Heavy Rain in Hawaii, April 11 to 18, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
129 PM HST Sun Apr 11 2021

.UPDATE...
A passing trough aloft has generated mid-level clouds that are
producing snow showers down to around 9,500 feet. Snow amounts
are to be modest, only around an inch this afternoon, but recent
web camera images show this snow accumulating on Haleakala, which
does not often experience snow. A Winter Weather Advisory has been
issued for both Haleakala and the Big Island through the
afternoon, since snow chances will decrease by evening.

&&

.SYNOPSIS...
Spotty heavy downpours and thunderstorms are possible for much of
the state through Monday as a trough aloft passes over the area.
Daytime sea breezes will cause heavy afternoon showers to favor
interior terrain as well as the leeward coasts of Oahu and Kauai.
Northeasterly trade winds will redevelop over Kauai and Oahu
later today and tonight, then gradually strengthen statewide
Monday and Tuesday.

&&

.PREV DISCUSSION.../ISSUED 941 AM HST Sun Apr 11 2021/
Spotty heavy showers and thunderstorms are expected today. A
surface trough to the northeast of the state has disrupted the
trade wind flow, though light northeasterly winds are hanging on
near Kauai. Meanwhile, a potent mid- to upper-level trough, with
500 mb temps as low as -17C, is slowly dropping over the islands
from the northwest. Aside from a batch of heavy showers passing
southeast of Kauai, radar is not showing much shower activity this
morning. As sea breezes form midday, convection should develop
along the heated slopes, and with ML CAPE values reaching around
500 J/kg, spotty heavy showers and thunderstorms are expected. The
greatest instability will be over Maui County and the Big Island,
but Oahu will still have a chance of thunderstorms, especially
over interior areas and the Ewa Plain this afternoon. Due to the
exceptionally cold temps aloft, small hail could occur in
thunderstorms, and the summits of the Big Island and the highest
elevations of Haleakala on Maui could have a brief period of snow
or hail.

Instability will linger tonight into Monday for the eastern half
of the island chain as northeasterly trade winds gradually build.
The mid- to upper-level trough axis will pass over Maui tonight
and slowly push east of the Big Island on Monday evening, placing
the highest probability of spotty heavy showers and thunderstorms
over the eastern half of the island chain. The surface trough just
northeast of the state will weaken, and a surface ridge several
hundred miles to the north will strengthen, causing northeasterly
trade winds to gradually build, especially on Kauai and Oahu.
Under the lighter winds, the Big Island and Maui could see the
bulk of shower activity push offshore tonight then redevelop
during the day. On Kauai and Oahu, precipitable water should fall
below an inch, and the moderate to locally breezy northeasterly
trade winds will be focus rainfall over windward areas by late
tonight.

A stable northeasterly trade wind flow will settle over the state
on Tuesday. The mid- to upper-level trough will move east of the
islands, leading to increased stability. Surface low pressure will
linger between Hawaii and California as surface high pressure
strengthens far northwest of the state. As a result, trades will
be from a more northeasterly than usual, and winds will be
strongest over the western end of the state. Precipitable water
values will be near to slightly below normal, and dew points will
remain in the upper 50s to around 60, suggesting modest rainfall
over windward areas and a continued cool feel.

Winds may back off on Friday and next weekend. The GFS and ECMWF
are showing a decayed front dropping near or over the state.

&&

.AVIATION...
Light northeast background flow will allow for daytime sea
breezes and nighttime land breezes to set up through tonight. An
upper level trough will move across the area resulting in a rather
unstable airmass.

VFR conditions currently prevail at all TAF sites. Daytime
heating along with sea breeze convergence, will allow for
increasing clouds to develop statewide with the potential for MVFR
conditions to occur. Shower activity will increase especially for
the central and eastern islands. The unstable airmass will also
allow for the possible development of thunderstorms especially
this afternoon and evening. AIRMET Sierra for mountain obscuration
is possible later today across some areas due to low clouds and
showers.

A broken layer of mid level clouds between 10k and 20k feet had
reports of light rime icing early this morning. AIRMET Zulu is
currently posted due to icing potential. As the axis of the
trough pivots across the area, the mid level deck of clouds
should shift N and E of the state later today.

&&

.MARINE...
Light to gentle northeast breezes prevail across the Hawaiian
coastal waters this morning in response to a trough of low
pressure located northeast of the islands. East to northeast
winds will remain light with smaller scale land and sea breezes
occurring over more sheltered coastal areas today. An upper level
disturbance traveling across the northern waters today will
increase the probability for mainly windward, eastern half of the
state showers and thunderstorms. There is the threat for locally
high, visibility-reducing rainfall within stronger rain bands.
Northeast trade winds will return through the first half of the
week. Early week moderate trades could potentially strengthen into
the fresh to locally strong category Tuesday and Wednesday.

As a small, west northwest swell arrives from a gale that
developed near the Kuril Islands a few days ago, north and west
facing shore surf will trend up tonight through the first half of
the week. A more moderate, shorter period north northeast swell
may arrive later this week from a fetch region generated by
stronger north winds produced from a tighter north pressure
gradient between strong high pressure northwest of the state and
lower pressure to the east. Surf along east facing shores will
remain small through the first half of the week. Surf may will
trend up along eastern exposures by the second half of the week
with the arrival of the aforementioned north northeast swell.
Incoming long period south southwest swell energy will provide a
boost to southern coast surf. This swell will peak surf below
advisory levels Monday and Tuesday with a slow decline through the
middle of the week.

&&

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

&&

DISCUSSION...Wroe
AVIATION...Burke
MARINE...Blood