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Hawaiian Islands Synoptic Discussion and Guidance

957
FXHW60 PHFO 230637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Sat Feb 22 2020

.SYNOPSIS...
Large scale winds will become lighter on Sunday as the high
pressure ridge north of the islands breaks down ahead of a
stalling cold front far to the northwest of the state. Land and
sea breezes will develop over each island through Sunday night.
Moderate trade winds will return by Monday morning as a migratory
high moves through the region north of the islands. Trade winds
will last through Friday. Expect typical scattered windward and
mountain showers in this weather pattern favoring the overnight
to early morning hours, only isolated showers are possible over
leeward areas. Long range guidance is suggesting increasing
rainfall trends over the Hawaiian Islands by the end of the week.

&&

.DISCUSSION...
In the large scale weather picture we see a strong high pressure
center far northeast of Hawaii with a weakening ridge just north
of the islands, and a weakening cold front approaching the state
from the northwest. This approaching cold front will continue to
weaken the ridge north of the islands causing the southeasterly
wind flow to weaken allowing for land and sea breezes to increase
on Sunday. The warmer land surfaces over each island and
convergent sea breezes will produce increasing clouds from Sunday
morning to afternoon only isolated showers are possible in this
pattern mainly in the afternoon to early evening hours. Changes to
the forecast will start on Monday as another high pressure center
moves into the Central Pacific basin.

On Monday, trade winds return as the cold front north of the
islands diminishes and another high pressure system tracks from
west to east far to the north of the island chain. Initially the
weakening front north of Kauai will slightly reduce surface
pressure gradients keeping trade wind speed in the more moderate
range. A slight bump in the trade wind shower activity may occur
by Monday morning as convergent wind speeds interact with mountain
ranges under the larger scale influence of a weak trough aloft.
Otherwise a building high pressure ridge aloft will keep rainfall
chances on the low side through the week. The current forecast
shows a trade wind weather pattern with scattered showers over
windward areas and drier weather over leeward areas of each
island through Thursday.

Long range forecast guidance is hinting at higher shower activity
by Friday morning as a relatively strong upper trough moves in
from the north. Both the American (GFS) and European (ECMWF)
models form a cold core low at the base of this upper trough on
Friday just east of the island chain. This low pressure center
will remain cut off from the upper level jet stream and tend to
meander towards the islands through the weekend. The unstable cold
core of this low may destabilize the low level cloud field
upstream of the Hawaiian Islands, potentially enhancing shower
activity with moderate to breezy trade winds through next Sunday.
Weather impacts for each island will be highly dependent upon the
strength and path of this upper level cold core low. Stay tuned.

&&

.AVIATION...
The low-level flow has shifted out of the southeast due to an
approaching front from the northwest. Light and variable surface
winds will be the result for most areas through Sunday night,
which will translate to overnight clearing as land breezes setup
and interior clouds and a few showers Sunday afternoon due to sea
breezes. Ridge of high pressure will begin to nose eastward behind
the stalled/diminishing frontal boundary to the north Sunday
night into Monday. Light northerly low-level winds will become a
possibility for the western end of the state by Monday morning.
Outside of a few clouds/showers that could lead to MVFR
CIGS/VSBYS, VFR conditions will prevail.

There are no AIRMETS in effect or anticipated at this time.

&&

.MARINE...
A front will approach from the northwest tonight and Sunday,
then stall out around 100 nm northwest of Kauai Sunday night. The
front will then slowly dissipate early next week as a new high
builds eastward, well to the north of the Hawaiian islands. A
Small Craft Advisory (SCA) remains in effect for some of the
waters around Maui and the Big Island, but should drop below SCA
levels prior to daybreak Sunday. The one exception may be the
windward/leeward Big Island waters where SCA level winds may hold
in place through noon Sunday. Winds should then remain below SCA
levels across the marine area Sunday afternoon through Monday as
the front stalls northwest of the state. Trades will begin to
return on Monday as high pressure strengthens north of the state,
with SCA level winds likely returning to some marine zones Monday
night. The trades will gradually strengthen Tuesday through late
next week, with SCAs likely being required for much of the marine
area by the end of the work week.

The current northwest swell will peak tonight, then gradually
lower Sunday through Monday. Surf from this swell will remain well
below advisory thresholds. A large northwest swell is expected to
arrive Monday night. Surf from that swell will peak well above
advisory levels Tuesday, and may reach warning levels along north
and west facing shores. The swell will gradually subside through
Thursday when a new northwest swell will arrive. This swell will
peak Thursday night and Friday, then gradually decrease next
weekend. Moderate surf along east facing shores will subside
through early next week as the trade winds weaken. East shore surf
will trend up late next week, and may reach advisory levels next
weekend. Surf will remain small along south facing shores.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Gibbs
MARINE...Jelsema

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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