High Surf in Hawaii, June 20 to 23, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
913 AM HST Tue Jun 20 2017

.SYNOPSIS...
A ridge of high pressure north of the state will keep moderate
trade winds in place today, before easing slightly tonight through
the weekend. The trades will then increase early next week as high
pressure strengthens north of the island chain. Showers will favor
windward and mauka areas through early next week, with a slight
increase in trade showers expected Wednesday night through the
weekend.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high centered around 1750
miles northeast of Honolulu, is driving moderate easterly trade
winds across the island chain this morning. Meanwhile, a weakening
cold front is located around 625 miles northwest of Kauai.
Infrared satellite imagery shows partly to mostly cloudy skies
across the state, with cloud cover most prevalent over windward
areas. Radar imagery shows scattered showers moving into mainly
windward areas, with the brunt of the activity over the central
and eastern islands. Main short term concern revolves around rain
chances.

Today through Wednesday,
High pressure will hold in place well northeast of the state,
while the weakening cold front northwest of the islands shifts
eastward to a location around 450 miles northwest of Kauai before
stalling out by late Wednesday afternoon. Moderate trade winds
will continue today, but are expected to east slightly tonight and
Wednesday as the weakening from edges closer to the area. The
airmass is expected to remain dry and stable, with precipitable
water values in the 1.0 to 1.2 inch range, and upper level ridging
holding in place over the island chain. As a result, showers
should remain light through the period and remain focused
primarily over windward and mauka areas.

Wednesday night through Sunday night,
Light to moderate trades will continue for the second half of the
work week and through the upcoming weekend, as a ridge of high
pressure remains north of the island chain. A mid-level trough
will approach from the east Wednesday night through Friday night,
then push eastward through the island chain over the upcoming
weekend. This will act to elevate inversion heights across the
area and increase precipitable water values into the 1.2 to 1.5
inch range. As a result, we should see a bit of an increase in
trade wind showers through the period, as well as some leeward
shower development during the afternoon and evening hours.

Monday through next Tuesday,
There are some timing differences with respect to the large scale
synoptic features by early next week, with the GFS slower to
shift the mid-level trough west of the islands than the ECMWF.
For now will utilize a blend of the two solutions for forecast
details. This suggests that trade winds will be on the increase
by early next week, and trade showers will decrease as a more
stable and dry airmass moves over the island chain.

&&

.AVIATION...
Moderate easterly winds will persist today. AIRMET TANGO for low
level turbulence has been cancelled, as widespread moderate
turbulence is no longer expected for the islands. Pockets of
isolated light to moderate turbulence can still be expected over
ridges and through valleys, where wind speeds can be accelerated.

Clouds and showers will favor the windward and mountain areas.
AIRMET SIERRA for mountain obscuration is posted over Molokai and
Oahu this morning, but conditions will be improving in the next
few hours. Expect the mountain obscuration to be cancelled by the
next routine issuance at 2200Z. Passing clouds and showers through
the day may produce brief periods of MVFR, otherwise, predominately
VFR conditions can be expected.

&&

.MARINE...
The moderate to locally breezy trade winds will continue through
today before easing slightly on Wednesday. A Small Craft Advisory
(SCA) is posted for the typically windier waters around the Big
Island and Maui through late this afternoon.

A long period south-southwest swell has arrived and will peak
tonight before gradually subsiding through Thursday. A high surf
advisory has been posted for all south facing shores through
Wednesday afternoon. This swell will be reinforced by another
south swell of about equal size, arriving Thursday afternoon, and
peaking on Friday. The afternoon high tides will steadily increase
each day this week reaching over 2.5 feet Friday and Saturday
afternoons. These "King" tides combined with the south swell may
translate to beach and street flooding in the typical vulnerable
locations.

The north and west facing shores are enjoying an off season
northwest swell. This small swell will gradually lower through
Thursday. A trade wind generated swell will provide small to
moderate size surf along the east facing shores, with a short
period southeast swell adding to surf heights starting Wednesday.
This southeast swell will continue into the weekend.

For more details, please refer to the Collaborative Surf Forecast
prepared by Pat Caldwell, under WMO header FXHW52 PHFO.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 4 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai-
Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-Kona-South Big Island-Big
Island North and East.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...Morrison