288 
FXHW60 PHFO 121322
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
322 AM HST Tue Dec 12 2017

.SYNOPSIS...
A cold front will move down the island chain today through 
Wednesday. Breezy north winds will bring cooler and drier 
conditions across the state behind the front tonight through 
Thursday, with unsettled weather possible from Friday through the 
upcoming weekend.

&&

.DISCUSSION...
The short-term guidance remains in good agreement through midweek
and brings a cold front (currently 100 miles northwest of Kauai) 
over Kauai early today, then east of the state by Wednesday night.
A better source of moisture associated with this boundary 
combined with upper height falls and isentropic ascent will 
translate to decent rainfall coverage across the state as this 
boundary moves through. 

Hi-res model reflectivities depict the main band of showers along
the front reaching Kauai by noon today, Oahu through the 
afternoon hours, Maui County this evening, then the Big Island 
tonight into Wednesday. Although periods of heavy rainfall can't 
be ruled out through this period, any widespread flooding 
potential remains low due to the front quickly advancing down the 
island chain. Another shot of cool and dry air will fill in (PWs 
down toward half of an inch; dewpoints into the mid/upper 50s) 
behind this front tonight through Wednesday. Winds will shift out 
of the north-northeast and become breezy Wednesday through 
Thursday (nearing wind advisory levels for some of the typically
windier areas by Wednesday night/Thursday).

Thursday through the weekend, forecast uncertainty remains high due 
to some notable differences shown between model solutions that have 
been consistent from run-to-run over the past few days. In general, 
all solutions indicate a shortwave trough will develop into a cutoff 
low near or just east of the islands on Thursday, then drift 
westward over the state Friday through Saturday before becoming 
absorbed as another trough approaches late in the weekend. The 
main difference is shown between the GFS and ECMWF, where the GFS 
suggests the bulk of the moisture hanging up east of the state 
(dry slot with PWs hanging around an inch through Saturday over 
the islands). The ECMWF, however, remains the wetter solution and 
depicts a deeper source of moisture along with breezy northeast
winds through this time. If this ECMWF scenario evolves, 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. The current forecast package through this 
period reflects a modest increase in rainfall chances with a 
slight chance of snow on the Big Island summits as a compromise 
between the two possible scenarios. The chance of thunderstorms 
may become included in later packages as confidence rises over the
upcoming couple of days.

&&

.AVIATION...
A few clouds and showers will linger over Oahu and Molokai this 
morning. A cold front approaching the Hawaiian Islands from the 
northwest will reach Kauai by 18Z and then move down the island 
chain to the Big Island by Wednesday afternoon. There will be 
widespread MVFR and localized IFR conditions along the frontal 
band as it moves through the northern islands, the front will
weaken with areas of MVFR once it reaches Maui and the Big Island.

No AIRMETs currently in effect. AIRMETs for mountain obscuration
are expected later this morning as the cold front moves into
Kauai and Oahu.

&&

.MARINE...
The incoming north-northwest swell has been building on schedule 
during the night at Buoy 51101. This swell is expected to arrive 
in the islands during the day, then peak tonight into Wednesday. 
It is expected to be larger than the last swell, and will produce 
warning-level surf on north and west facing shores of the smaller 
islands. Surf on the north facing shores of the Big Island is 
expected to reach advisory levels in some areas, but with a lot of
local variations due to the expected 320-330 degree direction of 
this swell. PacIOOS run-up forecasts for the Oahu north shore 
indicate possible overwash of some lower roadways near the coast 
during the peak of this next event. A High Surf Warning has been 
issued for most north and west facing shores of the smaller 
islands for today through Thursday. A High Surf Advisory has also
been issued for the north facing shores of the Big Island for 
tonight through Thursday.

This new swell will also bring seas above 10 feet across most of 
the marine area, beginning late this morning for the far 
northwest waters, and spreading across the rest of the area 
this afternoon through Wednesday morning. A Small Craft Advisory
(SCA) has been posted for most of the marine area. Seas are 
forecast to remain above 10 feet for most waters through Friday.

The next cold front is forecast to move down the island chain
today through Wednesday morning, with northerly winds increasing 
behind the front. Winds behind this front will likely reach SCA 
levels across much of the area by Wednesday afternoon or Wednesday
evening. These strong winds are forecast to continue on Thursday,
then decrease from east to west across the area Friday into 
Saturday.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from 6 AM this morning to 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 from 6 PM this evening to 6 PM HST Thursday 
for Big Island North and East.

Small Craft Advisory from 6 PM this evening to 6 AM HST Friday 
for Maui County Leeward Waters-Pailolo Channel-Alenuihaha 
Channel-Big Island Windward Waters.

Small Craft Advisory from 6 AM this morning to 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.

&&

$$

AVIATION...Bohlin
MARINE...Jacobson
REST OF DISCUSSION...Gibbs