Heavy Rain in Hawaii, June 29, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
810 PM HST Fri Jun 28 2019

.SYNOPSIS...
Easterly trade winds are slowly returning to the region tonight
and tomorrow as a low west of the islands drifts further away from
the state. More stable trends are also in the forecast as the
high pressure ridge north of Hawaii builds back in through the
weekend. Expect a more normal trade wind weather pattern with
periods of showers favoring windward and mountain areas through
Tuesday. Trade winds weaken next week Wednesday through Friday,
bringing another round of drier weather with more variable winds.

&&

.DISCUSSION...
Water vapor imagery shows an upper level low north of Kauai and an
upper trough axis running north to west of the Hawaiian Islands.
A low level convergence boundary that brought widespread heavy
rain to Kauai last night has reformed to the west of the state.
Radar imagery across the islands shows unsettled showers lingering
over Kauai this evening where a Flash Flood Watch remains in
effect through Saturday morning. More stable conditions with less
showers will gradually return to Kauai County by Saturday evening as
the high pressure builds in.

The low pressure system that produced widespread rain across the
western half of the state this past week will slowly drift
westward away from the Aloha State through the weekend. The
subtropical ridge will build back in north of Hawaii with stronger
easterly trade winds forecast through Tuesday. Expect less humid
conditions in a more stable atmosphere with periods of showers
favoring typical windward and mauka areas.

Wednesday through Friday, a large scale low pressure trough
moving into the Central Pacific region will break down the ridge
north of the state producing weaker east to southeast winds.
Weaker large scale winds will allow local scale heating to drive
land and sea breezes along leeward slopes of each island. The
combination of weaker surface winds and an upper level ridge aloft
will limit shower activity statewide. Global models are in good
agreement with this long range forecast.

&&

.AVIATION...
High pressure northeast of the state will keep moderate trade
winds in place across the island chain through Saturday. Clouds
and showers will favor windward and mauka areas, with a stray
shower affecting leeward areas from time to time. Lingering deep
tropical moisture will keep more showers and lower clouds in place
across Kauai tonight, with improving conditions expected Saturday.

AIRMET Sierra is in effect for mountain obscuration across Oahu
and Kauai. Conditions may improve over Oahu later tonight, but
will likely hold in place across Kauai through mid morning
Saturday.

&&

.MARINE...
Moderate to locally strong trade winds are expected to continue
across eastern and central portions of the forecast area into the
weekend. A Small Craft Advisory (SCA) remains in effect for the
Alenuihaha Channel and waters south of the Big Island. If winds
increase a bit more, the SCA may be expanded to include Pailolo
Channel, Maalaea Bay, and possibly other windward coastal waters
Saturday.

The forerunners from a new long-period south-southwest swell are
expected to arrive late tonight. Buoy observations south of the
area point to a potential for a High Surf Advisory along south
facing shores of all islands starting Saturday night and
continuing into Monday. Additional smaller swells arriving from
the southern hemisphere will continue to maintain small to
moderate surf along south facing shores from Monday night into
the middle of next week. Elsewhere, expect a modest increase in
short-period choppy surf along east facing shores this weekend
due to the return of the easterly trade winds.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 AM HST Saturday for Niihau-Kauai.

Small Craft Advisory until 6 PM HST Sunday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

&&

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Kinel