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Hawaiian Islands Synoptic Discussion and Guidance

037
FXHW60 PHFO 160656
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
856 PM HST Fri Jan 15 2021

.SYNOPSIS...
Although mostly dry conditions will continue into the weekend, a
few showers near and along a diminishing front will remain
possible around Kauai through the day Saturday. Rainfall chances
will quickly rise through the day Sunday as the winds shift out
of the south to southwest ahead of a stronger cold front
approaching. This front will quickly advance down the island
chain Sunday night, then stall around the Big Island Monday
through Tuesday. Heavy showers and a few thunderstorms will be
possible along ahead of this front. Cool and breezy northerly
winds are expected following the frontal passage for the western
end of the state early next week. Trade winds may return late
next week.

&&

.DISCUSSION...
Short-term guidance has initialized well with the current pattern
and depicts mostly dry conditions with light to moderate trade
winds continuing across much of the area through Saturday. The
exception will be over and around Kauai where showery conditions
will remain possible due to a diminishing frontal boundary that
has stalled nearby.

A transition period is expected Saturday night through Sunday as
a cold front associated with a gale currently north of Midway
approaches. Low-level winds will quickly shift out of the south
and southwest ahead of this approaching boundary - potentially
increasing to moderate levels over and around Kauai/Oahu later
Sunday. Increasing instability and deep layer forcing for ascent
out ahead of this front and its attendant shortwave trough aloft
will be enough to trigger spotty pre-frontal showers Sunday
afternoon over Kauai and Oahu.

The cold front is forecast to reach Kauai late Sunday, then move
down the island chain Sunday night through Monday. The front will
be accompanied by a period of showers. Model reflectivities are
in decent agreement and depict the leading edge of the main line
of showers along the front reaching Kauai late Sunday afternoon,
then to Oahu and Maui County Sunday night. Lowering upper heights
combined with 40-50 kt 0-6 km shear ahead of this upper trough
will support a few thunderstorms Sunday night into Monday. The one
limiting factor with regard to the strength of these storms could
be the phasing between the upper trough and deep moisture axis
along the front (appears to be a lag in the latest model cycle).
If these were to align, a few strong storms can't be ruled out -
especially over the coastal waters Sunday night.

Breezy northerly winds and cool conditions will follow the frontal
passage across the western end of the state early next week.
Guidance reflects this and depicts dewpoints dipping into the low
50s for Kauai and Oahu.

There remains subtle differences in the global models on how far
east the front will progress before stalling. The GFS moves the
front to just east of the Big Island on Tuesday while the ECMWF
has the front stalling over the Big Island. Either way, the Big
Island and possibly east Maui will remain rather wet on into
Wednesday, which could result in localized flooding concerns.

Winter weather conditions can't be ruled out over the Big Island
summits Monday night into midweek due to a combination of the deep
moisture axis associated with the front hanging up or stalling and
cold temperatures aloft arriving with the shortwave trough.

As we head on into Thursday, high pressure will build in northeast
of the area resulting in strengthening trade winds. Moisture
associated with the frontal boundary is then forecast to move back
westward likely providing for a wet trade wind pattern to set up
across the state. With the differences in the long range models
continuing, changes to the forecast our possible next week.

&&

.AVIATION...
A weakening front located northwest of Kauai will continue to
approach the islands this evening. The close proximity of this
feature is keeping a light wind regime over the western end of the
state, with weak low-level trade flow in the vicinity of the
eastern islands. Low clouds and showers ahead of the front, which
is expected to stall later tonight, will primarily affect Kauai
through early Saturday morning. This will likely result in periods
of MVFR ceilings / visibilities on Kauai. Elsewhere, expect low
clouds and isolated showers along some of the windward coasts and
slopes of the islands east of Kauai. This may result in brief
periods of MVFR conditions along some windward sections on these
islands. Otherwise, expect primarily VFR conditions to prevail
through Saturday morning elsewhere on the islands from Oahu to
the Big Island.

On Saturday, the front is expected to dissipate northwest of
Kauai. The weak trade wind flow will continue to transport some
low clouds and a few showers into windward sides of the islands.
But VFR conditions will again likely prevail over most areas
Saturday afternoon.

No AIRMETs are currently in effect. We will monitor conditions on
Kauai in case AIRMET Sierra is required for mountain obscuration
later tonight or on Saturday.

&&

.MARINE...
In summary, an active winter pattern this holiday weekend with
giant surf on Saturday, and a potent cold front arriving late on
Sunday.

The largest NW swell of the winter season so far will rapidly
build overnight into Saturday, with peak surf heights of 40 to 50
feet (with potentially higher sets) expected along exposed shores,
where a High Surf Warning is in effect. Long-period forerunners
(around 26 seconds) are arriving at NDBC buoys 51001/51101 to the
NW of Kauai, indicative of the powerful swell's arrival, although
the swell (as is typical of late) is running a little behind
WaveWatch III guidance. After peaking Saturday and Saturday night,
the swell will gradually diminish Sunday, but a new NW swell
arriving into Monday will likely push surf to warning levels once
again.

In the meantime, the giant NW swell has also prompted the issuance
of a Small Craft Advisory for all waters except Maalaea Bay as
seas build rapidly, and a Marine Weather Statement, which is
highlighting the potential for surges and significant wave action
in and near exposed harbors and their entrances. Due to the period
and size of the NW swell, some (much reduced) swell will wrap
into select S and E facing shores.

A surface ridge is near and E of Kauai, while a weak and
dissipating front is nearly stationary just NW of Kauai, leading
to light winds over Kauai waters, and light to moderate trades
elsewhere. The forecast anticipates the ridge moving N overnight
as the front dissipates, leading to a temporary increase in E to
SE winds on Saturday, potentially becoming locally strong. The
ridge will move S over the islands again on Saturday night as what
is expected to be a vigorous cold front approaches. The ridge will
move SE of the Big Island Sunday and Sunday night, leading to a
period of light to moderate SW winds, until the front's passage
ushers in near gale force N winds Sunday night. The front will
bring the potential for locally heavy showers and thunderstorms,
with the front expected to stall over the islands early next week.
A strong trough aloft is expected Monday and Tuesday, maintaining
the potential for thunderstorms along the stalled frontal
boundary.

&&

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

High Surf Advisory until 6 AM HST Saturday for Big Island North
and East.

Small Craft Advisory from 6 AM Saturday to 6 AM HST Sunday for
Maui County Leeward Waters-Pailolo Channel-Big Island Leeward
Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 AM 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.

&&

$$

DISCUSSION...Gibbs
AVIATION...Houston
MARINE...Birchard

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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