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Temp3.4 C
RH17 %
WindSW 7 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

525
FXHW60 PHFO 110145
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Sun Dec 10 2017

.SYNOPSIS...
Moisture associated with a dissipating front will bring clouds and a
few showers to windward areas of Oahu and Maui county this evening
before gradually dissipating overnight. Light winds will prevail
through Monday, then increase late Monday into Tuesday as a stronger
front moves down the island chain Tuesday. The front will bring a
period of showers, followed by cool, breezy and mostly dry north
winds on Wednesday and Thursday. There is the potential for
unsettled weather toward the end of the week as a low pressure
system develops near the islands.

&&

.DISCUSSION...
Shallow moisture associated with a stalled and dissipating front is
draped over the central portion of the island chain this afternoon,
fueling a few mostly windward showers from Oahu to Maui while Kauai
and the Big Island have been dry today. A fast-moving and relatively
weak high pressure cell is passing several hundred miles NE of the
islands, supporting a light to moderate low-level flow that is
currently veering from E to SE. Water vapor imagery shows a ridge
aloft building over the area from the W, while afternoon soundings
highlight a dry and stable atmosphere with PWAT < 1" and a
subsidence inversion based between 6 and 8 kft. A cold front about
875 miles NW of Kauai is moving rapidly SE.

As the high to the NE moves swiftly E in response to the rapidly
approaching front, an associated ridge will move over the islands
from the NW overnight, and the low-level flow over the area will
weaken and veer to the S. This will allow the remnant showers to
lift N, as developing land breezes likely preclude their ability to
move ashore after midnight.

Light S to SW winds Monday will strengthen somewhat near Kauai and
Oahu as the front draws closer Monday night, with increasing pre-
frontal showers. Frontal passage is depicted as occurring on Kauai
late Tuesday morning in latest GFS/ECMWF guidance, Oahu and Maui
county Tuesday evening, and Wednesday on the Big Island. With
limited upper-level support, the front is not expected to bring a
significant flood threat. However, moisture up to about ~12 kft will
allow for some briefly heavy downpours to develop both in the pre-
frontal bands that are expected to develop near Kauai and Oahu
Monday night, and along the frontal band as it passes over all
islands. Breezy, cool and mostly dry weather will move in from the N
and NE after the front passes, with this general weather pattern
expected to continue into Friday.

After Thursday however, forecast uncertainty increases dramatically
as forecast models have been exhibiting a fair amount of run-to-run
variability, due to the shortwave diving SE toward the islands by
mid-week. The 12Z GFS/ECMWF/CMC indciate that the shortwave will
develop into a cutoff low just E of the islands on Thursday, and
then drift W over the islands Friday and Saturday before gradually
dissipating Sunday and Monday. If this scenario develops, island
weather would become very active, with the potential for heavy rain
and thunderstorms, as well as snowfall and high winds at the Big
Island summits. However, if the NAVGEM is correct (and now the 18Z
GFS) the cutoff would remain E of the islands, and the impacts on
island weather would be significantly less. With confidence in the
large scale weather pattern so diminished, the details of the
forecast are highly uncertain, and no significant changes to the
longer range forecast are expected at this time.

&&

.AVIATION...
AIRMET SIERRA for MTN OBSC is in effect for Maui, Lanai, Molokai and
Oahu. An ENE-WSW frontal cloud band over Maui, Lanai, Molokai and
will produce widespread MVFR ceilings and scattered showers through
the evening. The cloud band is forecast to shift N ahead of another
front approaching from the NW. As the cloud band shifts N,
conditions are expected to improve over Maui and Lanai.

VFR will prevail at the terminals on Kauai, the Big Island and
leeward Oahu with MVFR on Maui, Lanai and Molokai and windward Oahu.

&&

.MARINE...
The current large north-northwest swell should be nearing its peak
this afternoon at least over the western islands, and this evening
near the Big Island. Warning level surf will continue on the north
and west facing shores of the smaller islands exposed to the surf,
and advisory level remaining on the north facing of the Big Island
tonight. This swell will continue to decline through Monday night.
Expect the warning to be downgraded to an advisory by morning. Surf
along the north facing shores of the Big Island are expected to be
below advisory by Monday morning.

This current swell is bringing seas in excess of 10 feet to exposed
waters, and an SCA remains posted through tonight. Seas are expected
to lower tonight from west to east. A few areas may need to have the
SCA extended tomorrow morning depending on how quickly the seas drop
tonight.

The next north-northwest swell is expected to peak Tuesday
night/early Wednesday and is expected to be even larger than the
current swell. This swell is expected to produce warning-level surf
on north and west facing shores. This swell will also bring seas in
excess of 10 feet. The current forecast brings 10 foot seas to the
Kauai northwest waters as early as midday Tuesday.

A new front will move down the island chain Tuesday/Wednesday, with
northerly winds increasing behind the front. Winds behind this next
front will likely reach SCA levels by Wednesday night.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Monday for North and West facing
shores of Niihau Kauai Oahu Molokai and Maui.

High Surf Advisory until 6 AM HST Monday for North facing shores of
the Big Island.

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

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...M Ballard

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