High Surf in Hawaii, January 18 to 20, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
410 PM HST Wed Jan 18 2017

.SYNOPSIS...
High pressure east northeast of the state will keep mainly moderate
to fresh trade winds through Thursday. Winds will become locally
light and variable in areas Thursday night and Friday as a front
approaches from the northwest. The front will pass over the
islands this weekend, then stall out near the Big Island early
next week, bringing increasing showers across the state.
Additionally, very strong trade winds are expected to overspread
the state this weekend, before easing early next week.

&&

.DISCUSSION...
A 1024 mb high is centered about 1200 miles east northeast of Hawaii,
with an east-west oriented ridge with the axis passing 250 miles
north of Kauai. The high is driving mainly moderate to fresh trade
winds across the state this afternoon, with some stronger winds in
exposed areas. Visible satellite shows mostly sunny skies with a few
clusters of low clouds banked against the windward slopes and on the
Kona side of the Big Island. The only area of showers this afternoon
impacted windward Oahu which has since moved northeast of the area.
00Z soundings at Lihue and Hilo show very low and strong inversions
at 4000 to 4500 feet and well below average precipitable water values.
The dry and stable trade wind driven weather will continue tonight
and Thursday. Stratocumulus clouds from upstream may fill in again
later on this evening over windward areas, but not anticipating
much rain out of these clouds.

A cold front is located nearly 850 miles northwest of Kauai moving
east towards Hawaii at 20 mph. As the front approaches, the high will
weaken, causing our trade winds to also weaken. Localized nighttime
land breezes and daytime sea breezes may set up over leeward areas
Thursday night and Friday. Winds aloft are expected to gradually
weaken over the next couple of days, and the Wind Advisory for the
Big Island summits has been cancelled.

Northeast winds will dramatically increase late Friday night into
Saturday as the cold front reaches the state and a strong high
builds behind. The strong pressure gradient between these two
features will result in very windy trade winds through the
weekend. Winds will likely be strong enough for a Wind Advisory
for all or portions of the state, though it is too early to issue
this product. We have highlighted this event with a Special
Weather Statement (SPSHFO) available on our website. Along with
the winds, an increase in showers is expected. Showers will be
most prevalent across windward and mauka areas, but with the
increasing trades and rising inversion heights, some of the
showers will reach leeward areas of the smaller islands as well.

The front will stall out near the Big Island Monday with winds
veering towards the east southeast ahead of another approaching
front. Both long range GFS and ECMWF keep most of the moisture
associated with this second front just north of the state as it
skirts by Monday night and Tuesday. However, a minor shift
southward may bring a bit more moisture over the state, so we'll
be watching it for changes in the coming model runs.

&&

.AVIATION...
Locally breezy trade winds will continue to transport low
clouds and isolated showers across the smaller island and onto
windward facing slopes and coasts of the Big Island. Weak sea
breezes and mild afternoon convection along the leeward coast of
the Big Island will both diminish after sunset. Localized MVFR
CIGS possible in passing showers. Otherwise, VFR conditions are
expected to prevail.

AIRMET Tango remains in effect for TEMPO moderate low-level
turbulence leeward of the mountains on all islands due to the
breezy low-level trade wind flow. This AIRMET will likely be
continued into tonight, but may not be necessary on Thursday,
since the trade wind speeds are expected to decrease over the
next 24 hours.

&&

.MARINE...
Data from buoys 51001 and 51101 northwest of the state are
showing the new west northwest swell has arrived and will gradually
fill in along north and west facing shores overnight. This swell
is expected to peak late tonight and early Thursday, then lower
gradually late Thursday and Thursday night. With the more
westerly direction of this swell, high surf will also reach west
facing shores of The Big Island later tonight and Thursday.
Another large northwest swell is expected to arrive Thursday night
and Friday with advisory level surf expected. This swell will
lower gradually over the weekend. A short period north northeast
swell is expected over the weekend behind a cold front moving
across the area. The swell will gradually swing around to the east
early next week. Advisory level surf is likely along north and
east facing shores with this new swell. Another large northwest
swell is expected for next Tuesday and Wednesday.

A Small Craft Advisory is presently posted for most Hawaiian
coastal waters mainly due to seas associated with the new west
northwest swell arriving. Winds around portions of The Big Island
are also reaching Small Craft Advisory levels. Winds are expected
to drop off later tonight and Thursday. Winds behind a cold front
moving across the chain this weekend will cause winds to exceed
advisory criteria and could reach Gale Warning levels across some
areas. Also seas will remain rather high through the weekend and
on into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for north and west
facing shores of Niihau, Kauai, for north facing shores of Oahu,
Molokai and Maui, and for west facing shores of the Big Island.

High Surf Advisory until 6 PM HST Thursday for west facing shores
of Oahu.

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian
waters except Maalaea Bay-

&&

DISCUSSION...Foster
AVIATION...Bedal
MARINE...Burke/Stall