High Surf Advisory 15, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
908 PM HST Tue Jan 31 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 1035 mb high is centered around 2150
miles north of Kauai, with ridging extending southward and over
the island chain. Meanwhile, a deep 992 mb low is located around
1400 miles northeast of the Big Island. The resulting gradient
associated with these features is driving a moderate to locally
breezy northeasterly trade wind flow across the state early this
evening. Infrared satellite imagery shows quite a bit of high
cloudiness over the island chain, with mostly cloudy skies in most
locales. 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 00Z observed soundings from PHLI and
PHTO which indicated 0.62 and 0.55 inches respectively. The very
dry and stable airmass in place has resulted in rain free
conditions across the state, with radar imagery remaining void of
any meteorological returns early this evening. Main short term
concern revolves around the north-northeast swell impacting the
islands over the next couple of days.

Tonight and Wednesday night,
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 moderate to locally breezy trades in place tonight
will diminish to light to moderate levels on Wednesday and
Wednesday night. A dry airmass will remain in place over the
state through the period, 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 is expected to slowly shift eastward
across the islands overnight, thinning out and pushing east of
the island chain Wednesday morning allowing for more Mostly sunny
to partly cloudy skies in most areas. An increase in upper level
cloud cover is then expected to move back in over the island chain
Wednesday night.

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.

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 as a
stable air mass remains in the island vicinity at least through
early Wednesday morning. Northerly trade winds will weaken
overnight and turn more east, but will 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 continue through the rest of
the evening and will probably be canceled early Wednesday as the
winds ease further overnight.

&&

.MARINE...
Over the last couple of hours, the north-northeast swell has
started to show up on the Kailua Bay buoy. This swell, originating
between the low far northeast of the islands and the high far
north of the islands, is expected to bring warning level surf to
north and east facing shores through Wednesday, before slowly
easing Thursday through Friday. The current warning goes through 6
pm Wednesday, and may then need to be lowered to an advisory
depending on how fast the swell subsides.

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
Wednesday.

A combination of winds and seas will maintain a Small Craft Advisory
over all waters through early Thursday morning, before some areas
begin to fall below criteria. 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 PM HST Wednesday for Niihau-Kauai
Windward-Oahu North Shore-Oahu Koolau-Olomana-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala-Big Island
North and East.

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

&&

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