Drought in Hawaii, September 14 to 30, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Mon Sep 13 2021

.SYNOPSIS...
An upper level low northeast of the islands will drift slowly
west-southwest through the week, weakening the trade winds from
Tuesday night through early Thursday morning. Localized sea
breezes may develop over sheltered western slopes on Tuesday and
Wednesday. Instability associated with the upper low could lead to
localized heavy rain at times. Drier weather should return from
Friday onward as the trade winds strengthen across the region.

&&

.DISCUSSION...
The satellite water vapor imagery this evening continues to show
an unstable upper low roughly 600 miles northeast of Maui drifting
slowly towards the west-southwest. Scattered thunderstorms are
currently brewing near the cold core center of the low. This
upper low is also producing fairly unstable clouds across the
region and these clouds are riding into each island on the trade
winds.

Broad instability associated with this low and a weaker high
pressure ridge north of the islands will keep the trade wind
temperature inversion in the 7,000 to 10,0000 foot range for the
next few days. Cumulus cloud heights will rise to the inversion
level keeping periods of enhanced showers in the forecast for all
islands through Thursday.

By Tuesday, moderate trade winds will decrease enough for
localized land and sea breezes to develop along terrain sheltered
western slopes of each island. This light to moderate trade wind
pattern with localized land and sea breezes will continue through
Wednesday.

On Wednesday and Wednesday night, the upper low will pass just
north of the Hawaiian Islands with the potential for a few heavy
showers in some locations. Expect slower moving shower bands in
lighter trade wind flow. Chances for locally heavy showers will
increase into Thursday.

Thursday will be a transition day as the trough begins to move
away from the state and high pressure builds north of the state.
Showery conditions are still expected as the breezy trade winds
fill back in starting from the eastern end of the state. By
Friday, trade winds will strengthen across the state with drier
weather trends into the weekend.

&&

.AVIATION...
Persistent high pressure northeast of the the state will allow
for moderate northeasterly trades to continue tonight across the
islands. Scattered showers will mainly be confined to the
windward coasts and slopes though a few showers may pass over
leeward areas. Expect brief periods of MVFR ceilings and
visibility associated with this activity.

There are currently no AIRMETs in effect.

&&

.MARINE...
High pressure north of the state will keep moderate to fresh
trade winds in place through Tuesday. The trades will briefly
ease Tuesday night and Wednesday as a surface trough slides by to
the north of the islands. As this trough exits to the northwest
of the state Wednesday night and Thursday, the trades will
gradually ramp back up to locally strong levels, with winds
reaching Small Craft Advisory (SCA) thresholds over the typically
windy Maui and Big Island waters by Thursday. The strong SCA
level trades are expected to expand to most if not all waters
Friday and Saturday, before dropping a notch Sunday into early
next week.

A series of long period south and southwest swells will keep surf
near the September average through Thursday, before dropping to
slightly below normal levels Friday through the weekend. Small
surf will prevail on north facing shores through most of the week,
with a small bump from a northwest swell possible this weekend.
East shore surf will remain well below normal through Wednesday.
As trade winds ramp up Thursday through Saturday, surf along east
facing shores will become rough and rise back close to seasonal
levels.

&&

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

&&

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Jelsema