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Forecast Discussion for Honolulu, HI

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FXHW60 PHFO 181956

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Sat Aug 18 2018

A low level trough will bring unstable air and convective showers
spreading from east to west today and tomorrow with isolated
thunderstorm activity. Frequent showers will develop along
windward and mountain slopes later this morning through the
overnight hours. Shower activity will trend drier on Monday and
Tuesday as more stable air arrives from the east. Hurricane Lane
remains too far away to determine any local impacts to the
Hawaiian Islands just yet. The latest forecast for Lane tracks the
Hurricane through the region near the islands through the middle
of next week.


A weak high pressure ridge north of the islands and a low level
trough moving into the state from the east today will keep much
of Hawaii in a moderate trade wind flow pattern through the
weekend. The low level trough appears as a low level vortex on
satellite imagery with deep convection and thunderstorms
developing on the eastern side of the trough axis. The trough
will pass from east to west across the state this weekend before
drifting towards the northwest away from the islands by Monday

Expect increasing shower activity spreading into the windward Big
Island and Maui this morning with unstable showers spreading
westward across the rest of the state later this afternoon and
evening. Precipitable Water levels will rise into the 1.75 to 2.00
inch range within the eastern side of this trough. Showers may
become heavy at times with isolated thunderstorms developing as
the trough passes through the island chain, elevating the
possibility for localized flooding. The heaviest showers
developing upstream of the islands are not numerous enough for a
flood watch at this time. However, flood advisories may be needed
later today as bands of showers move into each island. The highest
coverage for showers will continue to favor windward and mountain
areas. Consider postponing outdoor activities near windward and
mountain areas due to the potential for locally heavy rain and

Shower activity will then decrease by Monday morning and trade
wind speeds increase as the low level trough lifts towards the
northwest away from the state. Drier air will then filter in from
the east as enhanced subsidence north of Hurricane Lane with a
building East Pacific ridge will help to stabilize the atmosphere
across the state. PW values will fall into the 1.00 to 1.50 inch
range through Tuesday keeping cloud and shower coverage to a

Much higher moisture and humidity levels will move into the islands
on Wednesday morning as deeper tropical moisture surrounding
Hurricane Lane reaches the Hawaiian Islands. Expect warm and humid
weather through the rest of the week as the Hurricane passes
through the Hawaii region. Forecast uncertainty ramps up from
Wednesday through Friday as the Hurricane passes near or just
south of the islands. Hurricane Lane remains too far away to
determine any local impacts to the Hawaiian Islands just yet.
However, forecast confidence will steadily rise as Lane
approaches the islands.


High pressure far northeast of the islands will maintain light to
locally moderate trade winds across the state through tonight. A
deep band of moisture, embedded in the trade wind flow, will
reach the Big Island by noon and spread across the smaller islands
by early this evening.

Clouds and showers will favor windward coasts and slopes through
the period, but both shower coverage and intensity will increase,
beginning in the east and spreading to the west. Locally heavy
downpours, possibly in thunderstorms, are expected across Windward
Big Island and the east Maui uplands today. AIRMET Sierra, for
mountain obscuration, is in effect for north through east
sections of Maui and the Big Island.


Light to moderate trade winds are expected to continue today and
remain below Small Craft Advisory (SCA) levels. A weak trough
will pass through the islands today and tonight, bringing the
potential for some heavier showers and thunderstorms. High
pressure will then build well northeast of the state through the
middle of next week, while Hurricane Lane is forecast to pass by
to the south of the Big Island on Wednesday. As a result, trade
winds will increase again with SCA conditions returning first to
the typically windy waters around Maui and the Big Island Sunday
night, then possibly expanding to additional zones as moderate to
strong trade winds continue through the middle of next week. It is
still too early to determine the exact impacts that Hurricane
Lane may have on the coastal waters, so mariners are urged to keep
up to date with the latest forecasts.

No significant swells are expected through the weekend, so surf
will remain small on all shores. A small, north-northwest swell
is expected to arrive on Monday and continue through Tuesday. A
larger north swell is expected to arrive late Tuesday, peak
Tuesday night and early Wednesday, then slowly fade through
Thursday. We will continue to monitor the track and intensity of
Hurricane Lane as it moves closer to the islands, as an elevated
easterly swell is expected to move into the coastal waters Sunday
night through the middle of next week. This swell will likely
result in large surf for east and southeast facing shores of the
Big Island and potentially large surf for east facing shores of
Maui as well.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.






Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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