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Hawaiian Islands Synoptic Discussion and Guidance

331
FXHW60 PHFO 130648
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Tue Dec 12 2017

.SYNOPSIS...
A dissipating cold front located over the southeast portion of
the state will shift east of the Big Island through the day
Wednesday. Gusty north-northeast winds with cool and dry
conditions will fill in behind the front Wednesday through
Thursday, with unsettled weather possible Friday through the
weekend.

&&

.DISCUSSION...
The latest hi-res model guidance remains in good agreement tonight
through midweek and shows the frontal boundary over Maui County
shifting east of the Big Island by Wednesday night. Clouds and
showers along and ahead of this boundary will remain possible
through this time. Rainfall accumulations, however, will remain on
the light side as the front continues to dissipate as it moves
through. The latest rainfall summary across the state over the
past 24 hrs reflects this with peak amounts around and just over
an inch on Kauai, less than three quarters of an inch on Oahu, and
up to a quarter of an inch through 8 pm HST this evening for Maui
County.

Drier and cooler air will fill in (PWs down toward half of an inch;
dewpoints into the mid/upper 50s) behind this front overnight
through midweek. North-northeast will become gusty Wednesday
through Thursday (potentially reaching wind advisory levels for
some of the typically windier areas with this flow come Thursday).

Changes in the weather are afoot late in the week as global models
are consistently depicting an upper level low dropping south as it
cuts off from the jet stream to the north, reaching a position
northeast of the islands Friday. The upper low will bring
instability and cold temperature to the middle and upper atmosphere
allowing for increasing rainfall chances, and heavier showers. Near
the surface, a low is expected to form east of the state in response
to the upper low. The instability aloft, low pressure near the
surface, and increasing low level moisture will lead to unsettled
weather over the islands with the possibility of heavy rain and
thunderstorms, and the chance of snow for the Big Island summits.
However, models are still having a difficult time agreeing with
the location of the upper low and the timing of how long the low
stays in the island's vicinity. The current forecast has
increasing showers and rainfall totals Thursday night through
Saturday. We will likely introduce a slight chance of thunderstorms
to the forecast starting Friday over the eastern windward waters as
the upper low will mostly likely bring instability over those areas.
More fine tuning will continue as the details become clearer as
model solutions converge.

&&

.AVIATION...
A weakening cold front over the central islands will continue to
move down the island chain overnight and early Wednesday morning.
AIRMET Sierra remains in effect for mountain obscuration for Oahu,
Molokai and Lanai, and has begun for Maui. Low cloud coverage is
already on the decline over Oahu, and anticipate that the AIRMET
there will be canceled in the next few hours. Anticipate
conditions improving over Maui County during the second half of
the night. The frontal band is expected to reach the Big Island
during late tonight or early Wednesday morning, and depending on
how much of it hangs together, an AIRMET may be needed there.

Northerly winds pick up a bit behind this front, but at this time
do not anticipate any significant low level turbulence.

&&

.MARINE...
A High Surf Warning is in effect for north and west facing shores
of most islands with a High Surf Advisory for north facing shores
of the Big Island of Hawaii. The swell height this evening at buoy
51101, about 350 miles northwest of Honolulu, had reached over 19
feet, and had built over 10 feet at the Kauai buoy. This north
northwest swell will peak Wednesday, remain quite large on Thursday,
then subside through Saturday.

North winds are spreading over the Hawaiian coastal waters behind
a dissipating front near Maui. Wind speeds are below the 25 knot
Small Craft Advisory (SCA) threshold this evening, but a SCA is in
effect for most waters due to large seas associated with the north
northwest swell. Wind speeds will increase as high pressure builds
northwest of the area and a trough forms to the east. Winds will
reach SCA levels across much of the area by Wednesday evening and
peak with near-gale force winds in the windier areas around the
Big Island and Maui County on Thursday. Winds will subside Friday
into Saturday as the trough approaches from the east.

See the Collaborative Surf Discussion for details on surf and
swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday 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 PM HST Thursday for Big Island North
and East.

Small Craft Advisory until 6 AM 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-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters.

&&

$$

AVIATION...M Ballard
MARINE...Donaldson
REST OF DISCUSSION...Gibbs/Morrison

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