Drought in Hawaii, October 1 to 31, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST Thu Sep 30 2021

.SYNOPSIS...
Moderate to locally breezy trade winds will continue into next week
as high pressure persists to the north of the islands. The trade
winds will deliver clouds and showers to windward areas, with a few
showers spreading leeward on the smaller islands. A developing
disturbance aloft will bring the potential for some heavier
showers, and a possibly a few thunderstorms, from late Friday
into Sunday, especially over windward areas of Kauai and Oahu as
well as the Big Island. A wet trade wind pattern is expected for
much of next week, with the trades becoming breezy to windy during
the middle to latter part of next week.

&&

.DISCUSSION...
Currently at the surface, a 1031 mb high centered around 1800
miles northeast of Honolulu is driving moderate trade winds across
the island chain this evening. Meanwhile, aloft a deep upper level
trough is evident in water vapor imagery around 850 miles
northeast of the state, with weak troughing extending
southwestward and just to the north of Kauai. Infrared satellite
imagery shows mostly cloudy skies over the Big Island and windward
sections of the smaller isles, with partly cloudy conditions in
most other areas. Radar imagery shows scattered showers affecting
windward areas, with a few of these showers spilling into leeward
communities from time to time. Main short term concerns revolve
around the potential for thunderstorms and some heavier showers
during the next few days.

High pressure to the north and northeast of the state, and an
active Inter-Tropical Convergence Zone (ITCZ) to the south, will
keep a general moderate to locally breezy trade wind flow in
place through early next week, with only minor fluctuations in
strength. The trades appear to ramp up to breezy and possibly
even windy levels during the middle to latter part of next week
as a new strong high builds into to the north of the island chain.

As for the remaining weather details, a weakness aloft will
remain over the islands throughout the next 7 days, keeping the
island atmosphere more unstable than normal. The slug of moisture
which affected the Big Island and Maui earlier today will continue
to shift westward overnight and Friday morning, bringing an
increase in trade wind showers to Oahu and Kauai, particularly
windward areas, while scattered showers continue to affect
windward sections of Maui County and the Big Island as well. We
should see a fairly typical decrease in trade wind shower
coverage Friday afternoon across the smaller islands, however the
airmass will likely remain unstable enough to allow for a
thunderstorm or two to develop over the Big Island.

The airmass will become more unstable Friday night through Sunday
as an upper level low closes off a few hundred miles north of
Kauai. This should bring an increase in trade wind shower coverage
and intensity, particularly to windward areas. The elevated
inversion heights will also allow for a few more showers than
normal to reach leeward areas. Additionally, a thunderstorm or two
may develop each afternoon over the Big Island and a thunderstorm
or two could affect the nearshore coastal waters and possibly
even windward areas at times.

The airmass should stabilize enough to preclude thunder areawide
by Sunday night, but will remain unstable enough to keep a wet
trade wind pattern in place, which is expected to continue through
much of next week. Windward areas will continue to be the primary
recipient of these trade wind showers, although leeward ares will
see more shower activity than usual due to inversion heights
remaining elevated.

&&

.AVIATION...
A band of clouds moving slowly westward through the state will
bring periods of showers with MVFR CIG and VIS to the windward
and mountain sections of each island through the early morning
hours. Drier trends are forecast to fill in from east to west on
Friday with improving conditions. Trade winds in the moderate to
locally breezy range will continue for the next several days.

AIRMET Sierra for Mountain Obscuration remains in effect over the
windward and mountain sections from the Big Island through
Molokai. These low cloud ceilings will likely spread to Oahu and
Kauai later tonight.

AIRMET Tango is in effect for Moderate Turbulence between
FL250-350 from the Big Island through Molokai. This turbulence
AIRMET is caused by strong upper level winds and may expand over
Kauai by early Friday morning.

&&

.MARINE...
High pressure moving down into the North Pacific from the Gulf of
Alaska this weekend will keep the pressure gradient tight enough
across the region to support moderate to locally strong trade
winds. A Small Craft Advisory (SCA) is in effect for the typically
windier waters surrounding Maui County and the Big Island through
Friday afternoon with a possible extension needed into Friday
evening or early Saturday morning. An upper trough to the north
may destabilize the regional air mass enough to warrant a slight
chance for weekend thunderstorms across our local waters.

A small, medium period north swell is filling in tonight with a
secondary reinforcing small north northwest swell arriving early
next week. These swells will maintain low end north and west
facing shore surf the next several days. A small, medium period
west northwest swell generated by Typhoon Mindulle will overlap
these swells and likely reach the western islands Tuesday. A
small, medium to long period south southwest swell that will keep
south surf from going completely flat, is arriving tonight and
will peak Friday. Choppy wind wave surf along most eastern
exposures will remain small. A stronger trade fetch upstream of
the islands the next couple of days will provide a slight boost to
east facing shore surf this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Blood