Heavy Rain in Hawaii, September 23, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
954 AM HST Wed Sep 23 2020

.SYNOPSIS...
Trade winds can be expected for the next few days as high
pressure passes by far to our north. An area of enhanced moisture
will provide for a period of passing showers through tonight
primarily for windward and mauka areas. Winds will be lighter over
the weekend and on into early next week as a frontal system moves
closer to the area from the northwest. The airmass will be fairly
dry and stable so just a few showers are expected.

&&

.DISCUSSION...
Surface high pressure centered far to the north northwest of the
area is providing for moderate trade winds across the state. An
area of enhanced moisture is seen on satellite imagery moving into
the east end of the island chain. This is providing for passing
showers over windward Big Island and extreme east Maui. A few
light passing showers are seen elsewhere primarily over windward
slopes and summits. Most of this moisture is expected to pass
south of the state but the northern periphery will continue to
bring showers to the east end of the state today with some of this
moisture moving up across the central and eastern islands later
today and tonight. Thus we do expect some increase in windward and
mauka shower activity as this moisture moves through the area.

The surface high will move slowly eastward over the next few days
with an uptick in trade winds expected. Passing light showers will
continue to favor windward and mauka areas with most leeward
locations remaining dry.

As we head into the weekend, the high will have moved to a
position far to the northeast of the state while a frontal
boundary far to the northwest nudges the highs associated ridge
axis a bit south. This will cause the trade winds to become light
to moderate allowing daytime sea breezes and nighttime land
breezes to become widespread. The sea breezes will produce afternoon
clouds and spotty showers, but a ridge aloft will keep the atmosphere
stable, so rainfall will be light.

By early next week, the front will be located a few hundred miles
to the north northwest of the state. Winds will become rather
light with land and sea breezes dominating. The airmass will
remain rather dry and stable so shower activity will be rather
limited.

&&

.AVIATION...
With high pressure entrenched north of the islands, trade winds
will be moderate to locally breezy through Thursday.

An area of showers, and accompanying MVFR conditions, is moving
from the east and will mainly affect windward and mauka sections
across the state, with the Big Island, Maui, and Molokai seeing
the most activity this morning. Otherwise, VFR will prevail.

An AIRMET for mountain obscuration is currently in effect for
windward parts of the Big Island.

&&

.MARINE...
A secondary area of high pressure building in behind a eastern
Pacific cold front will maintain the ongoing north-south pressure
gradient across the state. This will result in moderate to fresh
trades with a slight strengthening through Friday. These anticipated
higher trade winds will become locally strong within the channels
surrounding Maui County, Maalaea Bay and those waters south of
the Big Island. Thus, a Small Craft Advisory (SCA) remains in
effect for these waters. Expect winds to weaken below SCA criteria
by Saturday with light winds expected into early next week.

As of this morning, the eagerly anticipated long period north
swell has yet to arrive at Buoy 51001. The models are still timing
the arrival of these building swells today, peaking tonight and
tomorrow while producing much higher, yet below advisory level,
surf for north-facing shores. These swells will begin to subside
through Saturday. A few long period south and southwest swells
will keep surf along south-facing shores near the seasonal
average. Slightly stronger trade winds over the next couple of
days will result in a minor upward swing for east-facing shore
surf through Friday. After a weekend of weaker trades and lowering
surf, eastern swell front runners may arrive from (what is left
of) Tropical Cyclone Lowell's East Pacific fetch.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Burke
AVIATION...Kinel
MARINE...Blood