be
indicating a higher potential for a long duration heavy rainfall
event would be over the eastern end of the state. Given that the
models have just begun to come into better agreement however,
prefer to wait another model cycle or two before ironing out the
details. As a result, will continue to keep rain chances in the
forecast statewide throughout the extended portions of the
forecast. Will also maintain the mention of thunder and locally
heavy rainfall along and in close proximity to the front as it
moves through the area Sunday through Monday night when
confidence in timing details is the highest. Future shifts may
need to extend the mention of thunder and locally heavy rainfall
to portions of the area through Wednesday and possibly next
Thursday, so stay tuned.


Showers will continue to threaten the southern half of Kauai until
shortly after sunrise as the convergent southerly winds gradually
become light and variable as the morning progresses. This light
and variable wind pattern will favor an onshore sea breeze flow,
but an inversion will be strong enough to hold down the
convective showers to isolated. The return of the trades by
tonight means there will be an uptick in showers favoring the
windward and mountain areas.

This shift in the winds is being caused by an approaching front from
the northwest. The front is forecast to stall and fall apart a short
distance from Kauai, but in doing so, some of this moisture may leak
over to Kauai tonight.

No AIRMETs in effect, and none are expected today.


Two cold fronts will move into Hawaiian waters over the next
several days. Winds will veer from the southeast and weaken
today ahead of a cold front approaching from the northwest.
Another, stronger front is forecast to move down the island chain
from the northwest Sunday through Monday, bringing strong
northerly winds to the western half of the state and possible
thunderstorms along the front as it passes through each island. A
Small Craft Advisory (SCA) remains in effect for most coastal
waters until Friday evening for elevated seas. An extra large
northwest swell will build into the northwest waters by early
Saturday morning with a return to SCA conditions for elevated

A series of large to extra large northwest swells will impact the
state through early next week. The current northwest swell will
slowly trend lower today. Surf heights will remain at advisory
levels today for north and west facing shores of the smaller
islands (north facing shores of Maui and the Big Island).

The first extra large northwest swell will arrive this evening,
quickly pushing surf well above warning levels by Saturday
morning. This new northwest swell will potentially become the
largest surf event of the season so far. Surf will trend slowly
lower Saturday night through Sunday, but remain well above warning
levels through the weekend.

Another extra large northwest swell is expected to build into
Hawaii waters on Monday, producing another round of warning level
surf. This swell begins to diminish Tuesday as another north
swell fills in, with moderate swell heights holding through

Some larger surf may wrap into south and east facing shores
exposed to large northwest swells. Otherwise, surf along south
and east facing shores will remain small through the first half of
next week.


High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

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




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