High Surf in Hawaii, June 14 to 15, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
939 AM HST Fri Jun 14 2019

.SYNOPSIS...
Moderate trade winds will pick up slightly this weekend as surface
high pressure strengthens far northeast of the state. Stable
conditions will continue, keeping showers focused over windward
slopes through most of the week. However, an area of moisture
will produce enhanced showers over the Big Island tonight and
Saturday. Trades will likely decline again Monday and Tuesday.

&&

.DISCUSSION...
A stable, moderate trade wind flow remains in place. A pair of
high pressure cells to the northeast and northwest of the state
are bisected by a stalled surface trough sitting roughly 250
miles north of Kauai. As a result, the somewhat weakened pressure
gradient over the islands continues to drive moderate trade
winds. A ridge aloft is holding firmly over the state, producing
stable conditions with an inversion hovering around 6,500 ft.
Moisture immediately upstream of most islands is unimpressive,
leading to scant rainfall over windward slopes and dry conditions
leeward. Expect little change through the day, except for
afternoon clouds and a few showers over the Kona slopes of the Big
Island.

While stable trade wind weather will persist over most islands
this weekend, wetter conditions are briefly expected on the Big
Island. The northern edge of a shallow area of moisture is
currently located about 100 miles southeast of Hilo. This moisture
will reach windward Big Island late tonight and will provide a
boost in windward rainfall through Saturday night. In addition,
expect enhanced clouds and scattered showers over leeward and
interior slopes of the Big Island Saturday afternoon. Trades may
pick up slightly as the local pressure gradient tightens in
response to the area of moisture passing by and high pressure
building far northeast of the state.

Trade winds will hold into next week, though at potentially weaker
speeds. The GFS and ECMWF models are showing another deep trough
developing several hundred miles northwest of Kauai. This may
shunt the surface high eastward, causing trades to weaken slightly
and shift easterly, or perhaps southeasterly, on Monday and
Tuesday. The ridge aloft will likely erode, but stable conditions
look to remain in place. Under this pattern, showers will continue
to favor windward slopes, though we could experience increased
leeward sea breezes with a few afternoon showers.

&&

.AVIATION...
The same weather pattern continues today with high pressure to the
northeast providing Hawaii with moderate easterly trade winds. A
weak boundary to the northwest of Hawaii will remain stationary
and eventually fall apart this weekend. It is not expected to affect
the weather across the state. The combination of stable conditions
and dry air over the islands means there is only a minimal chance
for showers, even on the windward slopes. The only exception to
this may be the Big Island where there is more moisture which may
lead to a few showers, especially on the windward slopes. All of
these weather conditions are expected to remain in place through
the weekend.

No AIRMETS are currently in effect.

&&

.MARINE...
Trade winds will hold at moderate to fresh strength today and
tonight, as a dissipating trough over the far northern offshore
waters keeps the local pressure gradient somewhat weak. As the
trough dissipates, high pressure to the northeast and northwest of
the islands will consolidate into a single surface high well to
the northeast of the state this weekend. This will result in a
slight increase in the trade wind speeds and a Small Craft
Advisory (SCA) may be needed for the typically windy waters around
the Big Island and Maui late Saturday into early next week.

Elevated surf is expected along south facing shores through the
weekend. A new south-southwest swell will continue to build
through the day today, then peak tonight through Saturday. A High
Surf Advisory (HSA) is in effect through Saturday along south
facing shores. The swell will gradually decline Sunday, with
south shore surf dropping back to average summertime levels on
Monday. Several reinforcing south-southwest swells will continue
to produce average summer surf through middle of next week.

Aside from wrapping southerly swell along west facing shores,
generally small surf will prevail elsewhere. A pair of small
northwest swells moving through today through the weekend will
give a boost to surf along north facing shores, with surf topping
out in the head high range late Saturday into Sunday. Additional
pulses of tiny northwest swell are expected early next week. Surf
along east facing shores will remain small through the weekend,
then rise close to the summertime average early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for south facing shores
of all islands.

&&

DISCUSSION...Wroe
AVIATION...Chevalier
MARINE...Kino