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Hawaiian Islands Synoptic Discussion and Guidance

989
FXHW60 PHFO 170658
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
858 PM HST Sat Jan 16 2021

.SYNOPSIS...
A cool and wet pattern is on tap for the islands during the first
half of the coming week. This comes about as a front moves down
the island chain Sunday night, then stalling around Maui and the
Big Island between Monday and Tuesday. The front, or trough, is
then slated to drift back to Oahu and Kauai between Tuesday night
and Wednesday. The front will have good support aloft from an
upper level disturbance, resulting in heavy showers and a slight
chance of a thunderstorm. Strong trade winds will accompany the
passage of the front, along with some cooler temperatures.

&&

.DISCUSSION...
A change in the weather is forthcoming. A cold front with strong
trade winds and cooler temperatures behind it will move down the
main Hawaiian Islands, starting with Kauai late Sunday afternoon,
Oahu early Sunday evening, Maui County during the rest of Sunday
night. The front is expected to come to a rest near Upolu Point
Big Island and the Alenuihaha Channel early Monday. Both the new
GFS model run and old ECMWF, are in pretty good agreement,
although the ECMWF has the front further west over Maui County.

The models have the front, or trough by this time, to drift back
west to Oahu on Tuesday, then Kauai Tuesday evening. As the front
or trough moves back west, the winds will become easterly and
lighter. As noted, the temperatures will be cooler on Monday and
Tuesday, lowering from a daytime high of 84 degrees, to the upper
70's and near 80. It could be possibly cooler, in the mid 70's on
Monday, with all the clouds and showers.

The rough weather is likely to occur between Monday and Tuesday,
that is when the front is slated to be in the vicinity of
Maui/Big Island, with increasing upper level support. The upper
trough will have a pool of cold air of about minus 16 degrees F at
18k feet. Normally, the temperature at that height is minus 8
degrees F.

In the coming hours, we will be assessing the need for some land
based advisories, such as a Flash Flood Watch, beginning as early
as Sunday night. The summits of the Big Island may receive some
wintry weather as well, which may warrant a Winter Storm Watch.
The strong trade winds will may warrant a Wind Advisory as well.

On Wednesday afternoon, the ex-front, now trough, will be west of
Kauai, and moving further away from the islands. The weather will
be somewhat better with less showers during the second half of
next week, but the air mass will still be unstable due to a
trough aloft. Trade winds will be in the moderate to strong range
across the region.

Currently, light south to southeast winds are present across most
of the smaller islands. A shower is noted just off Diamond Head,
and scattered showers along Oahu's windward shoreline. We do
expect isolated showers to pop up here and there the rest of
tonight across the smaller islands. Isolated showers have just
sprung up along parts of the Big Island's leeward coast, from
north of the airport to around Kawaihae/Kamuela. These showers
will dissipate by midnight or drift into the adjacent waters due
to a developing land breeze. The Hilo, Puna, parts of the Kau
district may get a shower or two overnight, carried ashore by a
trade wind flow.


&&

.AVIATION...
Winds have diminished considerably in advance of a cold front
approaching Kauai from the northwest. Mostly clear skies and weak
winds will allow land breezes to develop in favored areas tonight
as VFR prevails. Light wind will then emerge out of the southwest
as the front approaches during Sunday with continued VFR and a
low coverage of poorly organized shower activity for Oahu through
Big Island. Nothing more than brief MVFR is expected within the
heaviest showers for these locations.

Deepening moisture will bring the potential for increasing showers
to Kauai by late Sunday morning or early Sunday afternoon with
increasing probability for widespread showers after about 00z/2pm
HST. Prevailing MVFR and widespread showers, some locally heavy,
can be expected over Kauai as front moves across the island around
03z/5pm or shortly thereafter. In addition to the onset of more
widespread rainfall, the arrival of the front will be marked by a
sharp transition to gusty northerly winds.

No AIRMETs in effect.

&&

.MARINE...
In summary, expect giant surf to continue tonight, then gradually
decline tomorrow, with another giant swell Sunday night into
Monday. A potent cold front will move over most of the island
chain late Sunday night into Monday, and then stall over the
islands Monday and Tuesday, bringing the potential for heavy
showers and thunderstorms. Strong to near gale-force N to NE
winds will develop over most waters as the front passes,
potentially persisting into Tuesday.

A High Surf Warning remains in effect for exposed N and W facing
shores as the largest NW swell of the winter season continues to
propagate through island waters. The peak of the swell has already
occurred in Kauai and Oahu waters, and will likely peak in Maui
County and Big Island waters overnight. Buoy data indicate that
surf may drop below warning-levels from NW to SE during the day
Sunday, but another giant swell building Sunday night and Monday
will once again bring warning-level surf, but peak surf heights
are expected to be lower than today's swell. Very strong N to NE
winds will destroy any chances of quality surf along N facing
shores with this swell. A High Surf Advisory remains in effect
for exposed N and W facing shores of the Big Island, as most of
the swell's energy is blocked from upstream islands.

A Small Craft Advisory remains in effect for all waters except
Maalaea Bay, as the NW swell continues to produce seas greater
than 10 feet. A Marine Weather Statement is highlighting the
potential for surges and significant wave action in and near
exposed harbors, although the potential will be gradually
diminishing through Sunday. Due to the period and size of the NW
swell, some (increasingly reduced) swell may wrap into select S
and E facing shores.

A vigorous cold front will rapidly approach the islands from the
NW tonight and tomorrow, leading to a period of light to moderate
S to SW winds, with some pre-frontal showers developing within
this flow through Sunday. The front will move over Kauai Sunday
evening, and Oahu by midnight before stalling over the central
portion of the island chain early Monday, bringing some heavy
showers and the potential for thunderstorms. Strong to near gale-
force N to NE winds will follow the front, and a Gale Watch is in
effect for waters from the Alenuihaha Channel westward starting
Sunday night, due to the tight gradient between a strong high to
the NW and the lower pressure along the stalled front (that
eventually transitions to a trough). A developing sharp trough
aloft on Monday will likely invigorate the lingering moisture
through Wednesday as it drifts westward, keeping the potential
for heavy showers and thunderstorms.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM 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 Surf Advisory until 6 PM HST Sunday for Kona-Big Island
North and East-Kohala.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters except Maalaea Bay-

Gale Watch from Sunday evening through late Monday night 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-Maalaea Bay-Pailolo Channel-Alenuihaha Channel.


&&

$$

H Lau/JVC/Birchard

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