High Surf in Hawaii, March 21 to 22, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
319 AM HST Wed Mar 21 2018

.SYNOPSIS...
Cloudy conditions with breezy trades will continue into the second
half of the work week. Although most of the shower coverage will
focus over windward and mountain areas through Friday, a few passing
leeward showers, mostly light, will remain possible. Light winds
along with increasing rain chances are expected late Friday
through Saturday. Improving conditions will be possible Sunday
into early next week.

&&

.DISCUSSION...
Cloudy and breezy conditions continue across the state this morning
due to strong high pressure to the north and an upper trough
positioned several hundred miles west of Kauai near Midway. Water
vapor imagery showed plenty of middle- to upper-level moisture being
drawn northward east of this upper trough around a mid-level ridge,
which supports the cloudy conditions in place. Although most of the
shower coverage expected will continue to focus over the typical
windward and mountain locations, radar showed light showers, mostly
in the form of sprinkles, being carried eastward from the mid-level
cloud coverage (some of which are not reaching the ground). As a
result, rainfall accumulations are expected to remain on the light
side.

Guidance remains in decent agreement through the short-term period
(today through Thursday) and appears to be initializing well with
the current trends and patterns. Consensus supports breezy trades
holding as strong high pressure far north of the area shifts
southeastward across the northeast Pacific. The aforementioned
upper trough positioned to the west is forecast to hold in place
near Midway through Wednesday, then begin to dig southeast toward
the region Thursday. This upper pattern will continue to support
cloudy skies. Highest rainfall chances will remain over the
western end of the state where the best source of deeper moisture
will reside. Elsewhere, showers will remain focused mostly over
windward and mountain areas with a few passing showers leeward
periodically.

Conditions will begin to evolve Friday into the upcoming weekend
as a shortwave trough digs southeastward into the area bringing a
threat for heavy rainfall and even a few storms. Despite the
model spread between various solutions that has been shown in
each cycle over the past few days, most indicate weak low
pressure developing along an approaching trough in the vicinity of
the islands Saturday with an abundance of deep moisture being
drawn northward (2" precipitable water values). A combination of
falling upper heights, instability, lift, and moisture will lead
to a wet weekend with a few thunderstorms, especially late Friday
through Saturday.

Sunday through early next week, improving conditions are expected
from west to east as drier air fills in and the upper trough and
surface low lift northeastward and away from the area. Guidance
depicts light west winds, potentially giving way to overnight land
breezes and afternoon sea breezes. Forecast confidence, however,
remains low through this extended period provided the spread
between solutions shown.

&&

.AVIATION...
A surface ridge NE of the area will maintain low-level ENE winds
for Hawaii through the day today. Winds will increase through the
morning and afternoon hours with an increase in the pressure
gradient. Slight instability in some areas will produce clouds and
light showers over N and E sections of the islands with tempo
MVFR ceilings and vis into the morning hours. An increase in
showers is not expected until the weekend.

AIRMET SIERRA for mountain obscuration for N and E sections of all
islands.

AIRMET Zulu for light icing from 120 to FL230 for entire area
lingering into the early morning hours.

&&

.MARINE...
A large NE to N ocean swell is here. The swell was produced a day
or two ago from a strong low pressure system located NE of the
islands. An ASCAT pass Tuesday morning showed a good fetch of
severe gale to localized storm force winds aimed at us from
between 020-040 degrees. As of 3 am HST, both the Mokapu and
Pauwela buoys are reporting swell heights of 5 to 6 feet.
Ocean buoy 51000, located 215 nm NE of Maui, has maxed out at 13
feet, 14 seconds period a couple of hours ago. This matches up
well with the latest Wave watch guidance for that particular
point. Due to a small decay while in transit to the islands, the
Wave Watch has a max value of 11 feet for our north and east
facing shores. At 11 feet, this translate to a maximum surf
heights of 25 feet. The swell will be peaking this afternoon, and
gradually decline through Thursday.

As for other swells, there will be a series of small to medium
size NW swells through out the forecast period. Another NE swell,
but of a much smaller scale of 3 feet is due in over the weekend.
It is about that time of the year to start looking out for swells
from the south. There is a small pulse of 2 feet, 16 second
period, from 210 degrees, slated to arrive on Friday.

Small craft advisory (SCA) level winds will affect not only the
typical sensitive areas such as the Pailolo and Alenuihaha
Channels, but also the near shore waters NW of Kauai. Expect the
trades to start weakening early Friday morning as a front gets
into range to begin affecting the winds.

This combo moderate to strong trades and the large NE swell, has
result in the SCA covering all the near-shore coastal marine
zones.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for north and east
facing shores-

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

&&

AVIATION...Chevalier/Jelsema
MARINE...Lau
REST OF DISCUSSION...Gibbs