Flash Flood and Heavy Rain in Hawaii, May 1 to 2, 2022

Area Forecast Discussion
National Weather Service Honolulu HI
836 PM HST Sun May 1 2022

.SYNOPSIS...
Strong trade winds will decrease slightly by Tuesday but remain
breezy through the rest of the week. An upper trough will close
off into an upper level low tonight, then linger nearby through
the end of the work week. This will keep our airmass unstable and
maintain the potential for locally heavy downpours as well as
isolated thunderstorms. Drier trade wind weather is expected to
return next weekend.

&&

.DISCUSSION...
A 1030 mb high persists far northeast of the main Hawaiian Islands,
with a weakening front located several hundred miles northwest of
Kauai. The pressure gradient between the high and equatorial low
pressure is driving breezy trade winds across local waters this
evening. An upper trough resides over the central and eastern
islands. Satellite loop shows broken to overcast low clouds piled
up along windward areas of the islands, with mauka and leeward
areas also cloud covered within the strong trades. Radar shows
scattered to numerous showers moving into windward areas, with the
coverage the highest over the eastern end of the state. Spots of
deeper convection, possibly thunderstorms, persist across windward
Maui and across windward waters east of Maui County and the Big
Island.

Models show the high will remain in place well to the north and
northeast of the islands during the forecast period, keeping
moderate to breezy trade winds in place. the upper trough will
close off into an upper low in the vicinity of the central and
eastern end of the state tonight and Monday, then linger over or
near the islands through much of the work week. Persistent trades
and instability will lead to wet and showery conditions across
windward areas for much of the week, while elevated inversion
heights will allow showers to reach leeward communities more
regularly as well.

Isolated thunderstorms are possible across Maui County and the
Big Island tonight, and on the Big Island on Monday. Depending on
the timing and location of the upper low, the possibility for
isolated thunderstorms may need to be extended. Wintry weather
will impact the Big Island summits through late tonight, and a
Winter Weather Advisory remains in effect for for this area
through 6 AM Monday. Expect a return of more typical trade wind
weather to the island chain.

&&

.AVIATION...
Clouds and showers remain concentrated along windward facing
slopes, coasts, and within channels through the TAF period as
moderate to locally strong trade winds continue. An upper low in
the vicinity will continue the threat for thunderstorms across
much of the central and eastern islands tonight. Showers are
expected to increase across leeward areas through Monday as
moisture deepens and winds strengthen, while weaker gradient winds
in sheltered areas allows for sea breeze clouds and showers
Monday afternoon across interior leeward sections of the Big
Island and Maui.

MVFR cigs/vsby continue to expand across windward zones tonight,
along with some IFR cigs, as conditions continue to deteriorate.
AIRMET Sierra for mountain obscuration will remain in effect for
north through east sections of most central through eastern
islands through tonight

AIRMET Tango for low-level turbulence immediately south through
west of higher terrain remains in effect for all islands. In
addition, light icing is possible in the shallow layer between
FL180/280 through tonight, mainly for Maui through the Big
Island, as mid and upper level moisture combines with the upper
level low over the islands.

&&

.MARINE...
High pressure north of the state is producing strong trade winds
across the area. A Small Craft Advisory (SCA) is currently posted
for all Hawaiian Coastal Waters through Monday afternoon. Trade
winds are expected to remain elevated over the next several days.
This along with a late season northwest swell will keep SCA
conditions going across many areas through the middle of the
week. Also, an upper low over the area has destabilized the local
airmass. This instability is producing a cluster of thunderstorms
over windward Maui and a few around the Big Island this evening.
Most of the thunderstorms will continue to be over the windward
waters tonight, but a few over the leeward waters can't be ruled
out. This upper level low is expected to linger over the state
through at least Thursday.

A new long period northwest swell is expected to arrive tonight
and Monday. The swell will then get a reinforcement late Tuesday
into Wednesday with surf heights possibly reaching low end advisory
levels. This swell will then lower gradually through the remainder
of the week.

A new long period south swell has started to fill in this evening
and will continue to build on Monday. An even larger south swell
is expected to arrive Tuesday into Wednesday. Data from the
PacIOOS Aunuu Buoy in American Samoa indicates that this well is
running higher than model guidance. This swell may be large enough
for surf heights to reach low end advisory levels during its peak
Wednesday and Thursday. This swell will then lower gradually
through the remainder of the week.

Trade winds over and upwind of the state will continue to produce
short period choppy surf along east facing shores through all of
next week. Surf heights are expected to remain above normal the
next several days then lower during the second half of the week as
the trade winds weaken some.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM HST Monday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters-

&&

DISCUSSION...Powell
AVIATION...SWR
MARINE...Kino