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Hawaiian Islands Synoptic Discussion and Guidance

582
FXHW60 PHFO 170224
AFDHFO

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

.SYNOPSIS...
Wet weather is on the way as a strong cold front approaches the
state Sunday into Monday. This front will quickly advance down
the island chain Sunday night, then stall around the Big Island
Monday through Tuesday. Heavy showers and isolated thunderstorms
will be possible along and ahead of this front. Cool and strong
northerly winds are expected following the frontal passage over
the western end of the state early next week. Trade winds may
return late next week.

&&

.DISCUSSION...
As a strong cold front approaches the state Sunday, we will see
the winds shift toward the south and southwest on Sunday. By
Sunday afternoon, the front is expected just northwest of Kauai
and will begin to bring the threat of heavy rain and isolated
thunderstorms. The cold front is expected to continue to move
down the island chain Sunday night through Monday with the threat
of heavy rain and isolated thunderstorms along and ahead of the
front.

Right behind the front, we will see gusty northerly winds
(potentially reaching advisory levels beginning Sunday night) and
cool conditions will follow with dewpoints dipping into the 50s
for Kauai and Oahu.

There remains subtle differences in the global models on how far
east the front will progress before stalling. But the GFS and
ECMWF remains in fairly good agreement with he front stalling
near Maui or the Big Island. Either way, the Big Island and Maui
will remain rather wet on into Wednesday, which could result in
localized flooding concerns.

Winter weather conditions are also expected over the Big Island
summits as early as Monday, but more likely by 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. A Winter Storm Watch could be
possible for the summits if the boundary looks to set up over the
Big Island.

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...
Light to moderate easterly trades will persist through this
afternoon, then shift to southeasterly tonight ahead of an
approaching frontal system. Expect shower activity to increase
slightly tonight, as moisture deepens, with brief periods of MVFR
ceilings and visibility possible.

By late in the day on Sunday, the aforementioned front will begin
to push across Kauai and bring scattered to numerous showers and
a slight chance of thunderstorms.

No AIRMETs are currently in effect.

&&

.MARINE...
In summary, expect giant surf to continue through the island chain
tonight and decline tomorrow. A potent cold front will move down
the island chain late tomorrow night into Monday. Another large NW
swell will fill in Monday.

High Surf Warning is in effect as the largest NW swell of the
winter season rapidly moves down the island chain, with peak surf
heights of 40 to 50 feet (with locally higher waves) along
exposed shores tonight and into tomorrow. PacIOOS Waimea buoy
swell heights and period have been on the rise all day and
currently at 16 feet 18 seconds. Offshore NDBC 51001/51101 buoys
have started to show a slow decline in swell height which are now
running in the 18 feet 17 second range. The swell may have peak
already for Kauai. The swell is still anticipated to peak late
this afternoon to early evening for Oahu, and in the evening for
Maui. A High Surf Advisory remains in effect for exposed N and W
facing shores of the Big Island as the swell is expected to peak
locally overnight. The swell will gradually diminish from NW to
SE tonight through Sunday night, but a new NW swell arriving
Monday will likely push surf to warning levels once again, with
peak surf heights lower than today's swell.

The Small Craft Advisory is still in effect for most waters except
Maalaea Bay, as E/SE winds have eased below SCA criteria, but
seas continue to build around the islands due to the giant NW
swell moving through the island chain. A Marine Weather Statement
is in effect due to the potential for surges and significant wave
action in and near exposed harbors. Due to the period and size of
the NW swell, some (much reduced) swell may wrap into select S and
E facing shores.

A vigorous cold front will rapidly approach the islands from the
NW, tomorrow and tomorrow night, providing a brief period of
light to moderate S to SW winds. The front will initially bring
the potential for locally heavy showers and slight chance of
thunderstorms as it moves relatively quickly through Kauai and
Oahu Sunday night, and then slows to a stall and dissipates over
Maui County and the Big Island Monday. A Gale Watch is in effect
for all coastal waters around the smaller islands as a strong high
to the NW of the islands will continue to support strong to gale
force NE winds behind the front. The moisture from the dissipated
frontal boundary may retrograde back west Tuesday as an upper
level trough moves over the islands from the NW. This setup may
increase the potential for heavy showers trade showers and
thunderstorms along the remnant boundary through mid-week.

&&

.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.

&&

$$

DISCUSSION...Kino
AVIATION...Thomas
MARINE...Almanza

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

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