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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 171933

Area Forecast Discussion
National Weather Service Honolulu HI
933 AM HST Thu Jun 17 2021

Locally breezy trade winds will decline during the next couple of
days, then remain at moderate strength through early next next
week. Showers will remain focused along windward slopes, though a
nearby disturbance aloft could produce a few briefly heavy showers
over the western end of the state through Saturday. Weaker trade
winds are possible by Tuesday.


Locally breezy trade winds persist, while a nearby upper level low
produces some instability. A 1028 mb high is sitting about 1,500
miles northeast of the state, and an associated surface ridge
stretches to about 700 miles north of Kauai. These features are
driving the locally breezy easterly trade wind flow, which will
hold today. A narrow band of moisture has been focusing showers
along windward slopes from Oahu to the Big Island since last
night and will progress westward today. A weak upper level low
centered 150 miles northwest of Kauai is producing some
instability, and with the moisture band expected to reach the
Garden Isle this afternoon, a few heavy showers will be possible.

Trade winds will ease on Friday as the surface ridge to the north
weakens. Trades will then hold at moderate strength through early
next week. The upper level low will drift eastward through
Saturday then lift to the north Sunday and Monday. This feature
will maintain some instability through Saturday around Kauai and
Oahu, where a briefly heavy shower will remain possible, and over
the higher slopes of leeward Big Island which could see short-
lived convection each afternoon. Otherwise, showers will favor
typical windward slopes. Periods of high clouds will also occur.

A further decrease in trade winds is possible around Tuesday as
the surface ridge to the north is eroded. The upper level trough
will fill in and mid level ridging will strengthen, leading to
stable conditions and limited rainfall under a weak trade wind
flow with afternoon sea breezes.


Moderate easterly trade winds will continue to stream occasional
clouds and showers across north through southeast sections of the
islands today and tonight. Mid-morning satellite imagery reveals
that an abundance of moisture embedded within prevailing flow
continues to stream onshore over windward sections of the Big
Island and Maui. MVFR observations (and even isolated IFR
visibility in heavier showers) have been common throughout the
morning at Hilo and should continue throughout the day. MVFR
conditions have been more isolated for the remaining smaller
islands, but as the band of moisture spreads over the western half
of the state later this morning, an increase in clouds and
showers is possible here as well.

For now, AIRMET Sierra for mountain obscuration remains in effect
for windward Maui and Big Island, but this may need to be expanded
to include portions of Molokai and Oahu later this morning or
early afternoon.


High pressure northeast of the islands will keep moderate to
locally strong trade winds in place through tonight. A Small
Craft Advisory remains in effect through 6 PM this evening for the
windier waters around Maui and the Big Island, but may need to be
extended through tonight. Trade winds are expected to begin
easing on Friday, then lower into the light to moderate range this
weekend through early next week, as a couple of fronts approach
and stall out well northwest of the island chain.

Surf along south facing shores will remain near or above the
summer average during the next 7 days, as a series of long-
period south swells move through the island chain. South shore
surf will slowly build during the next several days, peak Saturday
night through Monday near High Surf Advisory levels, then
gradually lower into the middle of next week. Surf along north
facing shores will remain very small through the weekend. A couple
back to back small northwest swells could give north shore surf a
boost Sunday night through the middle of next week. Surf along
east facing shores will hold fairly steady at slightly below
seasonal levels through the middle of next week.


Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman