Drought in Hawaii, September 1 to 30, 2023

Area Forecast Discussion
National Weather Service Honolulu HI
902 PM HST Thu Aug 31 2023

.SYNOPSIS...
A high pressure center far north to northeast of the Hawaii
Region will continue to produce moderate to locally breezy trade
winds with slight decreasing trends through Saturday. Two upper
level lows moving into position north of the Hawaii region will
significantly decrease the wind speeds over the islands from
Sunday through Tuesday. Light large scale winds on Monday and
Tuesday will allow local scale land and sea breezes to expand in
coverage to include most leeward areas. Brief shower trends may
increase for the first half of next week due to upper troughing
and island heating.

&&

.DISCUSSION...
The infra-red satellite imagery this evening over the Hawaii
Region shows a weak low level trough moving from east to west
across the island chain. Expect Periods of showers along the
typical windward, north through east, slopes of each island into
the early morning hours. The GOES Water Vapor channel shows a
narrow upper level trough west through north of the state with
weak embedded lows forming along the trough axis. Local radar
imagery shows increasing shower activity drifting into windward
areas (north through east slopes) of all islands on the trade
winds. Leeward areas (south through west slopes) will remain on
the dry side with just a few of the stronger windward showers
possibly bringing a brief passing shower or two to leeward slopes
during the overnight to early morning hours.

A high pressure system far north to northeast of the Hawaii
Region will continue to produce moderate to locally breezy trade
winds with slight decreasing wind speed trends through Saturday.
A cold air mass moving down the eastern side of this high center
will approach Hawaii from Sunday to Monday. This cold air will
deepen an upper level low just north of the state, significantly
weakening the surface pressure gradient over the islands, and
producing light and variable winds over the region. In the absence
of large scale trade winds, local scale land and sea breezes will
expand in coverage from Sunday through Tuesday, producing
onshore/upslope winds each afternoon with offshore/downslope winds
at night, especially over leeward areas.

Passing windward shower activity will continue for all islands
during the overnight to early morning hours through Saturday
morning as the low level trough slowly drifts westward through
the state. Starting on Sunday, a combination of upper level
troughing with island heating and converging sea breeze winds may
increase shower potential over the typically drier leeward areas
each afternoon from Sunday through Tuesday. Downslope land breeze
winds will start after sunset and last through the early morning
hours, producing more stable conditions over all island slopes.
This land and sea breeze weather pattern will continue at least
through Tuesday.

Long range model guidance continue to hint at a slight strengthening
of the trade wind speeds on Wednesday. However, we are less
certain that the trade winds will come back on time as two surface
troughs will linger just north of the islands and once the land
and sea breeze pattern is established stronger trade winds are
typically needed to break this local scale wind cycle. Model
solutions differ on how strong the ridge builds back in north of
the island chain for the second half of next week. The timing in
the wind pattern shift from land and sea breezes back to trade
winds may slip a little from next week Wednesday to Thursday.

&&

.AVIATION...
Moderate to locally breezy trade winds will persist through
Friday. A slight uptick in low level moisture may be enough to
help to increase low level clouds and showers tonight through
tomorrow morning, mainly across windward and mountain areas...with
brief spillovers into leeward areas possible. Overall, VFR
conditions will prevail with brief periods of MVFR conditions
later tonight into Friday morning with the increased shower
chances.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain across all islands. This AIRMET will
likely be needed through most of Friday, but low level turbulence
will begin to trend downward as the pressure gradient weakens into
the weekend.

&&

.MARINE...
A robust 1030 mb surface high resides well north of the Hawaiian
islands and continues to produce strong easterly trade winds across
the local waters. Small Craft Advisories will remain in effect for
all marine zones through tonight, due to the hazardous conditions.
Trades are forecast to weaken slightly by Friday morning, but
remain fresh to locally strong. Sunday into Monday, a weak trough
approaching from the east will pass north of the area and
weaken trade flow. Expect sea breeze development near the coast
during the afternoons on Monday and Tuesday as flow remains light
to gentle.

Rough choppy surf along east facing shores will continue over the
next few days as the trades remain breezy. Additionally, expect
some swell mixed in from former Tropical Cyclone (TC) Irwin
through the weekend. Mokapu Point buoy has already begun to show
energy this evening in the 10 to 12 second bands. Surf heights
should stay below the 10 ft (faces) High Surf Advisory (HSA)
criteria during the peak of this event Friday through Saturday. A
downward trend is expected Sunday into early next week as the
trades weaken.

Surf along south facing shores will remain small through the
weekend as overlapping background level long-period southerly
pulses move through. A moderate south to south-southwest swell is
expected to slowly fill in Monday and peak around Tuesday into
Wednesday near HSA levels for south facing shores, then slowly
decline through rest of the week.

Surf along exposed north and west facing shores will trend up
slightly this weekend as a small west-northwest (300-310 degree)
swell arrives. This will be in response to a storm-force low
(former TC Damrey) racing eastward toward the Date Line from Japan
over the past couple of days.

&&

.FIRE WEATHER...
Traditional leeward areas, south through west of island mountains,
will see the lowest afternoon RH values in the middle to low 40
percent range lasting through the weekend. Sustained wind speeds
will likely approach, yet not exceed Red Flag Warning thresholds
in these leeward areas on Friday and Saturday afternoons. Wind
speeds will decrease significantly from Sunday through Tuesday
with lighter large scale trade winds allowing an expansion of
afternoon onshore/upslope sea breezes across most terrain
sheltered leeward areas.

Long range model guidance continues to hint at increasing trade
wind speed trends starting next week Wednesday. However, there
are still two surface troughs lingering just north of the islands
that may limit trade wind speeds towards the light to moderate
range by the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Bohlin
AVIATION...Farris
MARINE...JT/Shigesato
FIRE WEATHER...Bohlin