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Hawaiian Islands Synoptic Discussion and Guidance

393
FXHW60 PHFO 270602
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
802 PM HST Sun May 26 2019

.SYNOPSIS...
A strong high pressure system far northeast of the Hawaiian
Islands will interact with a stalled cold front northwest of
Kauai. Trade winds will decrease over the next few days,
strengthening land and sea breezes over leeward slopes of each
island. Scattered afternoon showers will develop over mountain and
interior sections of each island with decreasing trends each
night. By Wednesday the high pressure ridge north of the island
grows stronger pushing the stalled front away from the state
allowing moderate trade winds to return to the region through next
weekend.

&&

.DISCUSSION...
This evening's satellite imagery shows a few high cirrus clouds
passing over the state with stable low clouds drifting in along
the easterly trade winds. Isolated to scattered showers remain in
the forecast this evening favoring windward and mountain areas.
Daily high temperature records have been broken for several
locations this week due to this warm and stable weather pattern.

Across the East Pacific basin we see a high pressure center far
to the northeast of the Hawaiian Islands with a ridge from this
system just north of the islands producing moderate trade winds.
A stalled cold front lingers a few hundred miles northwest of
Kauai keeping our islands along the edge of the high pressure
ridge. The high center will drift slowly to the east over the
next few days. A subtle weakening in the ridge north of the
islands will decrease the trade winds from Memorial Day into
Tuesday. The trade winds will also veer towards the east-southeast
direction.

Memorial Day through Tuesday...Decreasing large scale trade winds
will allow local scale land and sea breezes to grow stronger.
Expect sea breezes to form during the day-light hours along
leeward slopes of each island. Clouds and scattered showers will
form over each island favoring mountain ranges, interior sections,
and sea breeze to background wind convergence zones. Model cross
sections are showing cloud depths up to 10,000 feet during this
period with precipitable water amounts over the islands increasing
through Tuesday. This means shower activity should trend higher
from Monday night into Tuesday.

Wednesday through Sunday...The high pressure ridge will push the
stalled front northward away from the islands on Wednesday allowing
pressure gradients over the state to increase into next weekend.
Expect a return to moderate easterly trade winds through next
Sunday with clouds and showers favoring typical windward and mauka
areas.

&&

.AVIATION...
High pressure northeast of the state will keep a light to moderate
trade wind flow in place tonight, with clouds and showers favoring
windward and mauka areas. The trades will ease on Memorial Day,
with sea breezes developing in many leeward areas. Showers will
favor windward and mauka areas during the morning hours, with
showers then transitioning to leeward and interior areas during
the afternoon.

No AIRMETs are in effect at this time.

&&

.MARINE...
Moderate to fresh easterly trades associated with a ridge north
of the islands will trend down into the light to moderate
category through the first half of the week. Localized land and
sea breeze conditions near the coasts will become a possibility as
the winds drop off. A return of fresh to strong trades is
forecast through the second half of the week as the ridge
strengthens.

Surf along south facing shores has been building all day today
due to a long-period south swell that has filled in. Local
observations from the lifeguards depicted advisory-level surf at
some locations today. This combined with observations at the
offshore buoys a couple of hundred nautical miles to the south
reflecting similar energy on the way has resulted in an evening
update to include a High Surf Advisory for south facing shores of
all islands through Monday afternoon.

As the current south swell eases late Monday into Tuesday, a new
long-period south swell (slightly smaller) will fill in Tuesday
and hold through midweek. This overlapping trend will continue
into the second half of the week another small south swell moves
through Thursday through Friday.

The late season small northwest swell that filled in over the
weekend will gradually lower through Tuesday. A similar northwest
swell (slightly smaller) will become a possibility by Thursday in
response to a decent sized pocket of strong breezes focused at
the islands this evening around 1500 nautical miles to the
northwest.

Surf along east facing shores will quickly lower through the
first half of the week as the trades back off. An upward trend
will be likely through the second half of the week as fresh to
strong trades return.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for south facing shores
of all islands.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Gibbs

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



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