Gale Warning 8, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
838 PM HST Thu Dec 19 2019

.SYNOPSIS...
Relatively quiet trade wind weather is expected through tonight,
with locally breezy trades delivering windward showers and a few
leeward spillovers. Strong high pressure passing north of the
islands will produce very windy conditions Friday through Saturday
night, and possibly into Sunday. An old front will also move
through the state late Friday through early Saturday, bringing
some wet weather to the islands. An area of low pressure west of
the state will cause mostly cloudy conditions across the area on
into the middle of next week. A wet weather pattern could develop
around the Christmas holiday, but confidence in the details
remains low at this time.

&&

.DISCUSSION...
Currently at the surface, a 1033 mb high is centered around 1400
miles north-northwest of Kauai, while an old front around 140
miles north of Kauai. The gradient is already beginning to tighten
up across the island chain this evening, with moderate to breezy
trades in place. Infrared satellite imagery showers partly cloudy
skies in most areas, with some leeward pockets of more extensive
cloud cover in windward areas as well as leeward sections of the
Big Island. Radar imagery shows bands of trade wind showers moving
into windward areas, with some of these showers reaching leeward
areas as well. Main short term concern revolves around the very
strong trade winds expected during the next few days.

A strong 1033-1035 mb high will shift eastward well to the north
of the islands during the next few days. Meanwhile, the remnants
of an old front will push steadily southward through the island
chain Friday afternoon through Saturday morning. Winds are
expected to really ramp up along and behind this old remnant
front. Winds will strengthen through the day Friday, with very
windy conditions expected Friday night through Saturday night and
possibly into Sunday. A High Wind Watch remains in effect for the
entire state beginning at Noon Friday and continuing through 6 PM
Saturday. Wind Advisories and possibly High Wind Warnings will be
required for much of the state with the morning forecast package.

More uncertainty in the wind forecast begins next week. The GFS
drags a front into the island chain during the middle to latter
part of the week, while the ECMWF maintains solid trade winds
through late next week. Given the continued uncertainty in the
forecast, will follow a middle of the road solution which results
in moderate to breezy trades through early next week, with winds
shifting around to the southeast and easing during the middle to
latter part of the week.

As for weather details, trade wind weather will continue through
early next week, with some fluctuation in shower coverage and
intensity. Typical trade wind shower coverage can be expected
tonight through much of Friday. We'll see a notable increase in
trade shower coverage and intensity late Friday through Saturday
as moisture associated with an old front pushes southward through
the island chain. The old frontal remnants are expected to push
through Kauai Friday afternoon and Friday evening, through Oahu
and Maui County Friday night, and through the Big Island after
midnight Friday through Saturday morning. Behind the frontal
remnants, it appears that conditions will dry out, with fewer
showers expected through Saturday night. We should see the airmass
gradually moisten up by Sunday, with a more typical trade wind
shower pattern then expected through the first half of next week.
Much uncertainty continues in long range portion of the forecast,
with the GFS indicating some heavy rainfall and thunderstorms
could be in store for the area Wednesday and next Thursday, while
the ECMWF continues to advertise a fairly typical trade wind
shower regime.

A considerable amount of mid and high cloud cover is expected to
move in tonight and continue through much of next week.

&&

.AVIATION...
A strong high pressure center northwest of the islands will
increase trade winds significantly statewide over the next 24
hours. Trade winds will increase through the day on Friday with
surface winds in the 25 to 35 MPH range with higher gusts by
Friday afternoon. Stronger wind speeds are possible over some wind
favored locations. AIRMET Tango remains in effect for moderate low
level turbulence below 080. This AIRMET will likely continue for
the next 48 hours.

Expect brief periods of low cloud ceilings and showers over Kauai
and possibly Oahu through the overnight hours as bands of unstable
clouds drift across the northwestern islands. A weakening cold
front will move into the islands from the north Friday spreading
periods of MVFR cloud ceilings and showers across the region from
Friday afternoon through Saturday morning. AIRMET Sierra in effect
for Tempo Mountain Obscurations above 020 for north through east
sections of Kauai.

&&

.MARINE...
Increasing winds and short-period seas are expected over the next
24 hours as a strong high pressure cell builds N of the islands.
A Small Craft Advisory (SCA) remains in effect for most waters due
to winds near 25 kt in the typical areas near Maui County and the
Big Island, and for seas near 10 feet in areas exposed to
diminishing NW swell. The SCA will expand to all zones Friday
morning as winds and seas increase, with a Gale Watch in effect
from Friday night through Saturday, as sustained open ocean winds
of 30 kt are expected to develop. This will likely lead to
sustained gale force winds in many of the channels between the
islands, while frequent gusts to 35 kt could warrant a Gale
Warning for all areas. Supporting this notion is that winds at
buoy 51001 NW of Kauai have had frequent gusts to 35 kt over the
last hour or two, where winds are averaged over an 8-minute period.

The wind waves outside the channels are expected to build to
13-15 feet by Friday night, with a period of 9 seconds making it
rather rough for boating. Seas will diminish slightly over the
weekend, as winds ease (and potentially veer) slightly Sunday-
Tuesday, but seas are expected to remain 10 feet or above into
Christmas, with SCA-level E-SE winds expected in most zones.

Surf along N and W facing shores looks to remain below advisory
levels through Christmas. However, surf along E facing shores will
rise as trade winds increase through Friday, and a High Surf
Advisory (HSA) is likely to be issued early Friday in anticipation
of the increasing rough and short-period surf. A small NNW swell
will shift to the N as it gradually diminishes over the weekend,
with a relatively small NW swell possible around Tuesday and
persisting into Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Friday afternoon through Saturday afternoon
for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui-Kona-South
Big Island-Big Island North and East-Kohala-Big Island Interior.

Gale Watch from Friday evening through Saturday afternoon for
all Hawaiian waters-

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

Small Craft Advisory from 6 AM to 6 PM HST Friday for Oahu
Leeward Waters-Maui County Leeward Waters-Big Island Windward
Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Birchard