Drought in Hawaii, September 15 to 30, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
829 PM HST Mon Sep 14 2020

.SYNOPSIS...
A weak high pressure ridge north of the Hawaiian Islands will keep
light to moderate trade winds in the forecast at least through
Thursday, with wind speeds increasing from Friday into the
weekend. Expect mostly dry weather conditions to continue this
week with periods of isolated to scattered shower coverage. A
slight increase in shower activity remains possible for the island
of Kauai on Wednesday and Thursday as an upper low drops in from
the north. Higher chances for showers are forecast by this weekend
across the entire island chain due to added moisture and instability.

&&

.DISCUSSION...
Several low pressure systems and associated cold fronts moving
across the North East Pacific basin will continue to keep the
high pressure ridge north of the islands in a weakened state
through Thursday. This means the trade winds will remain in the
light to moderate range for much of the week. These lighter large
scale winds will allow more local scale afternoon sea breezes and
overnight variable land breezes to continue. Afternoon convergence
between the sea breezes and the trade winds will produce cloud
bands over mountain and island interior sections. Some of these
convergent areas may produce isolated showers in the late
afternoon hours.

This mostly dry weather pattern will continue through Friday as
large scale subsidence will keep the trade wind inversion in the
6000 to 8000 foot range. Enough for isolated to scattered shower
activity, however precipitation amounts will stay on the lighter
side. A cutoff low drifts into the region on Wednesday, stalling
roughly 300 miles north of Kauai. Cold air aloft associated with
this low may destabilize the atmosphere around the island of Kauai
on Wednesday and Thursday possibly producing a slight increase in
shower activity for the Garden Isle. Confidence in these enhanced
showers for Kauai remains only moderate as this forecast hinges
on both the track and timing of the cutoff low.

On Friday, the high pressure ridge north of the islands
strengthens, while a low pressure system in the deep tropics moves
past the islands on a westward track. Both developing systems will
combine forces to increase regional pressure gradients and
produce moderate to breezy trade winds from Friday through Sunday.
Strengthening trade winds will bring more clouds and slightly more
showers to windward and mountain areas. Local scale sea breezes
will also diminish.

Longer range forecast guidance continues to suggest a more
unstable pattern developing over the Hawaii region this weekend
as trade winds strengthen. Model cross sections for Saturday and
Sunday are suggesting trade wind inversion heights around 8000 to
10000 feet. This higher thermal inversion height will allow
cumulus clouds to grow taller and produce more shower coverage. We
expect periods of scattered to numerous showers developing over
the weekend. The highest shower coverage in this range will favor
windward and mountain areas in the overnight to early morning
hours. Still far enough out on the horizon for some changes to
this long range weekend weather forecast. Stay tuned.

&&

.AVIATION...
Weak high pressure just northwest of the islands will keep light
to moderate trade winds in the forecast for the next several
days. The light broad scale flow will allow localized sea breezes
along leeward coasts in the afternoon and early evening hours then
weak land breezes in the late night and early morning hours.

The air mass remains mostly dry and mostly stable. Shower
activity will remain limited and favoring windward slopes and
coasts. Mid-level clouds over interior and leeward Big Island are
expected to clear out towards morning, but will likely redevelop
Tuesday afternoon.

No AIRMETS in effect or anticipated at this time.

&&

.MARINE...
A ridge north of the area will maintain gentle to fresh easterly
trade winds for the next few days. Winds are expected to strengthen
a bit late in the week as a high builds to the northeast. Winds
may get strong enough in the windy areas around the Big Island and
Maui to require a Small Craft Advisory Friday through early next
week.

Surf will remain well below advisory levels along all shores through
next week. A small, short-period northeast swell is expected to
arrive Friday along with a small northwest swell. A small south
swell is expected to persist into mid-week, with a slightly larger
sough swell possible by next weekend. A persistent small, short-
period southeast swell will continue.

&&

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

&&

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Donaldson