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Hawaiian Islands Synoptic Discussion and Guidance

616
FXHW60 PHFO 270628
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Thu May 26 2022

.SYNOPSIS...
Locally breezy trade winds will persist into Friday, followed by
weaker trade winds this weekend. In addition, stable and dry
atmospheric conditions will remain in place. High clouds will
start to cover skies across much of the state as we head into the
weekend, with minimal low clouds and trade showers along windward
facing slopes. The trade winds are expected to be weak enough by
Sunday to allow afternoon sea breezes and nighttime land breezes.
Expect the trade winds to gradually strengthen early next week.

&&

.DISCUSSION...
A 1020 mb surface high pressure system is near 32N 148W, or about
950 miles north-northeast of Hilo. The pressure gradient south of
this feature, which is moving slowly east, is maintaining locally
breezy trade winds across most of the state early this evening.
The atmosphere remains dry and stable over the island chain. Low
clouds cover most of the lower elevations of the Big Island at
dusk, but these are beginning to thin as nighttime cooling
becomes established. There are also scattered to broken low
clouds along most windward sections of the smaller islands. Radar
reflectivities show very limited rainfall within these clouds.
Note that in addition to the low clouds, a large area of high
clouds (mainly cirrus) is lurking over the open ocean south and
southeast of the Big Island. This is indicative of a subtropical
jet stream that is setting up south and southeast of the area.

The forecast guidance continues to show the surface high far
north-northeast of the area will move slowly eastward and weaken
from late Friday into this weekend. This will cause the trade
winds to gradually ease. At the same time, the large area of
cirrus clouds south and southeast of the Big Island will shift
northward over portions of the island chain as the subtropical
jet migrates closer to the islands. The dry and stable
atmospheric conditions will also continue to limit the amount of
low clouds and trade showers over windward facing slopes and
coasts. The greatest coverage of windward showers will remain
late at night and early in the morning.

By Sunday, the background trade wind flow is expected to be weak
enough to allow alternating local afternoon sea breezes and
nighttime land breezes. A weak mid-level trough located just west
of Kauai may phase with a much broader trough aloft to the north.
The subtropical jet will also likely remain near the eastern end
of the state. These features aloft may weaken the low-level trade
wind inversion slightly, but low- and mid-level moisture will
remain limited. The sea breezes may therefore allow some clouds
to develop over leeward and interior sections during the
afternoon, but only spotty showers are expected. The nighttime
land breezes will clear out most of the low clouds over these
same areas after sunset.

The global forecast models indicate the trade winds will
gradually strengthen from Memorial Day through early next week.
At the same time, a mid-level ridge will reestablish its
influence over the state, This will cause the atmosphere to
become stable again. The dry weather pattern will mean limited
low clouds and only modest trade showers over windward and mauka
sections next week.

&&

.AVIATION...
A high pressure ridge north of the islands will continue to keep
moderate to locally breezy easterly trade winds blowing across the
region on Friday. A slight moderation of these trade wind speeds
will occur this weekend with some afternoon sea breezes forming
over terrain sheltered western slopes of each island. Otherwise
fairly stable and dry conditions will result in limited cloud and
shower coverage.

A subtropical jet stream south of the region will drift northward
over the islands with potential for producing moderate clear air
turbulence over the islands from FL280/350 by Friday morning.
AIRMET Tango may be needed to cover this event. No AIRMETs are
currently in effect.

&&

.MARINE...
Fresh to strong easterly trade winds will continue through
Friday, then trend down into the moderate to locally fresh
category over the weekend as the ridge weakens and shifts
southward into the area. The wind direction will respond and
shift out of the east to southeast for a brief period Saturday
night through Sunday, which will lead to localized land and sea
breeze conditions near the coasts over the western end of the
state. A return of fresh to strong easterly trade winds is
expected early next week as the ridge retreats northward.

Surf along south facing shores will trend up Friday into the
weekend, then again next Tuesday through midweek as overlapping
south swells associated with an active pattern near New Zealand
move through. Surf heights could reach the advisory level of 10
ft (face value) late Friday through Saturday night, then to the
warning level of 15 ft (face value) late Tuesday through
Wednesday as the second swell moves in and peaks. Long-period
forerunners (23 second periods) associated with the first south
swell have already begun to show at our PacIOOS Barbers Point
buoy this evening. We will continue to monitor the offshore buoy
observations overnight and fine tune our numbers for Friday and
Saturday.

Surf along exposed north and west facing shores will remain up
into Friday due to a small, north-northwest swell moving through.
This should ease late Friday into Saturday as the swell moves
out. A similar northwest swell (medium period) is expected by
Sunday, which will linger into Monday before easing.

Surf along east facing shores will remain small and choppy each
day with the trades in place. A brief downward trend is
anticipated by Sunday as the winds ease and shift out of the east
to southeast.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Houston
AVIATION...Bohlin
MARINE...Gibbs

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



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