High Surf in Hawaii, December 16 to 17, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
350 AM HST Fri Dec 16 2016

.SYNOPSIS...
A weak cold front over the Big Island will move east of the state
later today. A deep low pressure system associated with the front
will move south and pass just west of the main island chain
Saturday and Sunday, introducing a slight chance for
thunderstorms. The low will drag deep moisture in from the east
Sunday through Monday night. Trade winds will return Monday night
into Tuesday.

&&

.DISCUSSION...
A ragged band of broken to overcast showery low clouds marking a
weak cold front extends northeast to southwest across the
northern third of the Big Island this morning and is moving
southeastward. Overcast low cloud cover persists up to 100 miles
west of the frontal boundary, including much of Maui, while
Molokai and Lanai have cleared out. Earlier pre-frontal cloud
cover across the southeastern two-thirds of the Big Island has
diminished dramatically as well. This cold front extends northward
to a deep well-defined low near 29N158W, about 530 miles north of
Oahu. This low has moved very little overnight. Behind the clear
slot immediately west of the frontal cloud band, broken stable
post-frontal cloud cover is moving southeastward across Kauai and
Oahu.

Northwest post-frontal winds continue to increase across Kauai,
Oahu and adjacent waters. Northwest winds get weaker toward the
frontal boundary, with light southeasterlies noted just east of
the Big Island. For this morning, expect dry and cool conditions
across the central portion of the state. Expect increased rainfall
across Kauai through today as moisture associated with the low
begins to merge with the post-frontal cloudiness, enhanced a bit
by cooling temperatures aloft. The front will move across the Big
Island today, so cloudy skies and showers are likely to continue
there over the next 12 to 18 hours before the front makes it east
of the state.

The forecast calls for the low north of the state to move
southward toward us, then pass just west of Kauai this weekend.
Models remain in good agreement with the associated cold pool
aloft arriving today across the western end of the island chain,
then building eastward. The resulting instability will introduce
a slight chance of thunderstorms to the smaller islands today and
Saturday. Precipitable water (PW) values will be average to below
average today as the islands sit in a dry slot between the frontal
boundary and the parent low. We expect breezy westerly winds, a
few showers, and a slight chance for thunderstorms today through
Saturday. Shower coverage will increase somewhat on Saturday as
models show PW values will increase slightly.

A High Wind Warning for Big Island summits remains in effect
through Saturday, thanks to increasing winds associated with the
southwesterly jetstream east of the low and front. It is likely
the High Wind Warning could be extended through Sunday before
winds weaken early next week.

Cold air aloft will push Big Island summit temperatures below
freezing into this weekend, but it remains to be seen if enough
deep layered moisture will make it over the Big Island for
snowfall to occur, at least until Saturday night or Sunday. If
thunderstorms form over the Big Island Friday through Sunday and
track over the summits, quick bouts of snowfall could occur. For
now, we will keep a chance for summit snow in the forecast but not
issue accumulation details due to low confidence.

As the low passes just west of the main islands, it will drag
deep frontal moisture hung up east of the Big Island back over
the state Sunday and Monday. Winds will shift to southeasterlies
and bring moist tropical air and vog across the state. We will
watch how this return moisture develops over the next few days to
see if a Flood Watch or Hydrological Outlook is needed. Trade
winds will build back in Monday night and Tuesday, returning our
weather to more typical windward and mauka showers.

&&

.AVIATION...
The cold front moved ashore of western Big Island shortly
after midnight HST. The front and its weather will continue
eastward, looking to push off the eastern shores this afternoon.
AIRMET SIERRA is in effect for the lee and northern portion of the
island, and is expected to be extended for the rest of the morning.
Clouds and showers will likely spread to the rest of the island,
that is the eastern part by mid-morning. To what degree is the
question. Showers appear to be abating across Kohala and Kona
now, hinting that it is a weak front. We will likely go with VFR
for this sheltered area of the island.

Although the showers have moved east of Maui, a band of
stratocumulus clouds /sc/ remains over the island with bases
above 5k feet, tops 8k feet. So no AIRMET SIERRA.

Another area of low clouds consisting of mainly stratocumulus has
spread eastward from Kauai to currently Oahu. It will continue
eastward through the rest of morning reaching into Maui County by
or shortly after sunrise. There are embedded showers with these
clouds which has a high ceiling base at 5 or 6k feet. Tops 8k
feet.

Located just off the northwest coast of Kauai is another band of
clouds but with tops to 15k feet and maybe heavier showers. Expect
this band to reach Kauai around sunrise and continue eastward to
Oahu by mid morning and Maui County this afternoon. The timing is
only an estimate and subject to updates. Widespread MVFR ceilings
and vis may prompt the issuance of AIRMET SIERRA for Kauai
initially.

Otherwise VFR conditions will prevail.

The westerly winds will become southwesterly this morning and
strengthen to moderately strong in the afternoon. This may warrant
AIRMET TANGO for some of the smaller islands. TANGO will be for
low level turb located over and north through east of mountains
below 6k feet. AIRMET TANGO for high level turb above the island
skies in association with the upper trough remains.

No other AIRMETs are expected.

&&

.MARINE...
The morning surface analysis and satellite imagery showed a large
gale-force low centered a few hundred miles north of Kauai that was
drifting southward. The overnight ASCAT pass depicted a decent sized
batch of 35 to 40 kt north and northwest winds to its west pointed
at the local marine waters. This pass also clipped the Oahu and
Kauai waters where 20 to 25 kt northwest winds were shown. A recent
altimeter pass within this fetch depicted maximum seas just above 20
ft, which is already being matched at the northwest buoy (51001, 21
ft) as of 3 am. The local nearshore CDIP buoy near Hanalei has
already climbed above the predicted levels by a couple of feet (16
ft) and will likely continue to rise through the morning hours today
before peaking. This general trend will continue across the
remainder of the Hawaiian waters today and will persist through
early Saturday morning before beginning to ease.

The Small Craft Advisory has been expanded to include all of the
exposed coastal waters through Saturday. Surf heights for Kauai have
been raised to reflect the current observations that are coming in
above the predicted levels. A High Surf Advisory has been included
in this morning package for the northern/windward coast of the Big
Island, which should fill in later today through tonight.

The latest model guidance has initialized well with this gale north
of the waters and shows it slowly tracking southward toward the
Kauai waters over the weekend, then southwest of the marine areas by
early next week. All guidance supports a gradual weakening trend as
this system approaches and moves through. A cold front associated
with this gale is currently over the Big Island waters and is
forecast to stall east of the Big Island through the weekend. High
pressure will build south early next week as the low moves away from
the waters and weakens further.

Local winds will steadily shift from the northwest to the west-
southwest today through Saturday...then toward the south-southeast
later in the weekend as the low drifts south-southwest. The best
chance for advisory-level winds will remain over the typically
windier channel waters and the northwest marine zones through
tonight.

A combination of the local winds, large surf and higher than normal
high tides could translate to overwash onto the typical vulnerable
roadways through Saturday morning and will need to be monitored.
This best chance for this will be around the times of high tides.

A new long-period west-northwest (320 deg) is expected to fill in
locally beginning Saturday night, peak through the day Sunday, then
ease into the upcoming week. This source is from a hurricane-force
low that was analyzed yesterday morning over the far northwest
Pacific near the Kamchatka Peninsula. Surf associated with this
source could reach advisory levels along north and west facing
shores Sunday if it comes in slightly larger than expected and will
need to be monitored.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

High Surf Advisory until 6 AM HST Saturday for Waianae Coast-Big
Island North and East.

High Wind Warning until 6 PM HST Saturday for Big Island Summits.

Small Craft Advisory until 6 PM HST Saturday 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.

&&

DISCUSSION...Powell
AVIATION...Lau
MARINE...Gibbs