Heavy Rain in Hawaii, August 16 to 17, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
336 PM HST Mon Aug 16 2021

.SYNOPSIS...
Light winds are expected to remain in place through Wednesday,
resulting in land and sea breezes. This will lead to afternoon
clouds and showers over the interior of the islands, and some
clearing overnight. A nearby trough will keep showers possible
tonight and possible Tuesday night for Kauai and Oahu. More
typical trade wind weather is expected to return Thursday through
Saturday.

&&

.DISCUSSION...
No big changes to the forecast with the afternoon package. We have
seen some showers develop over interior Oahu this afternoon in the
light wind flow, and along the Kohala mountains on the Big Island.
Will be monitoring those areas, but expect those showers to
diminish after sunset. Some additional increases in PoPs have been
added to the forecast for tonight and tomorrow based on the latest
blend and high resolution models.

A mid to upper level low and its associated surface trough remain
to the northeast of the islands this afternoon. Thunderstorms
remain active near the mid to upper level low center, while a
cloud band loosely associated with the system is moving towards
Oahu this afternoon. Model precipitable water increases over Oahu
and Kauai tonight, presumably as this feature reaches the area.
The inversion level near Lihue is expected to lift tonight, which
would allow more vertical growth in showers. However the
precipitable water value increase combined with the inversion
lifting at night and nighttime land breezes could limit rainfall
totals in any one location. As mentioned above, have boosted PoPs
some over the western end of the state, but still are not expected
much in the way of shower activity overnight.

The land and sea breezes will stick around through Wednesday, with
the impacts from the trough likely sticking around through Tuesday
night. The second half of the week should see trade winds
rebounding across the state, along with the more typical trade
winds showers focused over windward and mauka slopes.

Hurricane Linda in the eastern Pacific is expected to cross into
the Central Pacific Basin late Thursday as a weaker system. The
remnants of Linda will likely have an impact on the local weather
over the weekend, as it disrupts the trade wind flow, leading to
lighter winds by the end of the upcoming weekend. Its too early
to tell if the remnants of Linda will bring additional shower to
the islands, but if that were to happen, the moisture likely would
reach the islands until Monday or Tuesday next week.

&&

.AVIATION...
Light background winds have allowed sea breezes to fill in over
most areas this afternoon. Winds will switch to land breezes
tonight and back to sea breezes on Tuesday.

Sea breezes will bring low clouds and showers over the island
interiors in the afternoons and evenings, followed by clearing
overnight. Clouds and showers will may mountain obscurations,
MVFR ceilings and visibility restrictions through this evening,
and again during the afternoon and evening on Tuesday. AIRMET
Sierra is posted for tempo mountain obscurations across leeward
Big Island and leeward Oahu. Other locations are seeing
brief/isolated mountain obscurations.

&&

.MARINE...
As a weak, west-moving surface trough passes near the islands
over the next day or two, winds will ease and tend to vary from
northeast to southeast. After the trough leaves the area, a more
normal trade wind pattern will begin to take hold by Wednesday
night into Thursday.

South shore surf will be on the rise with the arrival of a large,
long period south swell. Several pulses within this swell will
have this south shore surf event lasting through mid week. A High
Surf Advisory has thus been issued for south facing shores through
Wednesday afternoon.

The ongoing small, moderate period northwest swell will be slowly
declining through Tuesday, as will the surf along north- and
west-facing shores. Surf along east facing shores will remain
relatively small the next several days. Swell within the captured
fetch from Hurricane Linda in the East Pacific might make it to
the east facing shores of the eastern most islands as small, long
period east swell by Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday for Niihau-Kauai
Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai
South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Southeast-
Molokai Leeward South-Lanai Leeward-Lanai South-Maui Central
Valley South-Kipahulu-South Maui/Upcountry-South Haleakala-Big
Island South-Big Island Southeast.

&&

DISCUSSION...M Ballard
AVIATION...Foster
MARINE...Kinel