Wildfire in Hawaii, February 1 to 4, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
403 AM HST Wed Feb 1 2017

.SYNOPSIS...
A dry and stable airmass will remain in place through Thursday,
with limited chances for rainfall and gradually diminishing trade
winds. A ridge of high pressure will settle over the island chain
Thursday night through Friday night, with overnight land breezes
and daytime sea breezes expected in most areas, along with a
continuation of rather dry and stable weather. The winds will
then increase and shift around to the south and southwest over
the weekend in advance of a strong cold front, with breezy to
locally windy conditions expected Sunday into early next week.
Stable and dry conditions are expected to remain in place on
Saturday, with rather unsettled conditions and the potential for
some heavier rainfall expected across the state Sunday night
through early next week.

&&

.DISCUSSION...
Currently at the surface, a large ridge of high pressure is
centered over southern Alaska, with ridging extending southward
and over the island chain. Meanwhile, a deep 991 mb low is
located around 1500 miles northeast of the Big Island. The
resulting gradient associated with these features is driving
moderate northeasterly trade winds across the state early this
morning. Infrared satellite imagery shows a mixture of high and
low clouds resulting in mostly cloudy skies across the island
chain. MIMIC total precipitable water (PW) imagery shows values
at or below 0.75 inches over and upstream of the state, which
correlates well with the 12Z observed soundings from PHLI and PHTO
which indicated 0.59 and 0.57 inches respectively. The very dry
and stable airmass in place has resulted in rain free conditions
across the majority of the state overnight, with only a few
hundredths of an inch observed at a couple rain gauges in
windward sections of the Big Island. Main short term concern
revolves around the large north-northeast swell impacting the
islands over the next couple of days.

Today and tonight,
High pressure north of the state will slowly and steadily weaken
through the period as it shifts southeastward to a location around
600 miles northeast of the Big Island by daybreak Thursday. As a
result, the trades are expected to gradually diminish through
through the period. A dry airmass will remain in place over the
state, with PW values remaining at or below 0.75 inches, well
below climatological norms. In addition to the dry airmass, low
and mid-level ridging will keep conditions stable with suppressed
inversion heights limiting cloud growth. As a result, only minimal
rain chances are expected through the period, with any showers
that manage to develop, limited to windward slopes. The high cloud
cover currently in place is expected to quickly thin out and
push east of the island chain this morning allowing for mostly
sunny to partly cloudy skies to be seen in most areas by late
morning and through the afternoon. An increase in upper level
cloud cover is then expected across the island chain once again
tonight.

Thursday through Friday night,
A weak cold front will slowly approach from the northwest
Thursday and Thursday night before stalling out around 150 miles
north of Kauai Friday and Friday night. This will suppress the
high pressure ridge axis southward over the island chain Thursday
night through Friday night. As a result, the trades will diminish
further on Thursday, with light to moderate trades holding on
across the eastern end of the state, while light trades and
localized sea breezes are expected over the western islands. By
Thursday night the gradient will diminish sufficiently to allow
for land breeze development in most areas, with light winds
featuring land and sea breezes expected to continue through
Friday night. A continuation of dry and stable weather is expected
through the period, with low and mid-level ridging continuing to
keep inversion heights suppressed. As a result, rain chances are
expected to remain low through the period. Any showers that
develop are expected to remain limited primarily to the interior
of the islands during the afternoon and evening hours Thursday and
Friday. Additionally, the light winds may allow some VOG to lift
northward into the smaller islands Thursday night through Friday
night, but given the light boundary layer flow, confidence in the
movement of the plume is lower than normal.

Saturday through next Tuesday,
High pressure will shift eastward and away from the state over the
weekend, as a strong cold front begins to approach from the
northwest. There remains some disagreement between 00Z GFS an 00Z
ECMWF regarding exact details of the frontal progression through
the island chain Sunday night through early next week. The GFS
indicates a more progressive and consolidated front moving slowly
down the island chain, while the ECMWF shows an initial front
pushing into the islands Sunday night and Monday, with a
secondary and stronger re-enforcing front then moving down the
island chain Monday night into Tuesday. Given the uncertainty in
the forecast this far out, will utilize a blended approach for
forecast details. As a result, we expect increasing south and
southwest winds to develop Saturday and steadily strengthen over
the weekend, reaching breezy to locally windy levels on Sunday.
There is more uncertainty regarding wind details Sunday night
through early next week given the differences in the models, but
there remains the potential for strong damaging downslope
southwesterly winds along and ahead of the front. For now will
keep breezy to windy conditions in the forecast, but this will
need to be monitored closely as the event approaches.

As for sensible weather details, the GFS and ECMWF are in good
agreement showing a dry and stable airmass holding in place
through Saturday, with rain chances then slowly increasing
Saturday night and Sunday, particularly over the central and
western islands. Both models due show deep layer moisture
increasing significantly beginning Sunday night in association
with the front pushing into the western islands. The specific
details then become more uncertain early next week given the
differences in the model solutions. As a result we will keep
conditions unsettled Sunday night through next Tuesday. Given PW
values increasing to around or in excess of 2 inches along the
front, some heavier rainfall will be possible as the front pushes
through the island chain.

&&

.AVIATION...
VFR conditions will prevail across much of the terminals today as
a stable air mass remains in the island vicinity. Easterly trade
winds will weaken slowly and continue to bring low clouds mainly
across windward and mountain areas.

AIRMET Tango for moderate low level turbulence over and south
through west of the mountains will be canceled later this morning.

&&

.MARINE...
The nearshore buoys have been running a bit higher than originally
anticipated, so have bumped the swell up a bit into tonight. Will
have to monitor the swell today to see how to handle it further.
The swell has arrived several feet above guidance, but the trend
is similar to the model which has the swell peaking at the near
shore buoys late today/tonight. The resultant High Surf Warning
for various north and east facing shores has been extended through
tonight. Depending on how quickly the swell subsides, an advisory
may be needed tomorrow. The direction of the swell will also bring
elevated surf to the west facing shores of Maui and the Big
Island. A High Surf Advisory has been posted for the west facing
shores of Maui, while the west facing shores of the Big Island
have been added to the High Surf Warning.

The orientation of the swell will also bring the possibility of
harbor surges and large breaking waves to Kahului and Hilo
Harbors. Extra caution should be exercised in these areas through
at least tonight.

A new northwest swell is expected to build on Friday, bringing
advisory level surf to north and west facing shores Saturday. A
large west-northwest swell is expected to arrive Monday.

A combination of winds and seas will maintain a Small Craft
Advisory over all waters except Maalaea Bay through early Thursday
morning, before some areas begin to fall below criteria. Winds at
Maalaea Bay are expected to drop below advisory by noon today.
Winds will gradually veer toward the east through midweek as high
pressure builds north of the state. Winds will weaken toward the
end of the week as high pressure shifts east and weakens in
response to another cold front approaching from the northwest.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Thursday for Niihau-Kauai
Windward-Oahu North Shore-Oahu Koolau-Olomana-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala-Kona-Big
Island North and East.

High Surf Advisory until 6 AM HST Thursday for Maui Leeward West.

Small Craft Advisory until 6 AM HST Thursday for all Hawaiian
waters except Maalaea Bay-

Small Craft Advisory until noon HST today for Maalaea Bay.

&&

DISCUSSION...Jelsema
AVIATION...Hui
MARINE...M Ballard