High Surf in Hawaii, March 12 to 13, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Mon Mar 12 2018

.SYNOPSIS...
Strong high pressure centered far to the northwest of the state
will maintain fresh to strong trade winds through midweek. A weak
front is forecast to move across the area through today accompanied
by a small increase in windward and mauka showers. Thick high
clouds will blanket most of the state through the next several
days. A wetter weather pattern is on tap beginning late today and
continuing through much of the week.

&&

.DISCUSSION...
A strong high pressure system currently northwest of the state
will maintain fresh to strong trade winds through the middle of
this week. The subtropical jet over Hawaii is pushing broken to
overcast high clouds over the area. A shallow cold front is
forecast to move across the island chain today. A slight uptick
in shower activity will accompany the front that will mainly
affect windward slopes. Winds at the Big island summits have
increased enough to warrant issuance of a Wind Advisory there.
Models show winds will likely remain blustery up there through
this afternoon.

An upper trough is forecast to take up a position just to the
west of the state today, and begin tracking east towards us.
Southerly flow on the east side of this trough will draw up deep
tropical moisture and increase the chances for rain across
portions of the state beginning this afternoon. The high clouds
will also thicken up and keep overcast skies over the state
through at least Thursday. The upper trough will weaken and lift
away from our area by late Wednesday.

Models indicate that moisture brought up from the south will
initially affect The Big Island and Maui beginning late today,
with rainfall chances increasing for the rest of the island chain
by midweek. However, models disagree on the placement of the
highest rain threat areas as well as the structure of local winds.
GFS consistently wants to develop a a low just southwest of the
islands while ECMWF wants to develop one farther to the west. Wetter
weather is coming, but confidence remains rather low as to where
the greatest amounts of rain will fall. Our gridded database
aligns closer to the ECMWF solution, which keeps the low west of
the state with showers pushed over the islands within easterly
flow through Thursday. Both GFS and ECMWF show the surface
features tracking away or weakening on Friday and Saturday with
remnant showers and southeast flow over the islands.

&&

.AVIATION...
High pressure northwest of the state will maintain moderate to
breezy trade winds to the islands today. Clouds and showers will
favor windward and mauka areas, bringing brief MVFR ceilings/VIS.
Conditions are expected to deteriorate particularly for the windward
areas of Maui County and the Big Island later today, with cloud
and shower coverage increasing hence MVFR conditions becoming more
widespread. AIRMET MTN OBSC may be needed for those areas.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. These conditions
will continue today. AIRMET Tango for moderate upper level
turbulence between 150 and FL280 will probably be canceled later
today as the strong winds aloft shift north.

Light icing over the area will persist due to the thickening mid
and upper level clouds. Conditions are expected to stay below the
AIRMET threshold for now, though with increasing moisture across
much of the area, that could change later today.

&&

.MARINE...
Fresh to strong northeasterly trade winds will persist as the
islands remain under the influence of a strong surface high
located far northwest of the state. A Small Craft Advisory (SCA)
is in effect for all waters due to winds and seas.

A large north swell will fill in, and north and east shore surf
from Kauai to Maui will build to warning levels today. During this
time, advisory level surf is expected along north and east facing
shores of the Big Island. Thus, a High Surf Warning (HSW) is in
effect for north and east facing shores of most islands. The north
swell will quickly decline Tuesday and Wednesday, as a rough,
short-period trade wind swell builds. As a result, a High Surf
Advisory will likely remain in place for east facing shores
through Tuesday or Wednesday.

The current SCA now covers all waters. Today, trade winds will
increase as a developing trough south of the state tightens the
local pressure gradient, and the incoming north swell will push
combined seas above 10 feet. As the north swell drops, the strong
trades will generate large and rough short-period seas, which will
likely keep the SCA in place through at least Tuesday. In addition,
the trough moving up from the south will bring the potential for
unsettled weather through mid week. Mariners should monitor the
latest forecast as the weather pattern becomes better defined.

&&

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

High Surf Advisory until 6 AM HST Tuesday for Big Island North
and East.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until 6 PM HST Tuesday for all Hawaiian
waters-

&&

DISCUSSION...Powell
AVIATION...Hui
MARINE...Kinel