Heavy Rain in Hawaii, August 27 to 28, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
316 PM HST Tue Aug 27 2019

.SYNOPSIS...
Gentle to locally moderate trade winds will continue to slowly
decrease tonight, becoming weak enough for localized leeward land
and sea breezes from Wednesday into Friday. Showers will continue
to be focused over windward slopes, though a clouds and a few
showers are expected over leeward terrain each afternoon. Trades
should rebuild slightly during the weekend.

&&

.DISCUSSION...
The stable trade wind flow continues its slow decline. The
subtropical ridge driving the trades is being eroded by a weak
North Pacific low digging southward about 1,000 miles north of the
state. Observations from the more wind-exposed stations show that
the trades have dropped into the 10 to 15 mph range this
afternoon, and the gradual downward trend will persist tonight. A
weakening mid level ridge overhead is maintaining stable
conditions, even though the strength of the low level inversion
has eroded slightly. Shower activity has been modest today and
largely confined along a diffuse band between Oahu and Maui.
Expect little change overnight, with the band bringing rainfall of
less than a half inch to windward slopes of Maui County and
perhaps northern Big Island.

Trades will gradually decline into Wednesday as surface high
pressure north of the state is further weakened. The North
Pacific trough eroding the subtropical ridge will also weaken the
mid level ridge over the islands. Conditions will remain generally
stable, but the small pockets of moisture moving along the trade
wind flow could produce briefly heavier rain rates. Trades will be
overpowered by day time sea breezes, which will produce afternoon
clouds and a few showers over leeward terrain. Daytime high
temperatures will continue to run a few degrees above normal,
which, under the suppressed trades, will lead to warm and
uncomfortable conditions.

Trades will rebuild a notch during the weekend, and the GFS and
ECMWF models point toward an increase in showers. Surface high
pressure will set up far north of the state during this time,
nudging the above mentioned area of low pressure eastward.
A dissipating front associated with the low will stall between
the high and the islands, keeping the local pressure gradient
somewhat weak and not allowing a significant increase in the local
trades. In addition, the forecast models are showing an area of
enhanced showers passing through during the weekend, though it is
too early for details.

&&

.AVIATION...
High pressure well to the northeast has weakened considerably.
This will keep the winds weakened and disrupted through this
forecast period. So expect much lighter winds tonight and through
Wednesday. Daytime sea breezes and nighttime land breezes may
develop in areas with the lightest wind speeds this afternoon and
tonight. Showers are possible near the mountains and through
interior sections most likely on Maui, Molokai and Oahu. These
should pass relatively quickly possibly causing a few isolated
areas of MVFR conditions. Expect VFR conditions elsewhere.

No AIRMETs are currently in effect or expected through the period.

&&

.MARINE...
A low far north of the main Hawaiian Islands will weaken the
subtropical ridge. Easterly trade winds will remain in the gentle
to moderate range. Winds will strengthen starting Friday as the
low moves away to the northeast, and a new high builds north of
the area. Trade winds could reach Small Craft Advisory speeds in
the windier areas of the coastal waters by Saturday.

A southwest swell from the Tasman sea forecast to build in today
has been slow to arrive. Swells from that area are partly blocked
by islands in the south Pacific so forecast confidence for these
swell is always reduced. The fetch was long-lasting, so once the
swell arrives it is expected to persist through Thursday and
subside very slowly through Saturday. A small south swell from
east of New Zealand is expected Friday and Saturday. The low north
of the area will produce a small and relatively short-period
north swell reaching the islands over the weekend. Choppy surf
along east facing shores will diminish as the trade winds weaken.

&&

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

&&

DISCUSSION...Wroe
AVIATION...Chevalier
MARINE...Donaldson