High Wind Watch 2, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Sat Feb 8 2020

.SYNOPSIS...
Cool and dry weather is forecast to spread eastward tonight as an
upper trough across the islands moves east. A strong and cold low
will move in from the north on Sunday night, bringing strong
northwesterly winds, colder temperatures, and additional bands of
showers through Tuesday. East to southeast winds will return by
Wednesday, with scattered showers expected.

&&

.DISCUSSION...
Have re-issued the Special Weather Statement to highlight the
strong winds and coastal flooding possibilities for Sunday night
into Monday. See the latest SPSHFO product for those details.

The main weather concern for the next 48 hours is the wind. A
surface low was analyzed about 650 miles north of Honolulu this
afternoon. This was the system that brought the cold front through
the islands a few days ago, and ushered in the cool northerly
winds. The models are in good agreement with a complex low
developing with the low to the north moving to the northeast and a
new surface low forming on the west side and digging south
towards the islands. It is this second feature, coupled with a
strong upper level low that will continue to bring in northerly
winds, and will increase winds for the islands as soon as Sunday
afternoon.

That being said, there are some significant differences in where
the global models place the low Sunday night and Monday, which
will play a big part in how strong the winds will get and what
exact areas will see the strongest winds. The highest likelihood
for the strongest winds (aside from the Big Island Summits) will
be over the western end of the island chain. Kauai and Oahu could
see the winds pick up sooner than Maui County and the Big Island,
with the earliest onset for Kauai being late Sunday afternoon.
Have begun to make some tweaks to the forecast grids to better
reflect the likelihood of advisory level winds making it into the
islands. Still making final decisions, but a Wind Advisory for
some lower elevations could be issued in the morning.

The High Wind Warning (HWW) for the Big Island Summits continues,
with observations from Mauna Kea continuing to support the
warning. The models are in good agreement with some weakening of
the winds at the summits through tomorrow (Sunday) afternoon, with
winds possibly dropping below warning levels, but then ramping
back up Sunday night. The current HWW goes through 11 AM Sunday
but will be extended, and strong winds there could continue into
Tuesday. There are differences in the ECMWF and the GFS for the
winds, so still exploring how long to extend the HWW for.

The placement of the low will also factor into the amount of
showers that move over the islands during the Sunday night and
Monday time period. Previous run of the ECMWF is wetter than the
GFS, but both move things through the islands relatively quickly,
so at this time not anticipating any flooding concerns.

By Tuesday afternoon, the GFS and ECMWF both had the surface low
over the islands, and moving to the southwest away from the island
during the second half of the week. As the low moves over the
islands, winds will weaken, and then as the system moves to the
southeast, light to moderate southeast to east winds will move
into the islands.

&&

.AVIATION...
Wind speeds at the surface and lower layers have decreased
significantly since earlier in the afternoon so AIRMET Tango has
been canceled for now. We expect the winds to increase again later
in the day Sunday into Sunday night. Anticipate AIRMET Tango for
low-level turbulence to be back in place during that time. With
these winds expected to be out of the west-northwest, this
turbulence should be possible in east through southeast areas
downwind of mountains.

Expect mostly VFR conditions across the state tonight into Sunday
with only brief, passing MVFR conditions possible with a few
isolated showers across northwest through northeast mauka
sections, especially for Molokai, Maui, and the Big Island. The
next chance for MVFR conditions may occur Sunday night into Monday
associated with an upper-level trough.

There are no AIRMETs currently in effect and none are anticipated
until Sunday afternoon into Monday.

&&

.MARINE...
Moderate to fresh northerly winds will continue tonight and
Sunday morning, then shift out of the west-northwest and increase
to strong and near gale-force speeds late Sunday through Monday
as a surface low drops southeastward to the north of the islands.
The surface low is then forecast to drop southwestward Monday
night through Tuesday night, passing through the eastern or
central portion of the state. This should keep fresh to strong
northerly winds in place across the western islands. Meanwhile
across the eastern half of the state, moderate to fresh
northwesterly winds Monday night will diminish and become light
and variable by Tuesday night. The surface low is forecast to open
up into a surface trough and shift southwest and away from the
islands Wednesday into next weekend, while high pressure builds
far northeast of the islands. This should bring a gradual return
of trade winds to the marine area beginning Wednesday and
continuing into next weekend.

Surf along north facing shores will remain rough through early
next week due to the northerly winds and large seas. A combination
of large, short-period seas out of the north and moderate to large,
long-period northwest swells moving through this weekend and again
Monday through Tuesday will keep north shore surf at or above
advisory levels much of the time. The current High Surf Advisory
remains in effect through 6 AM Sunday. Surf is trending down, so
this is expected to be cancelled with the early morning forecast
package. Surf will be back on the rise Sunday night, with warning
level surf expected Monday through Tuesday. Surf will trend down
Tuesday night, but may remain at advisory levels for north and
west facing shores. Small to moderate north and northwest swells
will then continue Wednesday into next weekend, with surf
remaining well below advisory levels.

Peak monthly tides combined with water levels running higher than
predicted, potentially gale-force west-northwest winds, and warning
level surf, will likely lead to coastal impacts for exposed
shores Sunday night through Monday. This could include significant
beach erosion, overwash onto vulnerable low-lying coastal
roadways, and surges within Haleiwa harbor. The peak high tide
will occur between midnight and daybreak Monday morning.

Surf along exposed east facing shores will remain well below
normal tonight through Wednesday. East shore surf will get a
notable boost Wednesday night into next weekend, as trade winds
return over and upstream of the islands. Surf along south facing
shores will remain small with mainly background long-period south
and southwest swells moving through. A moderate size south swell
will be possible next weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Wind Warning until 11 AM HST Sunday for Big Island Summits.

Gale Watch from Sunday evening through Monday afternoon for all
Hawaiian waters-

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

&&

DISCUSSION...M Ballard
AVIATION...Chevalier
MARINE...Jelsema