Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 190231 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
431 PM HST Sat Aug 18 2018

.SYNOPSIS...
A low level trough will bring unstable air and convective showers
spreading westward across the state tonight with isolated
thunderstorm activity. Showers will develop along windward and
mountain slopes 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.

&&

.DISCUSSION...
Frequent showers with locally heavy rains affected the Big Island
of Hawaii today with over 3000 individual lightning strikes
detected from thunderstorm activity. The heaviest rainfall was
estimated to range from 2 to 3 inches per hour and produced
mainly water ponds on area roadways and elevated stream levels.
Showers and storms started in the late morning hours and swept
across much of the Big Island through the day.

In the big picture, a weak high pressure ridge remains north of
the islands and a low level trough moving through the state from
the east today will keep much of Hawaii in a moderate trade wind
flow pattern through Sunday. The low level trough appears as
curved bands of clouds on visible satellite imagery with deeper
convection and a few thunderstorms developing on the eastern side
of the trough axis. The trough will pass across the state this
weekend before drifting towards the northwest away from the
islands by Monday morning.

Expect shower activity to continue spreading westward across the
state through the overnight hours. Precipitable Water levels will
remain in 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 convective showers are not numerous enough for a flood
watch at this time. However, flood advisories may be needed 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
thunderstorms.

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 combine with
a building East Pacific ridge to 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
minimum.

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.

&&

.AVIATION...
High pressure far northeast of the islands will maintain light to
locally moderate trade winds across the state into the evening
hours and again throughout the day on Sunday which could produce
light low level turbulence. There is still plenty of moisture in
place over the Big Island and this will continue to spread across
the smaller islands by this evening into the overnight hours.

With the moisture and instability in place, the chance for
increased clouds and showers will exist over most islands
spreading from the Big Island and Maui through the rest of the
smaller islands tonight and into the day on Sunday. The most
clouds and heaviest rain will favor windward coasts and slopes
through the period. The potential exists for locally heavy
downpours and thunderstorms across windward Big Island as well as
windward sections of Maui, Molokai and Oahu this evening and even
into Sunday.

AIRMET
Sierra, for mountain obscuration, is in effect for north through
east sections of Maui and the Big Island. Will monitor the
windward sides of Molokai, Oahu and Kauai through the period for
mountain obscuration and issue an AIRMET for those areas if
necessary.


&&

.MARINE...
Light to moderate trade winds are expected to continue tonight
and remain below Small Craft Advisory (SCA) levels. A weak trough
will continue to move through the islands tonight, for the
potential of 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 Tuesday night through 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Bohlin
AVIATION...Chevalier
MARINE...TS


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