Flash Flooding and Heavy Rain in Hawaii, March 29, 2021

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
820 AM HST Mon Mar 29 2021

.SYNOPSIS...
A weakened high pressure ridge north of the islands will produce
light to moderate trade winds with local scale day time sea breezes
developing over terrain sheltered areas through Tuesday. Trade winds
will strengthen from Wednesday onward, becoming breezy by the
weekend. A developing upper level low over the region today will
enhance shower activity a bit into the overnight hours. Otherwise
shower activity for the rest of the week will favor windward and
mountain areas, mainly in the overnight to early morning hours.

&&

.UPDATE...
The forecast package has been updated, focused on the central islands
and leeward Big Island for this morning, where there has been a general
clearing of the dense mid and high clouds. Windward Kauai has been
wet this morning, and these showers are expected to spread to the
western half the island this afternoon.

&&

.DISCUSSION...
Satellite imagery this morning shows a similar weather pattern with
an upper level trough moving over the Hawaiian Islands from the
southwest. A low pressure area just north of the state will weaken
the high pressure ridge far to the north of the region. Layered
middle and high level clouds, associated with the passing upper
trough, will keep cloudy conditions in the statewide forecast through
tonight. These layered clouds are producing ice and snow today over
the Big Island summits of Mauna Kea and Mauna Loa for elevations
above 12,000 feet. Strong winds aloft associated with this trough
will also produce strong southwesterly winds over both summits today.
A Wind Advisory and Winter Weather Advisory remain in effect for
both summit areas.

Today and Tuesday...A high pressure ridge located far north of the
Hawaiian Islands and a surface low pressure area just north of the
state will keep a weakened pressure gradient over the area. Light to
moderate trade winds are forecast with isolated to scattered showers.
Sea breezes may develop over some terrain sheltered areas along the
west and southwest island slopes of each island during this time
period. Converging trade wind and sea breeze boundaries will produce
brief showers over leeward and interior island slopes in the
afternoon to early evening hours.

Wednesday through Friday...The low pressure area will weaken as the
high pressure ridge builds in north of the region causing trade wind
speeds to gradually increase through Friday. Typical passing trade
wind showers remain in the forecast favoring windward and mountain
areas. Shower coverage will trend higher over these areas in the
overnight to early morning hours.

Saturday and Sunday...Breezy trade winds will return as the high
pressure ridge builds in north of the state. A stronger high pressure
ridge will keep a stronger trade wind inversion over the state on
Saturday, limiting shower activity. Low trade wind inversion heights
and breezy trade winds may reach the Wind Advisory range for some of
our wind favored terrain areas on Sunday.

&&

.AVIATION...
The trade winds will ease to moderate levels today and tonight,
allowing localized land and sea breezes to develop in the more
sheltered areas. Showers will favor windward and mauka areas this
morning, with the showers spreading to leeward areas this afternoon.
MVFR cigs/vsbys will be possible in showers, particularly in windward
locales.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai, Molokai, Maui and Big Island. These
conditions will likely hold in place though much of the day, and the
AIRMET will likely need to be expanded to include Oahu later this
morning.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. This AIRMET will
likely be cancelled later today.

AIRMET Zulu continues to mention light icing due to layered mid and
high clouds. Conditions may improve enough later this morning that
the mention of icing can be removed.

&&

.MARINE...
Recent satellite observations have depicted fresh to locally strong
trade winds funneling around and through the more eastern islands and
their respective channels. A trough developing into a closed low
northwest of the islands will weaken trades back to gentle to
moderate speeds later today and Tuesday. The Small Craft Advisory has
been extended until 11 AM HST this morning for these higher winds
across the Pailolo and Alenuihaha Channels, Maalaea Bay and the
waters surrounding the Big Island. As strong North Pacific high
pressure builds down across the island chain from Wednesday on
through the weekend, trades will restrengthen to fresh to locally
strong magnitudes.

The latest small, low period northwest swell is declining through
the morning hours. A couple of very small surf days are in store for
the northern and western coasts today and Tuesday. A couple of mid
week small north and west-northwest swells are expected to hold small
surf along north and west facing shores Wednesday and Thursday. No
anticipated northerly swells equate to very small north and west
shore surf Friday with nearly flat conditions continuing this
weekend. As trades ease up over and upstream of the islands, east
facing shore surf will slowly lower through Wednesday. Eastern shore
surf will slowly trend upward late in the week but flatten out and
stay under high surf advisory levels through Saturday. Surf along
southern shores will continue to run near or slightly above seasonal
normals the next few days. A series of overlapping south swells
arriving from the South Pacific will maintain fun, early spring
season waist to near head high surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

Small Craft Advisory until 11 AM HST this morning for Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Big Island Windward
Waters-Big Island Leeward Waters-Big Island Southeast Waters.

&&

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