Wildfire in Hawaii, August 4 to 9, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Sat Aug 4 2018

.SYNOPSIS...
Breezy trade winds will gradually decrease on Sunday then drop to
moderate strength for Monday and Tuesday. Rather wet trade wind
weather will trend drier late tonight, leading to limited
windward rainfall and dry leeward conditions Sunday through
Tuesday. Hurricane Hector will be crossing the region during the
middle of the work week, though it is still too early to determine
impacts on local weather.

&&

.DISCUSSION...
Breezy and wet trade wind weather is in place. The trades are
being driven by surface high pressure centered over 1500 miles
northwest of the islands, and the winds are being enhanced by an
active ITCZ to the south of the state that is tightening the local
pressure gradient. Thunderstorm activity within the ITCZ has
decreased since yesterday, causing the pressure gradient and trade
winds to decrease slightly. Shower activity is being enhanced by
an upper level low located about 250 miles west of Kauai. The
upper low is triggering isolated thunderstorms over western
offshore waters and is maintaining the slightly instability that
is producing rather active showers in the trade wind flow. Between
1/4 to 3/4 of an inch of rainfall was reported at most windward
stations last night, with standouts measuring about 1.5 inches.
Light accumulations were observed at leeward sites from Kauai to
Maui as the breezy trades periodically pushed showers over from
windward terrain.

Expect the breezy and somewhat wet trade wind flow to persist into
tonight. The local pressure gradient will relax only slightly,
and although winds will be down a notch from yesterday, breezy and
gusty trade winds will prevail. The upper level low will drift
slowly westward, away from the islands, and allow increasingly
stable conditions to slowly build over the state from the east. As
a result, a low level inversion, which is nearly absent at the
moment, will gradually build at around 7000 to 10,000 ft this
afternoon, then build down to about 5000 ft overnight. The
upstream trade wind flow still contains a good amount of shower-
bearing cumulus. This will keep windward areas somewhat wet, but
rainfall amounts and the intensity of the showers should be
trending down.

Stable and rather dry trade wind weather should dominate by Sunday
and persist into Tuesday. The pressure gradient over the state
will weaken slightly as the high to the northwest and the ITCZ
south of the state weaken. Expect locally breezy trade winds on
Sunday to drop a notch to moderate strength by Monday. A strong
ridge aloft will build over the state, leading to very stable
conditions with a solid inversion setting up below 5000 ft. The
field of stable stratocumulus currently seen east of 130W will
move over the islands, leading to a decrease in low level
moisture. This will result in rather dry conditions with limited
windward rainfall.

Heading toward the middle of the work week, the forecast hinges on
the location of Hurricane Hector. The system is currently centered
a little less than 1500 miles east-southeast of the state and is
moving toward the west. The National Hurricane Center continues
to indicate that Hector will enter the Central Pacific late
Sunday and be in the region around the islands by mid week. It is
still too early to predict what impacts Hector may have on our
local weather as average track errors are around 200 miles this
many days out. The National Weather Service urges everyone to have
a hurricane plan in place. A guide for hurricane preparedness can
be found at www.ready.gov/hurricanes, and the State of Hawaii and
the counties have preparedness information specific to the
islands.

&&

.AVIATION...
High pressure to the north-northwest will maintain moderate to
strong winds over the region. AIRMET Tango continues for
mechanical low level turbulent downwind of the mountains. This
will likely continue for the next few days.

Clouds and showers will be focused over the windward and mountain
areas, but the strong winds will carry some showers to leeward
areas at times. VFR conditions will prevail, but expect pockets of
MVFR in heavier showers. AIRMET Sierra continues for mountain
obscuration over the windward side of the Big Island.

&&

.MARINE...
Strong high pressure far NNW of the area will continue to produce
rather strong trade winds across the area. Thus a Small Craft
Advisory is posted for all Hawaiian Coastal Waters. The high is
forecast to weaken a bit tonight and Sunday. This will allow for
winds to diminish slightly from current levels but will likely
remain elevated over the typically windy waters around Maui County
and the Big Island into early next week.

A series of small south swells are expected through Monday with a
slightly larger south swell expected on Tuesday and Wednesday. A
small north swell is expected to arrive late Tuesday, peak Tuesday
night and Wednesday, then lower gradually Thursday and Friday. Short
period choppy surf will continue to impact east facing shores
into early next week.

The National Hurricane Center is issuing advisories on Hurricane
Hector in the East Pacific. Swells from Hector are expected to
reach the eastern coastal waters by Monday and then gradually
increase through Wednesday. Surf will increase gradually along
east facing shores of the Big Island early next week. Based on the
latest guidance, large surf is expected over exposed shorelines of
the Big Island by the middle of next week. Shorelines over the
smaller islands will be impacted less as they will be somewhat
shadowed by the Big Island. We are also entering a period of high
astronomical tides next week so low lying coastal areas may have
some coastal inundation issues to deal with. Stay tuned for the
latest updates on Hector, and its potential impacts on winds, seas
and surf across the area next week.

&&

.FIRE WEATHER...
Although the trade wind flow has been somewhat wet, gusty trade
winds are maintaining elevated fire danger. The Keetch Byram
Drought Index (KBDI) is approaching, but still remains short of
critical fire weather conditions. That said, the recent strong
trade winds have been driving elevated fire activity. Winds will
slowly trend down over the next couple of days but will still
remain breezy in places. In addition, conditions will be getter
drier, with leeward relative humidity values dropping to around 45
percent each afternoon starting tomorrow. Although critical fire
weather thresholds will not be crossed, ingredients will be in
place for elevated fire activity.

&&

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

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.

&&

DISCUSSION...Wroe
AVIATION...M Ballard
MARINE...Burke
FIRE WEATHER ...Wroe