High Surf in Hawaii, March 25 to 28, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
925 AM HST Sat Mar 25 2017

.SYNOPSIS...
A persistent ridge of high pressure north of the islands will keep
trade winds blowing through the next week, with locally breezy
levels expected this weekend. A dry and stable airmass will keep
showers limited and confined to windward slopes through early next
week. An increase in trade wind showers is then expected Tuesday
through late next week as upper level troughing sets up over the
island chain.

&&

.DISCUSSION...
Currently at the surface, a 1027 mb high is centered around 700
miles to the northeast of Honolulu, and this is resulting in
moderate trade winds across the state this morning. MIMIC total
precipitable water (PW) imagery shows a dry airmass overspreading
the island chain with PW values at or below 1 inch. This
correlates well with the 12z sounding from PHTO indicating a PW
value of 0.77 inches, while the PHLI sounding observed a PW value
of 1.23 inches in a slightly more moist airmass which has now
shifted west of the Kauai. Additionally the 12z soundings showed a
stable airmass working in from the east, with a sharp low level
inversion around 3kft at PHTO and a weaker inversion around 3 kft
at PHLI. Satellite and radar imagery also reflect the dry airmass
over the islands, with clear to partly cloudy skies in most areas
and little if any shower activity. Main short term concern
revolves around the building large northwest swell moving into the
islands.

Today through Tuesday,
A pair of high pressure areas will consolidate northeast of the
state through the weekend, leading to an increase in the local
pressure gradient across the islands. As a result, moderate to
locally breezy trade winds are expected to prevail through Sunday
night, with the trade wind speeds then dropping off a notch on
early next week as high pressure lifts further northeast and away
from island chain. A dry and stable airmass is expected to limit
shower activity across the island chain through the period, with
PW values remaining around or slightly below 1 inch. Showers will
favor windward and mauka areas, primarily during the overnight and
early morning hours.

Tuesday night through Friday,
Model solutions are in good agreement showing high pressure
remaining anchored well to the northeast of the state through the
period, with a weakening front approaching from the northwest late
in the week. Light to moderate trades are expected to prevail
through the period, with some decrease in trade wind speeds
possible by the end of the week depending on the proximity of the
approaching front to the island chain. Aloft, a weakness in the
upper level ridge is expected to move overhead during the middle
and latter part of next week, and this should de-stabilize the
airmass a bit. This in combination with increasing boundary layer
moisture resulting from the persistent easterly trades, should
lead to an increase in trade wind showers across the islands.
Showers will continue to favor windward and mauka areas, with
activity most prevalent during the overnight and early morning
hours.

&&

.AVIATION...
Locally breezy trade winds will continue. The morning sounding
from PHTO shows a strong low level inversion near 3 kft, and the
PHLI sounding shows a weaker inversion near 3 kft as well. AIRMET
TANGO for low level turb has been issued this morning for areas
over and south through west of the mountains. The first few
visible satellite images show wave clouds starting to set up
along the koolau mountains. Expect prevailing vfr conditions at
most terminals today.

&&

.MARINE...
The large west-northwest (WNW) swell approaching the area
continues to rise at NOAA buoys 51101 and 51001. Both buoys
reported the swell height at 7.5 feet every 19 seconds from the
WNW. Forerunners of this swell have reached the Hanalei, Kauai and
Waimea Bay CDIP buoys with reports of 20 and 22 second periods
respectfully. These observations are in excellent agreement with
the forecast models, and therefore no adjustments are needed to
the forecast.

Surf on Kauai is expected to build to warning levels (25 ft
north/20 ft west) during the day, then peak tonight into Sunday
morning. Warning level surf should continue through the day
Sunday, before gradually subsiding early next week. The more
westerly direction of the swell will likely cause Kauai to shadow
the other smaller islands. The current forecast continues to hold
surf for exposed north and west facing shores of Oahu and Maui
County just under warning levels. This will be monitored as more
observations come in from the northwest buoys, and a slightly
larger or more northwest swell could bring warning level surf to
Oahu/Maui County as well. Surf along west facing shores of the Big
Island is also forecast to reach low-end warning level (12 ft)
tonight through Sunday night. A high surf warning has been posted
for north/west facing shores of Niihau and Kauai, and for the west
facing shores of the Big Island. A high surf advisory has been
posted for north/west facing shores of Oahu and Molokai, and for
north facing shores of Maui.

Otherwise, small long period south swells will continue through
this weekend and into next week, with a slightly larger long
period south swell forecast for Wednesday night through Friday.
Another large west-northwest swell will be possible from next
Friday into the following weekend.

The Small Craft Advisory (SCA) remains posted for the typically
windier waters around the Big Island and Maui County through
Sunday night. High pressure strengthening northeast of the state
will provide locally breezy conditions through the weekend. The
expansion of the SCA to cover most of the other coastal waters
late this morning, and the remaining waters early this evening,
due to building seas of the large incoming west-northwest swell is
on track.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from noon today to 6 AM HST Monday for
Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui Windward
West-Maui Central Valley-Windward Haleakala.

High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward.

High Surf Warning from 6 PM this evening to 6 AM HST Monday for
Kona-South Big Island-Kohala.

Small Craft Advisory until 6 AM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST Monday
for Oahu Leeward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...Evans