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Area forecast discussion
National Weather Service Honolulu HI
835 PM HST sun Jul 22 2018

Synopsis...
a high pressure ridge north of the islands will keep moderate
trade winds in the forecast through the week. Periods of elevated
moisture will move up into the state from the tropics allowing
clouds and enhanced showers to develop across all islands. The
first of these wet trade events is ongoing and will continue
through Monday night for the eastern islands and Tuesday morning
for the western half of the state. A brief drying trend remains in
the forecast for Tuesday and Wednesday.

&&

Discussion...
in the big picture, a high pressure ridge remains in place far to
the north of the state keeping moderate trade winds in the
forecast through the week. An upper level trough is also located
just north of the islands with a subtropical jet stream streaming high
level cirrus clouds across the region.

In the lower levels a deep plume of moisture from the tropics has
moved into the islands from the southeast and a low level trough
shown on satellite is currently moving across the eastern half of
the state. Expect continued higher humidity levels with mostly
cloudy skies, and numerous windward showers through Monday night.
Periods of scattered showers will also drift over leeward areas.
The lava moisture plume on The Big Island, created by lava
flowing into the ocean, will interact with the additional moisture
and atmospheric instability to produce elevated precipitation
amounts near Pahoa through the overnight hours. This unstable air
will pass through the eastern islands by Monday night and the
western half of the state by Tuesday morning with drier air
filtering back into the islands from the east.

Winds aloft within the boundary layer of the atmosphere between
the 3000 to 10000 foot level has shifted to a more east-southeast
direction as shown on 00z upper air soundings from Hilo, and in
forecast model solutions through Monday night. This wind shift
will trend Kilauea volcanic emissions more towards the west-
northwest wrapping round the northern slopes of The Big Island and
lingering just south of Maui County. There is the potential for
some of this vog (volcanic smog) to reach the island of Maui by
Monday morning and we will re-evaluate the vog forecast
trajectories tonight making adjustments the forecast if needed.
East-northeasterly boundary layer winds return on Tuesday, pushing
any vog emissions towards the southwest away from the northern
slopes of The Big Island and the rest of Maui County.

Tuesday and Wednesday, drier and more stable air moves into the
state from the east with a noticeable decrease in island shower
activity. A lower trade wind subsidence inversion and decreasing
moisture levels will help to limit cloud development.

Wednesday night through Thursday, this brief drying period will
not last, as a second tropical moisture plume and weak low level
trough moving in from the east reaches The Big Island on Wednesday
night. This next round of higher moisture levels and instability
appears similar to the current round of clouds and enhanced shower
trends. Precipitable water (pw) amounts are similar to the first
wet trade wind event ranging from 1.5 to 2.0 inches. Therefore we
expect similar weather impacts with numerous showers returning
over windward and mountain areas along with periods of scattered
showers over leeward sections through Thursday night. The best
shower activity with tend to favor the overnight time periods.

Another drying trend returns on Friday and Saturday as more stable
air moves back across the region from the east. Scattered showers
will linger over windward and mountain slopes of all islands with
only isolated showers across leeward sections.

A third round of elevated moisture will arrive on Sunday. The
deeper moisture levels will remain south of the state with only
precipitable water amounts hovering near 1.5 inches in the latest weather guidance
with only minor increases to windward and mountain showers in the
forecast.

&&

Aviation...
locally breezy east winds will persist through the coming week.
Clouds and showers will increase over the smaller islands tonight as
a weak trough with additional low level moisture spreads over the
islands from the northeast. MVFR conditions could become
widespread over east facing slopes. An airmet for mountain
obscuration may become necessary by morning. The cloudy, showery
conditions are expected to continue throughout the day tomorrow.

&&

Marine...
surface high pressure far north of the islands will continue to
produce locally breezy trade winds this week. A Small Craft
Advisory (sca) remains in effect for Maalaea Bay and the pailolo
and alenuihaha channels through Monday afternoon. The Small Craft Advisory will
likely continue into mid-week for these same areas, and may
be expanded to include additional waters starting Monday night or
Tuesday.

The current long-period south swell will persist through early
Monday morning, and then gradually lower during the day on Monday.
A high surf advisory remains in effect through Monday afternoon
for south facing shores of all islands. Otherwise, only small
south swells are expected to maintain background surf along south
facing shores from Tuesday into next weekend. The trade winds
will continue to produce moderate short-period choppy surf along
east facing shores this week.

&&

Hfo watches/warnings/advisories...
high surf advisory until 6 PM HST Monday for Niihau-Kauai
windward-Kauai leeward-Oahu South Shore-Waianae coast-Molokai-
Lanai makai-Kahoolawe-Maui leeward west-Maui Central Valley-
leeward Haleakala-Kona-south Big Island-Big Island north and
east-kohala.

Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

$$

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