Heavy Rain in Hawaii, May 6 to 7, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Thu May 6 2021

.SYNOPSIS...
Cloudy skies and increasing moisture associated with an upper
disturbance and surface trough in the area will lead to increasing
rainfall chances statewide overnight through Friday night. Heavy
showers and a few storms will remain possible through this time,
which increases the threat for localized flash flooding. In
addition to the potential for heavy rainfall, a wintry mix is
possible at the summits of the Big Island overnight into Friday.
Conditions should gradually begin to improve and trend back toward
a typical trade wind pattern over the weekend through early next
week.

&&

.DISCUSSION...
Guidance remains persistent and continues to advertise a wet
pattern unfolding over the state overnight through Friday night
as an upper disturbance and surface trough influence the local
weather. Some showers may become heavy and lead to localized
flooding concerns where they become focused for any duration. In
addition to the rainfall, a few thunderstorms will be possible as
instability associated with the upper low increases and the trade
wind inversion continues to erode. The low-level flow has veered
out of the east-southeast direction - which will keep the land and
sea breeze regime in place over sheltered leeward and interior
locations into the weekend.

High-resolution rainfall guidance shows the highest accumulations
over windward and mountain locations with peak 48-hr totals
reaching the 3-5 inch range. As typically observed with this
pattern, convergent plumes setting up downstream of the smaller
islands could lead to persistent showers anchoring over windward
terrain for long durations. If this occurs, streams will quickly
rise and lead to localized flash flooding.

For the summits of the Big Island, deep tropical moisture lifting
northward into the area east of the upper low combined with
freezing temperatures will likely translate to a wintry mix later
tonight into Friday. Conditions should gradually improve at the
summits over the weekend.

Conditions should improve and trend back toward a typical trade
wind pattern statewide over the weekend through early next week
as drier air moves in and the upper disturbance lifts northward
away from the state. Some residual moisture lingering over the
western end of the state will keep the rainfall chances up into
Saturday.

&&

.AVIATION...
Overcast mid and upper level clouds out ahead of an upper trough
west of the islands will continue to stream over the area.
Widespread showers and isolated thunderstorms will mainly affect
windward and southeast facing areas, though wet conditions are
also possible on leeward sides. Expect periods of MVFR ceilings
and visibility to occur with this activity. Heavier showers and
thunderstorms may bring brief IRF conditions.

AIRMET SIERRA for mountain obscuration above 2500 feet will go
into effect later tonight. AIRMET ZULU remains in effect between
between 15000 and 24000 feet due to light icing.

&&

.MARINE...
Strong high pressure northeast of the islands has created a tight
enough pressure gradient back toward the state to support fresh
trade winds across windward central and eastern island waters. An
upper level low moving into the area from the southwest, along
with its associated surface trough draped over the west offshore
waters, has weakened and veered winds southeast across leeward and
west island waters. Little movement of this surface trough, along
with the further strengthening of northeast high pressure, will
generally produce fresh east trade flow over the central and
eastern island windward waters while maintaining gentle to
moderate east southeasterlies across more leeward or western
island waters. The upper low will continue to be the focus for
showers and isolated thunderstorms through Saturday as it slowly
meanders northeast and over the islands.

A small north northeast swell will keep small surf in along north
facing exposures through Friday. An area of strong to near gale
force winds along the southern periphery of a broad low near the
west Aleutians will be focused along a northwest great circle path
directed at the islands. This small, short to medium period
northwest swell is scheduled to arrive and boost north and west
facing shore surf late Saturday night into Sunday morning, hold
through Monday and then decline on Tuesday. Surf along east facing
shores should gradually trend up Friday and hold through early
next week in response to trade wind wave swell from strengthened
easterlies over and upstream of the islands Friday and Saturday.
South facing shore surf will remain small through Friday. The
arrival of a series of small, long period background south swells
will provide a small boost to south facing shore surf this
weekend. A slightly higher, long period south southwest swell
arriving Tuesday may hold southern coast surf to near seasonal
levels through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late Friday night for all Hawaii
islands-

Winter Weather Advisory until 11 AM HST Friday for Big Island
Summits.

&&

DISCUSSION...Gibbs
AVIATION...Powell
MARINE...Blood