High Surf in Hawaii, January 6 to 16, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
336 AM HST Mon Jan 6 2020

.SYNOPSIS...
A broad area of strong high pressure, stationed far northeast of
the islands will be shifting west, closer to the islands, during
the next day or so, thus maintaining breezy and gusty trade winds
through the new work week. Low level moisture riding in with the
trades will focus the showers to the windward and mountain areas,
but the strong trades will carry some of these showers well into
the lee areas of the smaller islands. A wetter trade wind pattern
is likely during the second half of the week in combination with
an upper level low moving over the area from the east.

&&

.DISCUSSION...
A 1038 MB surface high is currently sitting a short distance off
the northern California coast. It is slated to move west to a
point of 1100 miles north- northeast of the islands by Tuesday
night, with a 1033 mb central pressure. The high then holds there
through Friday, only to be replaced by a new and stronger high
over the upcoming weekend. Although the high weakens during its
transit west, it will still be enough to generate and sustain
strong and gusty trade winds through the second half of the week.

A Wind Advisory is in effect for the lower elevations for parts of
Maui and Hawaii counties, good through 6 pm HST Tuesday. This
advisory is in addition to the wind advisory for the 2 Big Island
(BI) summits. More than likely, this advisory will be extended
into the second half of the week. We will be watching closely if
more areas need to be added to this advisory.

In terms of showers, it will be pretty much the normal distribution
of trade showers, with a higher frequency during the nights and
mornings through Tuesday. Windward Big Island has been the
wettest thus far, as the island is in the path to a stream of
small pockets of showers. The GFS and ECMWF points to a slightly
drier trend for windward BI today. Models also suggest that
tonight is likely to be wetter tonight than this morning.

Discrepancies still exist between the new ECMWF (EC) and GFS
during the second half of the week and the upcoming weekend. The
EC is faster in bringing in the higher precipitable water, as
early as Tuesday night and Wednesday, compared to the GFS, not
until Thursday. The EC solution calls for the upper low east of
the BI linking up with a deep trough to the west of Kauai Thursday
night, creating a new upper low some 400 miles southwest of Kauai
on Friday. This new low becomes dominant and drifts north to 700
miles NW of Kauai on Sunday. GFS solution is a much slower, with
the upper low nudging over the BI Friday evening, then bridging
with the upper trough to the west of Kauai on Sunday. Another
difference between the two, is with the low level winds. The GFS
generates a low level inverted trough just to the north of Maui on
Sunday, interrupting the trade wind flow in the process, where
the EC maintain a breezy trade wind flow through the upcoming
weekend. In essence, both models maintain an unstable environment
during the second half of the week. And that the breezy trades
remains. Any threat of thunderstorms still remains with the BI, as
early as Thursday. It is not officially part of the forecast,
yet. Just waiting for another model run or two to confirm the
threat.

&&

.AVIATION...
A strong high pressure system north to northeast of the Hawaiian
Islands will produce breezy to windy easterly trade winds through
the week. Winds this strong will produce low level moderate
turbulence along and south through west of mountain ranges of all
islands.

An upper level low centered just east of the island chain will
continue to produce bands of showers spreading westward in the
trade wind flow mainly during the overnight and early morning
hours. Periods of mountain obscurations are possible for north
though east slopes of each island. Upper level winds around this
low are strong enough for moderate turbulence aloft between
FL280-400 across all islands.

AIRMET Sierra remains in effect for tempo mountain obscurations
along north through east facing slopes of the Big Island. This
AIRMET may be expanded later tonight for windward slopes of the
smaller islands.

AIRMET Tango remains in effect for tempo moderate turbulence
below 090 over and south through west of mountains. Another AIRMET
Tango remains in effect for moderate upper level turbulence due
to strong winds aloft. These turbulence AIRMETs will likely
continue through at least the next 24 hours.

&&

.MARINE...
A strong surface ridge to the N will support strong to near gale
force ENE trade winds near and upwind of the islands this week,
with winds to gale force in marine zones where they are most
accelerated by island terrain. See the Marine Weather Message
(MWWHFO) for details on the ongoing Gale Warning and Small Craft
Advisory.

A High Surf Advisory (HSA) is in effect for E facing shores of
most islands, and will very likely remain posted through the
week. As short-period seas build (to near 15 feet) in response to
the persistent upstream fetch of trade winds, surf along E facing
shores could approach warning levels later in the week. See the
Coastal Hazard Message (CFWHFO) for details. The combination of
high surf, strong onshore winds and elevated ocean water levels
could result in beach erosion and wave runup along E facing
shores later in the week when higher high tides coincide with
larger surf.

Little in the way of significant swell is otherwise expected. A
pair of small NW swells are expected to rise and fall between now
and Wednesday, with a larger long-period NW swell around Friday
that could produce advisory-level surf into the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Friday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST Tuesday for Lanai-Kahoolawe-Maui
Central Valley-South Big Island-Kohala-Big Island Interior-Big
Island Summits.

Gale Warning until 6 PM HST Tuesday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

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

&&

H Lau/Bohlin/Birchard