High Surf in Hawaii, July 6 to 9, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
954 AM HST Sat Jul 6 2019

.SYNOPSIS...
Light winds will gradually strengthen today into Sunday, with
locally breezy trade winds developing. With winds still on the
lighter side today, a few interior and leeward showers are possible
this afternoon. Otherwise, the trade winds will deliver a few brief
showers to windward areas through Sunday. Deep moisture associated
with the remnants of Barbara is expected to move over the islands
from the east, reaching the Big Island as early as Sunday night,
and spreading over the other islands on Monday. This deep moisture
will bring the potential for locally heavy rainfall, especially
windward. Windy conditions are possible on Monday, especially on
the Big Island, as the remnants pass south of the islands. Drier
and more typical trade wind weather will return Tuesday into
Wednesday, and continue for the remainder of the week.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is located
around 275 miles west-northwest of Kauai. Meanwhile, a 1028 mb
high is centered around 1650 miles northeast of Honolulu, with a
surface ridge extending southwestward to a location around 225
miles north of Kauai. Additionally, the remnant low of former
Tropical Cyclone Barbara is located around 985 miles east of
Hilo. The gradient locally remains quite weak, with light and
variable winds present in many areas. Visible satellite imagery
shows mostly clear to partly cloudy skies in place in most areas,
with a bit more cloud cover moving into southeast sections of
Kauai. Meanwhile, radar imagery shows a few showers moving into
windward areas. Main forecast concerns revolve around the
potential for heavy rainfall in association with the remnants of
Barbara.

Today through Sunday,
The weak trough of low pressure west of the islands will gradually
dissipate today, and this will allow the surface ridge axis north
of the islands to lift northward through the remainder of the
weekend. As a result, moderate trades are expected to return from
east to west across the island chain today, although some leeward
sea breezes will be possible, particularly across the western
islands. The trades will then strengthen tonight and become
locally breezy by Sunday.

A few showers will continue to move into windward areas through
the remainder of the morning, with a few leeward and interior
showers possible this afternoon as the trades begin to return. A
more typical windward and mauka shower pattern is expected tonight
and Sunday, as more solid trade winds fill in. No significant
rainfall is expected through the weekend.

Sunday night through Tuesday night,
Model solutions are in good agreement showing the remnant low of
former Tropical Cyclone Barbara tracking steadily eastward toward
the islands Sunday night, then weakening into a surface trough as
it passes by just to the south of the Big Island Monday. The
surface trough is then expected to continue to track westward to
the south of the state Monday night through Tuesday night. Breezy
to locally windy trades appear likely Sunday night through Monday
night, with the trades easing to moderate and locally breezy
levels Tuesday and Tuesday night as the gradient relaxes. In fact,
the trades may be strong enough to warrant a Wind Advisory for
portions of the state, particularly the Big Island, late Sunday
night through Monday night.

Deep tropical moisture with precipitable water (PW) values of 2.0
inches or greater will begin to move into the Big Island Sunday
night, then spread westward across the rest of the island chain
Monday and Monday night. Drier air will then begin to work into
the region from east to west Tuesday and Tuesday night. Overall,
with trade winds prevailing through the period, we should see wet
conditions over windward areas as the deepest tropical moisture
moves through, with showers reaching leeward areas more regularly
as well. Some of the rainfall could be heavy, particularly over
the Big Island and windward terrain of the remaining islands. As a
result, a Flash Flood Watch may eventually be needed for portions
of the state. The most likely time frame appears to be Monday
through Tuesday, with the Big Island appearing to have the highest
probability for flash flooding given the proximity to the track of
the remnant low. Additionally, given the moisture depth reaching
around 20,000 feet, and the enhancement from the terrain of the
Big Island, a few thunderstorms can't be ruled out.

Wednesday through Friday,
The GFS and ECMWF are in good agreement showing the deep tropical
moisture associated with the remnants of Barbara exiting to the
west of the island chain by Wednesday morning. This should bring a
return of more typical trade wind weather featuring windward and
mauka showers and the occasional leeward spill over.

&&

.AVIATION...
Trades will gradually strengthen through the day today as a
trough west of Kauai dissipates and a surface ridge lifts
northward and further away from the island chain. However, winds
are still light and variable this morning which will allow for
some afternoon interior and leeward cloud build ups until the
trades kick in. A few light showers are also possible in these
areas along with typical windward/mauka locales. VFR conditions
are expected with only brief periods of MVFR ceilings for windward
sections.

While the trades will strengthen into the evening hours, do not
anticipate them being strong enough to warrant the issuance of
AIRMET Tango for low level turbulence at this time.

&&

.MARINE...
A surface ridge building north of the area is expected to produce
slightly stronger easterly trade winds across most of the coastal
waters this weekend. A Small Craft Advisory (SCA) is in effect for
the Alenuihaha Channel for today. As the trades strengthen later
today, additional areas such as the typical windy areas around
Maui County and the Big Island will likely be added to the SCA
starting tonight.

East Pacific Tropical Cyclone Barbara has weakened to a post-
tropical gale low this morning. The latest forecast guidance
remains in decent agreement that the pressure gradient will
tighten across the state from Sunday night through Monday as this
post-tropical low passes near or just south of the state from
Monday through Tuesday. This will result in strengthening
northeast trade winds and elevated rough seas. Therefore, expect
SCA conditions to develop over additional waters by early Monday.
Note that gale conditions have been added to the latest forecast
for the Alenuihaha Channel on Monday. If the guidance continues to
suggest these hazardous conditions may develop, a Gale Watch may
be issued for this channel later today or tonight.

The large east swell produced by Tropical Cyclone Barbara filled
in overnight last night and continues to hold steady this morning.
The greatest energy is expected along east facing shores of the
Big Island with less energy as you get closer to the western end
of the island chain. Observations from the nearshore buoy at Hilo
and Mokapu Point confirms greater energy towards the Big Island. A
High Surf Advisory is in effect for east facing shores of all
islands exposed to this east swell. Early next week, the strong
northeast trade winds will also keep elevated rough surf along
east facing shores from Monday into Tuesday.

In addition to the large east swell, a long-period south-
southwest swell has arrived in the islands according to
observations from the buoys. This new swell is expected to
continue to fill in this morning, which will likely cause surf to
reach the HSA criteria along south facing shores through Sunday.
Surf is expected to trend down along south facing shores during
the first half of next week. However, the recent period of strong
southeasterly trade winds in the South Pacific should continue to
maintain surf near or slightly above the summer-time average
through mid-week along shorelines with an exposure to the
southeast that are not in the shadow of the Big Island. Elsewhere,
a small north-northwest swell may cause a slight bump in surf
heights along north facing shores of some of the islands early
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Oahu
Koolau-Olomana-Molokai-Lanai Makai-Kahoolawe-Maui Windward West-
Maui Leeward West-Maui Central Valley-Windward Haleakala-Leeward
Haleakala-Kona-South Big Island-Big Island North and East.

Small Craft Advisory from 10 AM this morning to 6 AM HST Monday
for Alenuihaha Channel.

&&

DISCUSSION...Jelsema
AVIATION...Dye
MARINE...Kino