Wildfire in Hawaii, May 15 to 18, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Mon May 15 2017

.SYNOPSIS....
A strong surface high far northeast of the islands will move northeast
slowly and weaken through early next week as a front approaches from
the northwest. A trough aloft will linger over the islands through
Tuesday evening with a low aloft forming northeast of the islands.
The trough will then move slowly northeast over the second half of
the week as a ridge aloft builds over the islands from the northwest.

&&

.DISCUSSION...
Surface observations show breezy trade winds across the main
Hawaiian Islands. Satellite loops show a band of showery low
clouds extending far east of the islands. The trade winds are
carrying clouds from this band over the Big Island of Hawaii, with
some of the clouds spreading over the smaller islands. Radars show
numerous showers moving over the windward side of the Big island
and scattered showers over windward east Maui. Showers are
isolated elsewhere.

Surface analyses show a strong 1035 mb high about 1220 miles
northeast of the main Hawaiian Islands near 36N145W. The pressure
gradient south of this high is producing breezy easterly trade
winds over the islands. Satellite loops show a 120 mile wide band
of showery low clouds extending over 500 miles east from over the
windward side of the Big Island. The band has been shifting south
slowly but there are a series of weaker bands upstream to the
northeast of the islands.

A northeast to southwest trough aloft over Molokai is forecast to
dig southeast over the next 24 hours with a new low forming about
850 miles northeast of the islands by late tonight. The trough
aloft will make the atmosphere unstable and enhance showers
moving over the islands in the cloud band mentioned above.
Windward areas will remain quite showery through tonight with
locally heavy showers possible. There is a slight chance of
thunderstorms on the Big Island today. Freezing drizzle produced
icy conditions on Mauna Kea and Mauna Loa last night, and more ice
is possible on the summits tonight.

As the trough aloft weakens and moves off to the northeast
starting tomorrow the atmosphere will become more stable. Also,
the active shower band now over the islands will shift south of
the islands. The increased stability and decrease in showery low
clouds should bring drier weather.

Over the second half of the week, a front will approach from the
northwest. A surface ridge extending southwest from the high
northeast of the area will shift south closer to the islands.
Winds over the islands will weaken and shift out of the southeast
Friday and Saturday. Land and sea breezes will prevail. The sea
breezes will bring afternoon clouds and showers but the atmosphere
is expected to be fairly stable, so heavy rainfall is not
expected.

.AVIATION...
VFR conditions prevail at most TAF sites with occasional MVFR
conditions occurring at PHTO in SHRA BR. AIRMET SIERRA remains
posted over windward areas of The Big Island as moisture embedded
in the trades interacts with island terrain. Expect these
conditions to continue through the day though there may be some
improvement this afternoon. Brief MVFR conditions can also be
expected over the windward slopes of the smaller islands today as
well. There may also be some brief MVFR conditions over the
leeward slopes of The Big Island this afternoon and evening as
daytime heating causes clouds to build up over the slopes with
a slight chance of thunderstorms.

AIRMET TANGO remains posted over and W of mountain ranges for low
level turbulence due to the rather strong trade wind flow in place
across the area.

&&

.MARINE...
Large high pressure northeast of the state will slowly move east
and gradually weaken. ASCAT passes from last night showed a wide
swath of 20-25 knot winds extending from the local coastal waters
to over 1000 nm upstream of the state. This wide and long fetch
will keep trade wind swell large enough for advisory level surf
along east facing shores through Tuesday. Beyond then, the trade
swell will gradually diminish through the end of the week.

A small craft advisory is also in effect for a larger than normal
portion of the coastal waters, based in part in the ASCAT pass
from last night and high resolution model data showing these
strong winds continuing through Tuesday as well. We actually had
good coverage from ASCAT, which showed concentrated areas of 30kt
winds extending downstream from the Alenuihaha Channel and from
South Point.

A gale south of the Aleutians produced a small northwest (320
deg) swell that will bring very small surf along north and west
facing shores today. A recent ASCAT pass showed a decent sized
batch of strong to gale-force northwest winds associated with
this feature that may bring  small to moderate surf along north
and west facing shores Wednesday night through Friday. Another low
forming west of the dateline may produce more small to moderate
surf along north and west facing shores through the weekend.

A small south swell arriving tomorrow, which will support a
slight increase in surf along south facing shores through
Wednesday. Another south swell is forecast for the coming weekend.
For the long range, a more active pattern is forecast to develop
south and southeast of New Zealand that may produce larger surf
along south facing shores by the Memorial Day weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 600 PM Tuesay for east facing shores of
Kauai, Oahu, Molokai, Maui and the Big Island of Hawaii.

Small Craft Advisory until 600 PM Tuesday for Oahu Windward
Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

DISCUSSION...Donaldson
AVIATION...Burke
MARINE...Bravender