High Surf in Hawaii, March 20 to 22, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM HST Fri Mar 20 2020

.SYNOPSIS...
A sea breeze and land breeze weather pattern will continue across
most of the state through Saturday with afternoon shower
development over interior and mauka areas. Trade wind weather will
return late Saturday afternoon, with breezy to windy conditions
expected Sunday through late next week. Showers will favor
windward and mauka areas once the trades return, although showers
will reach leeward areas at times due to the strength of the
trades.

&&

.DISCUSSION...
Latest radar and satellite imagery is showing isolated
thunderstorms over the eastern and southeast slopes of the Big
Island this afternoon. These storms are slowly moving eastward and
is already beginning to weaken late this afternoon. These
thunderstorms are expected to dissipate by this evening as the
sun sets. Elsewhere across the state, we have scattered showers
developing mainly over the interior portions of the state. These
showers are fairly slow moving, thus a brief period of moderate to
heavy rain will be possible through this evening. A downward trend
in shower activity is expected this evening and tonight as a land
breeze weather pattern develops.

A broad area of low pressure northwest of the state will keep
winds relatively light across the state through most of day
Saturday. This will allow for a sea breeze weather pattern with
interior and mauka showers developing in the afternoon. Starting
late Saturday afternoon into the evening, high pressure will build
north of the state and strengthen into next week. This will bring
back our typical trade wind weather with showers favoring
windward and mauka areas. Trade winds are expected to become quite
breezy by Monday and this will likely bring some showers across
leeward areas as well. The trade winds are expected to gradually
strengthen through mid week and this could cause advisory level
winds across portions of the state, especially the windier
locations over Maui County and the Big Island.

&&

.AVIATION...
Scattered showers will remain possible through tonight, but the
low is beginning to weaken and move out of the area. Enough
instability is in place and overall there is chance that the rain
this evening will cause MVFR conditions. Expect a few isolated to
scattered showers through the rest of the afternoon and into the
evening until this low weakens more and moves out of the area.
Most of the showers should not warrant an AIRMET but showers which
will be over the mountains may cause mountain obscuration. Rain
showers may continue tonight, especially over Kauai. These
overnight showers may cause mountain obscuration and possibly
AIRMETs for reduced visibility.

&&

.MARINE...
As the surface (kona) low that has been affecting the area over
the past several days continues to move away to the north,
conditions have been stabilizing. However, winds in the light to
moderate range will remain with a southerly component through the
rest of the day, with Big Island waters seeing returning trades
by tonight and the rest of the island chain by Saturday. Trades
are then expected to strengthen through Sunday night, reaching
breezy to strong area-wide as a robust surface high settles in
far north of the state. A Small Craft Advisory (SCA) for winds
will likely be issued for waters around Maui County and waters
south of the Big Island as early as Saturday night. Further
expansion of the SCA is likely by Sunday night, and it will likely
continue through most of next week. The trades may even reach
near gale force in some of the channels Sunday night or Monday.

A west-northwest swell is on the way to the main Hawaiian Islands.
At this time, it appears to be a low-end High Surf Advisory (HSA)-
type swell for the western islands. Thus, an HSA has been issued
for the north and west facing shores of Niihau, Kauai, and Oahu. This
swell will be lowering slowly through Monday after peaking
Saturday. Also, an SCA remains in place for the waters around
Kauai and the windward waters of the smaller islands, mainly due
to the elevated swell. This SCA is in effect through early
Saturday morning.

The significant boost in the trades will likely result in an HSA
for the east facing shores beginning about Monday. In the mean
time, a small north-northeast swell will maintain moderate size
surf along the east facing shores. Surf along south facing shores
will remain slightly up along coasts exposed to the southeast
winds. Otherwise, expect mainly small background south swells
through this weekend. Model data are suggesting an out of season,
long-period south-southwest swell reaching the south facing
shores this coming Monday night. The swell is expected to peak
Tuesday with a maximum swell height of 3 feet and a period of 16
to 17 seconds. This may lead to a HSA for the south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau.

Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters.

&&

DISCUSSION...Kino
AVIATION...Chevalier
MARINE...Kinel