Heavy Rain in Hawaii, May 3 to 4, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Mon May 3 2021

.SYNOPSIS...
Trade winds will weaken into Tuesday and showers will become more
active as an upper level low develops just west of the state.
Trade winds will strengthen Tuesday night and Wednesday as the
upper low sends thick high clouds overhead. The weakening upper
level low will slowly move over the state late Thursday into the
weekend, bringing potentially wet and unsettled weather.

&&

.DISCUSSION...
Trades are on a slow decline, and the atmosphere is becoming less
stable. A digging upper level trough centered several hundred
miles northwest of Kauai is invigorating a decayed front
stretching well north of the islands. This is weakening the
pressure gradient over the state and causing trade winds to slowly
decline. The upper trough is also eroding the upper level ridge
over the state, which is leading to decreased stability. As a
result, shower activity in the plumes west of the islands is
somewhat active, and showers that developed over leeward Big
Island this afternoon are persisting. Expect the showers over
leeward Big Island to hang on for several hours before dissipating
late tonight. With the trades decreasing, we do not expect a
significant amount of showers currently east of the islands to
move onto windward slopes, but the mass of clouds and showers
southwest of Kauai and Oahu should persist.

On Tuesday, trade winds will become weak enough for sea breezes
to develop over sheltered leeward areas in the afternoon, possibly
triggering a few briefly heavy showers over leeward terrain.
Windward areas will experience some showers, but the GFS and ECMWF
are suggesting that a band of enhanced moisture will remain just
east of the state.

On Tuesday night into Thursday, trade winds will rebuild and high
clouds will thicken overhead, as the upper level trough cuts off
about 600 miles west of Kauai and surface high pressure passes far
north of the state. The GFS and ECMWF show windward showers
picking up as a narrow band of moisture moves in on Wednesday,
but with the high clouds limiting daytime heating, leeward showers
appear less likely each afternoon.

The greatest potential for increased showers and unsettled
conditions state-wide looks to be Thursday night and Friday when
the weakening upper level low drifts over the islands. Breezy
trade winds should hold, keeping the highest chances for rainfall
over windward slopes, but spotty, locally heavy afternoon showers
will be possible over leeward areas, especially on the Big Island
and Maui.

The weak upper level trough will linger over the state through the
weekend, creating some uncertainty. Surface high pressure to the
northeast will maintain locally breezy easterly trade winds,
strongest over the Big Island and Maui. Although the GFS and ECMWF
show some drier air pushing in from the east, we could still have
some threat for unsettled conditions. Both models show
increasingly stable weather next Monday.

&&

.AVIATION...
A surface high northeast of the state will allow light easterly
trade winds to continue through Tuesday before strengthening
again Tuesday night and becoming breezy for the rest of the week.
Clouds, showers, and occasional MVFR conditions will favor north
through east facing sections of the smaller islands throughout the
day Tuesday, but VFR conditions will prevail for most area
airports. Additionally, land/sea breezes can be expected over
leeward Big Island, where cloud cover will diminish by the
morning but return during the afternoon. A low aloft will increase
rainfall chances beginning tomorrow night across the state.

There are currently no AIRMETs in effect, though mountain
obscuration may become an issue at times throughout the next 24
hours for interior locations and north through east facing slopes.

&&

.MARINE...
Moderate easterly trade winds, supported by surface high pressure
northeast of the state, will weaken tonight in response to a
deepening trough northwest of the islands. As the surface trough
over the western islands moves farther west of the area Wednesday,
a surface high will build in north of the chain. This will
restrengthen easterly trades to fresh to locally strong magnitudes
through next weekend.

Surf will remain well below high surf advisory levels along all
shores this week. The current northwest swell is slowly declining.
Surf will continue to fall accordingly along north and west
facing shores. Surf along east facing shores will diminish through
Tuesday as the trade winds slacken, but will rebuild Wednesday
and Thursday as winds increase again. Low background southern
hemisphere swell energy will maintain small south shore surf
through the week. Some additional south-southwest swell energy
arriving in a couple of days will provide a minor boost to
south shore surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Wroe
AVIATION...Vaughan
MARINE...TS