High Surf in Hawaii, August 1 to 3, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Wed Aug 1 2018

.SYNOPSIS...
High pressure north of the state will keep breezy trade winds in
place through at least Friday. A fairly typical trade wind shower
regime is then expected through Friday, with showers favoring
windward and mauka areas. An upper level trough is expected to
bring an increase in shower coverage and intensity late Friday
through the weekend, with shower activity expected to be most
prevalent in windward areas. A drier and more stable trade wind
pattern is then forecast to overspread the state late Sunday
through early next week.

&&

.DISCUSSION...
Currently at the surface, a 1031 mb high is centered around 1200
miles north-northeast of Honolulu, with a new strengthening 1033
mb high building around 1250 miles north of Kauai. This is
resulting in breezy conditions across the island chain early this
morning. Meanwhile, the remnant trough of former tropical cyclone
Gilma is located around 300 miles south-southeast of South Point.
Aloft, the state remains in the transition zone between an upper
level ridge southwest of the islands and an upper level trough to
the northeast. Infrared satellite imagery shows mostly cloudy
skies across the western islands, with partly to mostly cloudy
skies over the eastern end of the state. Radar imagery shows
scattered showers moving into windward areas, with a few showers
spilling leeward from time to time. Main short term concern
revolves around rain chances over the next few days.

The area of high pressure north-northeast of the state will
become absorbed into the strengthen high north of Kauai today.
This new high will then strengthen and lift northward further
away from the island chain tonight through the weekend, then will
weaken well north of the area early next week. Meanwhile, the
weak trough associated with the remnants of former tropical
cyclone Gilma will pass by to the south of the Big Island today.
As a result, we expect a further tightening of the pressure
gradient today and tonight, with breezy conditions expected to
continue through at least Friday, and possibly right through the
weekend.

As for sensible weather details, a fairly typical trade wind
shower pattern is expected over the next few days. This should
result in mainly windward showers and a few leeward spillovers,
particularly during the overnight and early morning hours. Due to
weak instability lingering over the island chain, an isolated
thunderstorm or two will be possible each afternoon across the
leeward Big Island slopes each afternoon through Friday.

The forecast becomes more interesting late Friday through the
weekend. Both the GFS and ECMWF show a digging upper level trough
over the western end of the state on Friday, closing off into a
low Friday night, then retrograding west of the islands Saturday
through early next week. In response, deep layer moisture
will increase beginning on Friday across the eastern islands,
then spread northwestward over the remainder of the state Friday
night and Saturday, before exiting from southeast to northwest
Saturday night and Sunday. The GFS remains the most aggressive
with the deep moisture return, while the ECMWF shows the deepest
moisture remaining to the south and west of the state. We will
continue to follow a middle of the road solution until the models
converge. As a result, we will advertise a wet trade wind pattern
developing late Friday over the Big Island, transitioning up the
island chain Friday night, holding in place Saturday, then drying
out and returning to more of a typical trade wind shower regime
from southeast to northwest Saturday night and Sunday. Given the
instability over the islands, some of the showers could be heavy
and we will keep a mention of locally heavy rain in place across
windward areas Friday night and Saturday when the deepest moisture
and strongest forcing for ascent most closely align.

Sunday night through next Tuesday the models are in good agreement
showing precipitable water values dropping back to near or below
normal levels for this time of year. As a result, we should see a
drier than normal trade wind pattern return across the island
chain. In addition, the latest forecast from the National
Hurricane Center in Miami shows tropical cyclone Hector
reaching 140W as a Hurricane Sunday evening, so we will be
monitoring this system closely as it tracks westward toward the
Central Pacific.

&&

.AVIATION...
Locally strong trade winds will prevail as high pressure persists
far north. AIRMET TANGO for low level turbulence over and
immediately south through west of the mountains will likely stay
posted at least for the next couple of days.

Trade winds will focus most clouds and showers over windward and
mountain areas. Brief MVFR CIG/VIS can be expected in passing
showers. The need for mountain obscuration AIRMET will continue to
be monitored, but is not warranted at this time. Airmass in the
island vicinity will remain somewhat unstable enough for TCU and
a slight chance of thunderstorms over the interior areas of the
Big Island this afternoon. High clouds, at or above FL200, will
continue to stream over the state today and tonight.

&&

.MARINE...
High pressure north of the islands will maintain strong trade
winds through at least Friday. A Small Craft Advisory (SCA) is in
effect for all coastal zones through Friday afternoon, but will
likely need to be extended through Saturday due to the persisting
strong trade winds. A slight decrease in trade winds is expected
on Sunday, but breezy trade winds are expected for the foreseeable
future.

The elevated trade winds will produce choppy surf along east
facing shores, with wave heights increasing through the day
today as the winds strengthen. Surf is expected to hold steady
through the rest of the week due to the persistent strong trade
winds. Small southeast and south swells will maintain small
background surf along south facing shores through the week. Small,
short-lived west and northwest swells are expected to arrive
today, providing a very small bump along exposed north and west
facing shores. However, Kauai will help block most of the west
swell energy from the other islands.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters-

&&

DISCUSSION...Jelsema
AVIATION...Hui
MARINE...Kino