High Surf in Hawaii, November 9 to 11, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
351 AM HST Fri Nov 9 2018

.SYNOPSIS...
A shear line just northwest of Kauai will move over Kauai this
morning and continue moving southeast. The shear line will move
past the Big Island on Sunday. A new high will build east behind
the front.

&&

.DISCUSSION...
The latest surface analysis shows a 1032 mb surface high centered far
northeast of the area near 44N 136W with a ridge extending to about
200 miles northeast of the islands. The ridge is producing gentle
to moderate east northeast winds over the islands. The ridge will
move northeast and erode from the west over the next few days. A
poorly defined northeast to southwest shear line lies northwest of
Kauai. The shear line marks the boundary between weaker east northwest
winds southeast of the shear line and stronger northeast winds northwest
of the shearline. There are some showers along the line, but the shower
band is currently rather diffuse.

A surface trough about 200 miles southeast of the Big Island is forecast
to move north and sharpen. There is a large area of layered clouds and
embedded thunderstorms southeast of the Big Island associated with
this trough. The GFS and ECMWF show these clouds and showers remaining
east of the Hawaiian Islands.

With a weak pressure gradient over the area, land breezes prevail over
the Big Island and Maui County with clear skies over most areas. The
east northeast winds are carrying some showers over windward areas
of Oahu and Kauai.

Showery low clouds associated with the shear line are expected to reach
Kauai later this morning. These showers will move across Oahu tonight,
across Maui County on Saturday and across the Big Island of Hawaii
Saturday afternoon through Sunday morning. Locally strong north northeast
winds will spread over the islands behind the shear line. There will
be cooler, drier air behind the shearline, so rainfall will subside.
Differences remain between the GFS and ECMWF with the timing of the
shear line and our forecasts may need to be updated to reflect changes
in timing.

A surface high will build east behind the shear line. Early next week
winds will shift out of the northeast. As the winds shift to a more-
typical northeasterly trade-wind direction, showers will be focused
over mainly windward areas with mostly dry weather over leeward areas.
By the middle of next week as the high moves away to the northeast.
Winds will weaken and increasing moisture may bring an uptick in
showers during the second half of the coming week, but it is too
early to be very confident about that.

&&

.AVIATION...
There will be light east northeast flow across the island chain
through this afternoon. VFR will prevail through most of the
morning. The shear line will reach Kauai by late morning, then
move southeast, reaching Oahu between 0200-0400Z. Clouds and
showers are forecast to increase over Kauai and the surrounding
waters ahead of the shear line. The shear line is likely to
produce enough MVFR coverage to require AIRMET Sierra for MTN
OBSC on Kauai by late morning and on Oahu by this evening. Strong
NE winds will spread over the area behind the shear line. AIRMET
Tango for turbulence in the lee of the mountains may be needed
tonight.

&&

.MARINE...
Fresh to strong northerly winds are expected to fill in across the
Kauai and Oahu waters through the day today, then across the rest
of the state tonight through the weekend as a shear line moves
down the island chain. Seas have already responded at the Hanalei
buoy this morning with heights nearing the advisory mark (10 ft).
As a result, a small craft advisory has been issued over the
Kauai and Oahu waters to account for a combination of winds and
seas. This advisory will likely be expanded across most exposed
waters later today or tonight, which will likely hold through the
weekend. Winds will shift to a more typical trade wind direction
Sunday night into early next week, potentially holding in the
fresh to strong category as a ridge of high pressure noses
southeast over the state.

A combination of strong northerly winds and a 12 sec northerly
swell filling in above predicted levels will likely drive surf
heights toward the advisory mark for north facing shores of Kauai
today, and possibly for north facing shores of Oahu tonight into
Saturday. This source will hold through Sunday, then slowly ease
early next week. A small northwest swell is expected to fill in
Monday with a larger reinforcing north-northwest swell expected
Tuesday through midweek. This reinforcement will likely support
advisory-level surf for exposed north and west facing shores by
Tuesday night.

A moderate long-period southwest swell is expected to fill in
later today, peak Saturday at heights around the summer average,
then gradually decline into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory for waters around Kauai and Oahu.

&&

DISCUSSION...Donaldson
AVIATION...Lau/Thomas
MARINE...Gibbs