Heavy Rain in Hawaii, April 28, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
948 AM HST Wed Apr 28 2021

.SYNOPSIS...
A weakening cold front will bring wet conditions and increasing
northeast winds to Kauai, while the rest of the state continues to
experience light winds with spotty afternoon showers. The front
will fall apart near Oahu and Maui County Thursday, and building
high pressure north of the state will produce increasing trade
winds. Strengthening trade winds and a typical pattern of mainly
windward showers is due Friday through the weekend.

&&

.DISCUSSION...
One last day of variable winds is in store for much of the state.
A shallow and weak front is poised to move onto northern Kauai
this morning, keeping local winds light and variable. There are a
few pockets of lingering moisture this morning that have produced
some showers over mainly leeward Big Island and Molokai, though
shower activity is diminishing as of mid morning. The front will
move onto Kauai shortly, bringing showers to mainly interior and
northern areas and causing increasing northeasterly trade winds on
the Garden Isle. Across the rest of the island chain, afternoon
sea breezes will produce clouds and spotty showers over land. A
building ridge aloft will lead to increased stability and limit
shower intensity.

Trade winds will slowly build state-wide tonight and Thursday as
the front gradually dissipates over the central islands. Expect
the somewhat enhanced showers along this feature to be focused
over Oahu tonight then spread to Maui County Thursday. The
atmosphere will remain stable as a ridge aloft builds overhead,
and significant rainfall is unlikely. Surface high pressure
passing north of the state will spread trade winds across the
entire state by Thursday.

Moderate trade winds and a typical pattern of windward rainfall
is expected for Friday and the weekend. Moisture that was pooled
along the dissipated front will fuel modest windward showers, but
most leeward areas will remain dry under a stable and locally
breezy easterly trade wind flow.

In the long range, trade winds will likely drop a notch early
next week as a disturbance develops west and northwest of the
islands. This feature could enhance showers over mainly the
western end of the state, though it appears unlikely based on the
latest GFS and ECMWF runs.

&&

.AVIATION...
Clouds, showers and trade winds along an old frontal boundary will
move onto Kauai today. Showers will focus over the windward side
of Kauai with likely AIRMET Sierra for tempo mountain obscuration/MVFR
conditions by afternoon. Light winds will bring sea breezes and
cloud build ups over interior sections of the other islands.
Brief/isolated mountain obscurations will be possible for
the remainder of the islands.

Frontal remnants will reach Oahu by early evening, and Maui
County Thursday morning. Strengthening trade winds will continue
to focus clouds and showers over windward slopes.

&&

.MARINE...
A weak front just north of Kauai as of 8 AM will move south
slowly and dissipate through tomorrow. Winds are light and
variable from Kauai south across the rest of the coastal waters.
Moderate to fresh northeast trade winds north of Kauai will spread
south through tomorrow. The winds will shift out of the east and
strengthen Friday as a high builds northeast of the area. Trade
winds will likely reach Small Craft Advisory strength around the
Big Island and Maui County by Friday evening and continuing
through the weekend. The trades will subside through the first
half of next week as a front approaches from the northwest.

A small and relatively short-period west northwest swell will
continue today. Kauai and Niihau will partly block this swell from
reaching Oahu and Maui county. The swell will build tonight and
shift out of the north northwest, reducing the blockage. A larger
and longer-period northwest swell is expected to arrive Friday.
Surf along north and west facing shores will peak near the High
Surf Advisory threshold Saturday morning, then slowly subside.
Surf will remain small along south and east facing shores for the
next few days. Surf along east facing shores will build over the
weekend as the trade winds strengthen.

&&

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

&&

DISCUSSION...Wroe
AVIATION...Foster
MARINE...Donaldson