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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 230904 AAA

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
1104 PM HST Tue Jun 22 2021

Forecast updated for leeward Big Island where showers have
developed in the Kawaihae area.


A ridge of high pressure 320 miles north of Kauai this evening
will continue to move through Wednesday. The ridge is expected to
stall around 220 miles north of Kauai Thursday, then head north
Friday. This action translate to trade winds continuing over the
area to Thursday, then give way to lighter winds with a slight
south component on Thursday night and Friday. Trade winds return
and strengthens Friday night through Sunday.


A 1026 mb surface high is centered 1350 miles far northeast of
the islands this Tuesday evening. A ridge extends southwestward to
about 320 miles north of Kauai. The high will be heading toward
the northeast, then stall off the Oregon coast over the weekend.
The surface ridge will be moving south in the mean time, becoming
nearly stationary 220 north of Kauai Thursday. This action will
cause the light to moderate trade winds to weaken and shift to the
east and east- southeast on Thursday night and Friday. With this
wind flow, Maui and Big Island will bringing on a mix of a
background easterly flow over water and a daytime sea and
nighttime land breeze over the remaining islands. The convergent
sea breeze overland, especially the lee and interior areas, will
result in some afternoon clouds and showers there.

Latest satellite and weather radar imagery shows more low clouds
and showers upwind of the islands than last night. So it will be
wetter than last night mainly across the windward and mountain
areas, which cover all islands exposed to the trade wind flow.
Do expect a few lighter showers to reach downwind into the lee
areas of the smaller islands. The frequency of these clouds and
showers will be less by mid or late Wednesday morning, with
isolated to scattered showers continuing over the windward and
mountain sections Wednesday afternoon. We will likely be cycling
through this process Wednesday night into Thursday, followed by
slightly less showers Thursday night into Friday.

The showers coming in now could be aided by an approaching upper
level short wave trough from the north, which stalls over the
islands between Thursday and Friday. Some weakening to the low
level inversion is expected, which rises to around 10k feet from
the current 6 to 7k feet areawide. This environment will tend to
support some isolated to scattered afternoon showers.

The upper trough moves south of the Big island Friday night,
followed by a weak ridge aloft. At the surface, a new surface
comes into play with trades returning to the islands Friday night
and Saturday, and strengthening to locally strong by Sunday.
It will hold at this range through early next week. The rain
pattern switches back to trades with pretty much a normal night
and morning routine. Models indicate it won't be dry as of late.


Light to locally moderate easterly trade winds are forecast to
remain in place throughout the next 24 hours. An area of surface
high pressure remains rooted far northeast of the state with weak
ridging north of Kauai. Clouds and showers will favor east and
northeast facing slopes and coasts throughout the period,
especially in the late night and early morning hours. VFR
conditions will prevail for the most part, with only isolated MVFR
conditions in heavier showers that do occur. Sea breezes are
possible in sheltered leeward areas Wednesday given the relatively
light background flow, which would promote increased cloud cover
during the afternoon hours.

There are currently no AIRMETs in effect.


Light to moderate trades will prevail through Thursday, then turn
light east to southeast over the open waters Thursday night into
Friday as a surface ridge stalls some 180 nm north of Kauai.
The ridge will begin to head north Friday with trade winds
returning Friday night into Saturday. The returning trade will
then strengthen to moderate to locally strong levels Sunday
through early next week. Small Craft Advisories may be needed for
the windier waters around Maui and the Big Island at that time.

The current south swell will continue to lower tonight and
Wednesday. A series of small south swells will build surf up near
the summertime average again Thursday through early next week. A
series of small northwest swells will keep some small surf in
place along north facing shores through early next week, with the
peak day looking to be Saturday, where slightly overhead surf
will be possible. Surf along east facing shores will remain small
and below the summertime average through early next week.






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