High Surf Advisory 12, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Sat Jan 28 2017

.SYNOPSIS...
A cold front moving across the Kauai Channel early this evening
will be reinforced by another front and push down the island
chain through Sunday night. Clouds and showers over Kauai will
spread to Oahu and Maui County overnight, and to the Big Island
Sunday as the front progresses. Gusty north winds will increase
with and behind the front, and persist into Monday as a cooler and
drier airmass moves over the islands. High pressure passing north
of the state early next week will bring a dry and stable airmass
to the islands with moderate trade winds. Winds will decrease and
shift southerly late Wednesday and Thursday as another front
approaches the state from the northwest.

&&

.DISCUSSION...
Satellite/radar data and surface observations show a cold front
located over the Kauai Channel and approaching Oahu early this
evening, with scattered mostly light showers located along and
behind the front. Six hourly rainfall amounts on Kauai have been
mostly a half inch or less, with a couple of stations receiving
up to an inch of rain. Mainly dry conditions prevail across the
remainder of the state ahead of the front. A second reinforcing
cold front is analyzed a couple hundred miles to the northwest of
Kauai. The 00Z Lihue sounding was just ahead of the front, and
showed the inversion rising to about 8K feet and PW of 1.03
inches, while the 00Z Hilo sounding remained quite dry and stable
with a strong inversion near 5K feet and PW of 0.81 inches. MIMIC-
TPW satellite imagery shows that PW values on Kauai have increased
to around 1.4 inches along the front, while PW values drop off
sharply behind the front to between 0.5 and 0.7 inches.

Overnight, the front will continue moving southeast across Oahu
and into Maui County, bringing clouds and scattered showers.
Rainfall amounts will remain light to moderate, with the best
rain chances on windward and north facing slopes. Northerly winds
have already increased on Kauai early this evening, and will
increase on Oahu and into Maui County as well later tonight as the
front arrives. However, wind speeds are expected to remain below
advisory levels through the night. Rainfall will begin to taper
off on Kauai late tonight as the drier airmass begins to move in,
although showers will likely persist on north facing slopes.

Winds on the Big Island summits have increased this evening, and
model guidance suggests that they will probably increase further
overnight into Sunday. A Wind Advisory may be needed for the
summits during that time.

On Sunday into Sunday night, the reinforcing front is expected to
merge with the leading front, pushing a drier and cooler airmass
southward through Maui County and the Big Island. Scattered
showers will continue along and for a period of time behind the
front. Rainfall amounts should remain mostly light, although more
substantial amounts will be possible in areas favored under
northerly post-frontal flow such as the Hamakua coast on the Big
Island. Strong north/northeast winds will be the main weather
concern behind the front, and wind advisories may be needed for
Sunday into Monday. Winds will be strongest over mountains,
downslope of mountains and through mountain gaps. Will evaluate
expected winds further overnight and may issue an advisory early
Sunday morning. It should be noted that wind speeds are expected
to be significantly lower than those observed last weekend, when a
widespread high wind event impacted the state. A Special Weather
Statement will be issued shortly to highlight the expected windy
conditions coming up for Sunday into Monday.

Otherwise, a notably dry and cooler airmass will move in behind
the front, with dewpoints dropping well into the 50s and daytime
highs only in the 70s to near 80 for most locations.

Winds will begin to turn more northeasterly on Monday, with a
moderate to locally breezy trade wind pattern in place Tuesday
and Tuesday night as a high pressure system passes north of the
state. Expect rain chances to remain low to nil during this time,
with the dry and stable airmass remaining in place over the
islands.

Another front will approach the state later part of next week. The
GFS and ECMWF are still in good agreement that trade winds will
decrease on Wednesday then shift southerly by Thursday as the
advancing front drives the another ridge near the islands. The
airmass near the islands is expected to become somewhat more
unstable, so we could see a few more showers. Southerly winds
could draw up vog across the smaller islands. The front is
forecast to pass mainly north of the state Thursday night and
Friday, with the surface ridge remaining nearby and rather light
southerly winds continuing over the islands. A stronger front
approaching from the northwest may cause increasing southwest
winds over the state next weekend.

&&

.AVIATION...
The frontal cloud band will reach Oahu in the next hour or so and
will continue down the rest of the island chain rest of the evening
into Sunday. Still expect more MVFR ceiling with showers conditions
to affect mainly windward and mountain areas. Winds will continue to
increase across the area and will become quite strong northerly
overnight.

As the winds increase behind the front, AIRMET for low level
turbulence will be posted for Kauai. Conditions will spread to the
rest of the islands overnight into Sunday. AIRMET for mountain
obscuration along windward areas is still a possibility with the
increasing low clouds.

&&

.MARINE...
A large northwest swell will peak this evening, then subside
briefly overnight. Surf from this swell will remain above the
advisory level along north and west facing shores of the smaller
islands into Sunday. A larger swell from a more northerly
direction will build Sunday and Sunday night, then gradually
subside. Surf from this swell may reach warning levels along north
and west facing shores of the smaller islands Sunday night. The
swell will shift out of the north northeast Monday night and
Tuesday. As the swell direction shifts, east facing shores,
including east facing shores of the Big Island, may get surf above
the 15 foot warning level.

A small to moderate size northwest swell is on tap for the
Wednesday through Friday time period. An east swell will keep
surf elevated on east facing shores, especially Sunday through
Tuesday, when heights may rise to near advisory level.

The large northwest swell will cause seas to exceed the 10 foot
Small Craft Advisory (SCA) threshold over waters exposed to the
swell. Winds will strengthen overnight as a front near Kauai
pushes down past the Big Island Sunday night. SCA is posted for
all waters through Monday afternoon. Winds will reach near-gale
strength in the windier areas on Sunday and could reach gale force
on Monday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for north and west
facing shores of Niihau Kauai Oahu and Molokai and north facing
shores of Maui.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters.

&&

DISCUSSION/MARINE...Jacobson
AVIATION...Hui