High Surf in Hawaii, February 12 to 13, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Mon Feb 12 2018

.SYNOPSIS...
Daytime sea breezes will overpower developing southeast winds,
leading to afternoon clouds and spotty showers over interior
sections today. A front and its associated rainfall will slowly
move down the island chain Tuesday through Wednesday, eventually
stalling across the eastern end of the state Thursday and Friday.

&&

.DISCUSSION...
The latest surface analysis showed a weak surface trough positioned
near the islands and a cold front a few hundred miles west of
Kauai. Outside of a few showers developing around the islands over
the waters, mostly dry conditions with land breezes will hold
through the early morning hours. The rest of the day will be more
of a transition period as the low-level flow shifts out of the
south ahead of an approaching front and upper-level trough.
Increasing moisture with dewpoints climbing toward the upper 60s
and precipitable water values rising toward 1.3" will support
better cloud and shower coverage through the day. The best chance
for showers will come through the afternoon hours over interior
locations as sea breezes develop. Rainfall, however, will be
limited due to an abundance of dry air located above a shallow
moist layer.

Tonight through midweek, guidance shows the previously mentioned
cold front, currently located a few hundred miles west of Kauai,
approaching and slowly moving down the island chain. Hi-res model
reflectivities depict a narrow line of showers along and ahead of
the front impacting Kauai and its adjacent waters late tonight
through Tuesday, then continuing eastward Tuesday night into
Wednesday toward and over Oahu and Maui County. Some pre-frontal
shower bands developing within the confluent southerly flow out
ahead of this boundary are also shown lifting northward and
impacting Oahu and Maui County through the day Tuesday. Despite
the impressive dynamics characterized with strong 0-6 km shear
profiles (40-50 kt) and a sufficient amount of instability that
will accompany this front and upper low, a limiting factor will be
the lack of available deep layer moisture. Model PWs only climb
into the 1.25 to 1.5". Despite this uncertainty, a slight chance
of thunderstorms remains in the forecast beginning early Tuesday
morning over the western end of the state, then Oahu and Maui
County Tuesday into Tuesday night.

For the second half of the work week, forecast confidence remains
low due to various model differences shown, which means additional
fine tuning of the forecast will be likely over the coming days as
conditions evolve. For now, a blend was taken which supports a
gradual drying trend from west to east over the western end of the
state with light westerly background low-level flow filling in. The
weakened front could hang up or remain in the vicinity of the Big
Island. This combined with an upper low/trough in the vicinity of
the islands will likely keep the rain chances in the forecast over
the eastern end of the state.

Heading into the upcoming weekend, the GFS and ECMWF are pointing
at another upper level trough dropping into the area from the
northwest. This could trigger heavy showers, though details on
timing and location are lacking at this time.

&&

.AVIATION...
A weak surface trough just north of Hawaii will move slowly
northwest. A surface front located about 300 nm west-northwest of
PHLI will continue to move toward the islands through tonight.
As a result, expect light east-southeast to southeast flow in the
vicinity of the islands today. This will allow local sea breezes
to become established over the individual islands starting around
mid-morning, and continuing through this afternoon. Convergence
of these sea breeze circulations will produce low clouds and
spotty showers over interior sections of many of the islands,
especially on the Big Island. Brief MVFR conditions due to low
ceilings and reduced visibilities are possible over some areas.
There is also a chance of mountain obscuration in some locations.

As the front continues to approach Kauai later this evening,
expect clouds and showers to increase over the coastal waters
west of Kauai. In addition, isolated thunderstorms may develop
west of Kauai after midnight HST tonight, with showers moving over
Kauai at the same time. This may result in widespread MVFR
conditions and mountain obscuration on Kauai before daybreak
Tuesday morning.

No AIRMETs are in effect at this time.

&&

.MARINE...
South and southeast winds are expected today as a front
approaches from the northwest. The front will reach Kauai early
Tuesday morning and then continue moving slowly east through the
week. Winds ahead of the front will remain from the south and
southeast, and behind the front winds will turn from the west.
Moderate northwest swells will continue through the week.

A moderate northwest swell will increase across the islands
Monday and Tuesday, with surf reaching near advisory levels for
north and west facing shores and seas reaching near SCA levels
by midweek.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

AVIATION...Houston
MARINE...Morrison
REST OF DISCUSSION...Gibbs