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WindSW 32 mph
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Hawaiian Islands Synoptic Discussion and Guidance

732
FXHW60 PHFO 241342
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
342 AM HST Mon Feb 24 2020

.SYNOPSIS...
Light and variable winds in the morning will transition into
moderate trade winds later this afternoon as high pressure builds
back in north of the Hawaiian Islands. Trade winds will increase
through the week, becoming breezy on Tuesday, and Windy from
Friday on through the weekend. Clouds and scattered showers will
favor windward and mountain areas through Wednesday, then trend
towards a wet trade wind pattern with numerous showers developing
as an upper level disturbance moves into the eastern half of the
state from Thursday through Sunday. Long range forecast guidance
suggests wet trades may linger into the first half of next week.

&&

.DISCUSSION...
A rapidly weakening cold front has stalled just north of the
islands this morning as a migratory high center builds in north of
the state. Light and variable winds will continue through the
morning hours as we transition to moderate trade winds later this
afternoon. A slight increase in cloud cover and shower activity
will develop due to convergent trade winds flow under the
influence of a weak upper level trough. Breezy trade winds are
expected from Tuesday through Thursday with gusts around 30 MPH in
some wind favored locations. Rain shower activity will follow a
more normal trade wind pattern with isolated to scattered showers
along windward and mountain areas with drier more isolated
activity through Wednesday.

On Thursday the weather changes towards a wetter and windier
pattern lasting through the weekend. A stronger high will build in
far north of the islands as a fairly strong upper level trough
deepens and moves into our area from the north. This upper trough
will destabilize the region by lifting the trade wind inversion
heights allowing for deeper cumulus clouds to form across the
state. These more unstable cumulus clouds will be caught up in the
strengthening trade winds and pushed up over the mountain ranges
of each island resulting in an extended period of wet weather for
all islands. Windward areas will see the highest rainfall amounts,
however even leeward slopes will see periods of shower activity
in these unstable conditions.

On Friday, an upper level cold core low forms at the base of this
upper trough and settles in just east of the island chain. Cold
temperatures and dynamic forcing aloft will continue to destabilize
trade wind clouds upstream of each island through Sunday. Global
weather models remain in good agreement with this blasting wet
trade pattern, and our confidence levels are good for this next
wet weather event. Rainfall amounts remain highly dependent upon
the strength and location of this upper level cut off low as it
meanders towards the Hawaiian Islands this weekend. Higher
rainfall amounts will tend to favor the eastern Hawaiian Islands
due to their closer proximity to the upper low. Snowfall on the
highest peaks of the Big Island are possible with several inches
of snow forecast over Mauna Kea and Mauna Loa by the end of this
weekend.

Meanwhile windy trades remain in the forecast as high pressure
continues to build in north of the state from Friday through
Sunday. Surface wind speeds will likely exceed wind advisory
thresholds and approach high wind warning conditions for the low
elevation sites. The highest mountain peaks on the Big Island will
reach wind advisory conditions and possibly exceed high wind
warning thresholds. A High Wind Watch may be required for the wind
favored areas of the Hawaiian Islands including the slopes of
Mauna Kea and Mauna Loa from Friday through Sunday.

Extended forecast guidance for early next week suggests the upper
level low will drift westward over the Hawaiian Islands potentially
keeping wet weather trade winds in the forecast for a few more
days. Stay tuned.

&&

.AVIATION...
Light northerly winds will allow the land breezes to hold through
the early morning hours today for most areas, before moderate to
breezy trades quickly fill in through the afternoon hours. There
may be a small window of time for a few hours of sea breeze
conditions around and by noon before the trades become
established. Brief periods with MVFR CIGS/VSBYS will remain
possible as showers develop and move ashore within the light
northerly flow this morning, mostly for northern facing slopes
and coasts. As the trades return, the cloud/shower coverage will
transition back to the typical windward areas.

AIRMET Tango is in effect across the entire state for tempo
moderate turbulence between FL200 and FL250. This AIRMET will
likely be dropped later this morning.

&&

.MARINE...
A stalled front will slowly dissipate northwest of Kauai today,
as high pressure builds eastward along 30N. Light trade winds
will prevail this morning, with the trades then ramping up to
moderate and fresh levels during the afternoon. High pressure will
continue to build northeast and strengthen tonight through next
weekend, resulting in gradually strengthening trade winds. SCA
level winds will likely develop around the typically windy areas
around Maui County and the Big Island beginning tonight. Winds
are expected to strengthen further through the week, so the SCA
will likely expand to additional marine zones through the week.
Most marine zones will experience SCA conditions by the weekend,
with Gales possible across some of the typically windy areas.

The current northwest swell will gradually lower today. A large
northwest swell is expected to arrive tonight, peak late Tuesday and
Tuesday night well above advisory levels, and could potentially
reach marginal warning levels along north and west facing shores
at the peak. The swell will then gradually subside Wednesday
through Thursday, with a new large northwest swell arriving late
Wednesday night into early Thursday. This new swell will peak
Thursday night and Friday, with surf likely reaching advisory
levels, then gradually decrease through the weekend. Surf along
the east facing shores will gradually trend up through the week as
the trades increase, and will likely reach advisory levels next
weekend.

&&

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

&&

$$

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

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