Wildfire in Hawaii, June 27 to 29, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
902 PM HST Wed Jun 27 2018

.SYNOPSIS...
Breezy trade winds will continue into Friday as a surface ridge
remains nearly stationary north of the islands. Trades will then
weaken to light to moderate speeds over the weekend. Expect trade
winds to deliver mainly night and morning showers to windward
areas. Muggier conditions may be moving through the islands
between Sunday and Monday night with some elevated showers in
association with a passing upper level trough.

&&

.DISCUSSION...
A 1028 MB surface high is located 900 miles NNE of Oahu this
evening, with a ridge extending westward along 34N latitude.
The pressure gradient is tight over the islands, resulting in
moderate to strong and gusty trades. This gradient will be
relaxing Thursday night through Friday that will lead to lighter
trades for the weekend.

The evening satellite imagery shows an east to west oriented band
of showers that will be moving through the main Hawaiian Islands
overnight and into Thursday. The band is will first affect the
smaller islands first and the Big Island (BI) after midnight.
Expect drier conditions by Thursday afternoon for all islands
except Kauai where passing showers will persist.

An upper level low is expected to develop some 540 miles north of
Kauai later tonight and stays there into the weekend. It will have
little affect on the islands. The GFS for Friday, has an east to
west mid level ridge over the islands with the marine layer capped
at 6 to 8k feet over the smaller islands and at 10k feet over the
BI.

Things get a little complicated over the weekend as the models
shows an upper level low/trough with some tropical moisture
approaching the islands from the east. Both the ECMWF and GFS have
this feature at 148W or about 350 miles east of the BI on Sunday.
This is close enough where the air mass destabilizes over the
islands, with the marine layer rising to 10k to 14k area wide.

The precipitable water (PW) outlook is showing a lowering from
the current 1.35 to 1.50 inches to 1.00 to 1.25 inches by Friday
afternoon. The PW then gradually increases over the weekend
topping 1.80 inches next week Monday. With this said, expect
muggier conditions along with some enhanced, elevated, showers to
move over the eastern islands as early as Sunday afternoon.

Stronger trades with a drier and more stable air mass will be
spreading westward across the islands Monday evening as a broad
surface high with a 1040 MB central pressure moves to a position
far north of the islands. Breezy trades will persist through
Independence Day.

&&

.AVIATION...
A high pressure ridge remains north of the Hawaiian Islands this
evening resulting in continued breezy to locally windy trade winds
through Thursday. An upper level disturbance and low level cloud
band will enhance showers across the state through the early
morning hours. Low cloud ceilings with tempo MVFR conditions and
showers will favor mountain areas and windward slopes. Expect
brief periods of shower drifting over leeward areas tonight.

AIRMET Sierra in effect for mountain obscurations for Kauai,
Oahu, Molokai and Maui this evening mainly over north through
east slopes due to low clouds and showers

AIRMET Tango is in effect for moderate low level turbulence below
9000ft.

&&

.MARINE...
A large band of fresh breezes locally and upstream of the islands
south of high pressure centered over the northeast Pacific will hold
through Thursday. Terrain-induced accelerations across the channels
and typically windier waters will continue to support strong trades
and rough boating conditions. Guidance, however, continues to
advertise a transition period beginning Friday and continuing
through the upcoming weekend that will be characterized by a
weakening pressure gradient at the surface in response to an upper
low tracking westward over the region and deepening. Trades are
forecast to weaken into the light to moderate range as a result.
Seas will respond and trend down over the weekend.

Surf along east facing shores will remain rough through Thursday,
then trend down Friday into the weekend as the trades relax locally
and upstream of the islands across the eastern Pacific. Some exposed
beaches could near the advisory threshold for east facing shores due
to a combination of a background southeast swell and the trade wind
source.

Surf along south facing shores continues to trend up this evening,
which was reflected at the Barbers and Lanai nearshore PacIOOS
buoys with a spectral peak observed within the 14-16 sec bands.
Expect this trend to persist overnight into early Thursday before
this reinforcing south swell begins to slowly ease into the
weekend.

Looking ahead into the upcoming weekend and the first week of July,
the active trend for south facing shores will persist due to another
system that passed south of the Tasman Sea and New Zealand this past
weekend where gales setup within Hawaii's swell window. The Tasman
source (210-220 deg) is expected over the weekend, which will
become reinforced early next week from the same feature out of
190-200 deg as it passed south of New Zealand earlier this week.
These sources are expected to be smaller than the current
overlapping south swells. &&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-
Oahu Leeward Waters-Kaiwi Channel-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

Lau/Bohlin/Gibbs