High Surf in Hawaii, March 12 to 14, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
403 AM HST Thu Mar 12 2020

.SYNOPSIS...
A trough of low pressure will remain nearly stationary in the
vicinity of Oahu through Saturday, bringing scattered showers to
the island chain, and even a slight chance of thunderstorms over
Oahu and Maui County Friday night and Saturday. The trough of low
pressure will shift west of the state Saturday night and Sunday,
with a Kona Low then developing west of the islands on Monday.
The weather will become increasing unsettled Sunday through the
middle of next week, with the development of the Kona Low
bringing the potential for heavy rainfall, flash flooding, strong
southerly winds and even strong thunderstorms to portions of the
island chain.

&&

.DISCUSSION...
Currently at the surface, a 1035 mb high is centered around 2100
miles northeast of Honolulu, while a very weak front has stalled
near Kauai. Meanwhile, a trough of low pressure is developing in
the vicinity of Oahu and Maui County. Infrared satellite imagery
shows mostly cloudy conditions across the state, with radar
imagery showing scattered showers affecting the islands. Main
forecast concern revolves around the potential for a Kona Low
bringing some unsettled weather to the state late in the weekend
through the middle of next week.

Tonight through Saturday night,
The weakening front will dissipate near Kauai today. Meanwhile a
surface trough of low pressure will continue to develop across
the central islands today, then linger in the vicinity of Oahu
and Maui County through Saturday. The surface trough is then
expected to begin to shift westward Saturday night as a sharp
shortwave trough digs rapidly southeastward near the Date Line.
This pattern will keep light to moderate northeasterly winds in
place along and to the west of the surface trough, while light to
moderate east to southeasterly winds prevail to its east. As for
weather details, we should see a continuation of scattered showers
across the state during the next several days, with showers
favoring areas near the coast at night and interior/mauka
locations during the afternoon and evening. That said, a shower
could pop up just about anywhere and at anytime during the next
couple days, due to the nearly stationary surface trough setting
up through the central islands, increasing boundary layer
moisture, and shortwave disturbances moving through the region
aloft. Additionally, increasing instability will bring a slight
chance of thunderstorms to Oahu and Maui County, Friday night
through Saturday night.

The Wind Advisory has been upgraded to a High Wind Warning for
the Big Island summits through 6 PM Friday as winds are expected
to increase to warning levels around noon today.

Sunday through Wednesday,
There remains some differences amongst the reliable model
solutions in the extended ranges of the forecast, but solutions
have come into much better agreement showing a Kona Low developing
to the west of the islands in the vicinity of 165W early next
week. The surface trough is expected to sharpen up as it shifts
west of the state on Sunday, with a Kona Low developing Sunday
on Monday, then holding relatively stationary to the west of the
state in the vicinity of 165W through the middle of next week.
It's still too early to determine which locations may see the most
significant impacts from this Kona Low, but it's beginning to
look more probable that heavy rainfall, flash flooding, strong
southerly winds and the potential for strong thunderstorms will
eventually impact portions of the island chain, and perhaps the
entire state, from late Sunday through the middle of next week.
Stay tuned for updates as the forecast details will continue to
become more clear as the event draws nearer.

&&

.AVIATION...
A poorly defined frontal boundary appears to have stalled near
Kauai early this morning. A surface trough is forecast to develop
in the vicinity of the smaller islands later today. These
features will keep the background flow light and variable over
most of the state today. Loops of satellite imagery show patches
of low- to mid-level clouds moving up from the south and southeast
over portions of the island chain. Radar reflectivity data depict
scattered showers embedded within some of these clouds,
especially along the southeast coast of the Big Island, and in
the vicinity of the smaller islands. As these showers move up
over land areas, brief periods pf MVFR conditions are possible.
In addition, AIRMET Sierra remains in effect for mountain
obscuration over the upslope sections of the Big Island from
Upolu Point to Hilo to South Cape.

With the light background flow in place, daytime heating is
forecast to allow the development of local sea breezes from late
this morning through this afternoon over most islands. This may
produce broken to overcast low clouds with embedded showers over
some islands this afternoon. This may result in MVFR conditions
developing, especially over interior sections of some of the
islands. In addition, AIRMET Sierra may be needed for mountain
obscuration on some of the higher terrain on some of the smaller
islands, such as Oahu, later today.

Looking ahead to this evening, the developing surface trough may
allow low clouds and showers to persist over some areas of the
state after sunset. Local land breezes may also develop, which may
cause some of the low clouds and showers to dissipate over parts
of the island chain from later tonight through early Friday
morning.

&&

.MARINE...
Light to moderate north-northeast winds associated with a
dissipating front moving into the area are expected over the Kauai
waters this morning, then potentially over both the Kauai and
Oahu waters later tonight through Friday. A surface trough will
develop and remain nearly stationary later today through Saturday
over the central islands. Northeast winds are expected over the
western end of the state west of this trough, while southeast
winds will hold over the eastern end to its east. A transition
period appears likely Sunday through early next week as a cutoff
low forms west of the islands. Guidance shows the potential for
strong southerly winds setting up across most areas Monday through
Tuesday as a surface low develops to the west. In addition to the
winds, this pattern will likely translate to deteriorating
weather conditions early next week. Confidence in the details will
steadily improve over the next few days as things begin to
evolve.

Surf along north and west facing shores will trend up today as a
northwest swell builds down the island chain and the small
northeast swell eases. Observations at the offshore buoys to the
northwest reflect this new swell (11 ft at 12-14 seconds as of
midnight), which is near or around its predicted peak. Guidance
shows this swell peaking over the Kauai waters today, then over
the Oahu and Maui County waters tonight through Friday. Surf will
reach advisory levels as it peaks for the smaller islands, then
continue through Friday afternoon before slowly lowering over the
weekend. Seas will likely reach the Small Craft Advisory threshold
of 10 ft today through Friday over the waters exposed to
northwest swells. The Maui County windward waters will likely be
added later today.

Surf along east facing shores will trend down through the second
half of the week as the small northeast swell fades. Expect heights
to remain small into the weekend due to the lack of trades locally
and upstream of the state across the eastern Pacific.

Surf along south facing shores will remain small with mainly
background south swells expected. An upward trend will be
possible early next week due to the potential for strong
southerly winds setting up over the island waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Wind Warning until 6 PM HST Friday for Big Island Summits.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Houston
MARINE...Gibbs