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Hawaiian Islands Synoptic Discussion and Guidance

895
FXHW60 PHFO 221340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Fri Jan 22 2021

.SYNOPSIS...
Strong and gusty trade winds will become especially strong over the
weekend, delivering passing clouds and showers, some briefly heavy,
mainly dampening windward areas. More widespread rainfall may
develop early next week as winds remain strong and shift to the
southeast.

&&

.DISCUSSION...
Windy and locally wet trade wind weather will continue through the
weekend, although shower frequency may be somewhat reduced today.
The strong winds will shift slightly to the SE early next week, with
increasing moisture potentially leading to more widespread clouds
and showers from Sunday night into Wednesday.

A broad mid- and upper-level low is nearly stationary to the S of
the area, with some of the associated layered clouds over the Big
Island and parts of Maui. Cooler than normal mid-level temperatures
associated with the low are supporting a somewhat unstable island
atmosphere, with radar VWPs indicating the low-level moisture riding
in on the trade flow extends as high as 10 thousand feet. With fresh
to locally strong trade winds being supported by a 1035 mb high to
the distant NE, orographic forcing is helping to generate showers
over and immediately downstream of the terrain. Meanwhile, many
leeward communities remain mostly dry.

While some drying of the upstream flow may lead to some reduction in
windward shower frequency today, it is not expected to last long,
and somewhat showery trade wind weather is anticipated over the
weekend. Mid-level temperatures will remain cooler than normal, and
strong trade winds will strengthen as the high to the distant NE
moves closer to the islands. The high will be reinforced Saturday
and Sunday, reaching a central pressure over 1040 mb, while a deep-
layer low develops to the distant W and NW of the islands. A Wind
Advisory may be required for parts of the state over the next couple
of days, potentially as early as tonight.

The development of the low to the distant NW of the islands is
expected to cause the winds to veer to the SE, while a moisture
convergence band extending SE from the low creeps towards the
islands from the S. Latest guidance indicates this band will move
over portions of the islands from Monday into Wednesday - bringing
PWAT close to 2", layered cloudiness, and widespread rainfall. Deep
moisture and modest instability are expected, so the chance of
thunderstorms appears too low to mention thus far. A drying trend is
loosely expected thereafter as the band shifts W of the area. High
pressure to the distant NE will continue to support locally strong
trade winds.

&&

.AVIATION...
Breezy to locally strong trades will persist through the forecast
period. This will facilitate frequent windward showers and
occasional leeward drift, particularly overnight. VFR is forecast to
prevail at all forecast points with nothing more than brief MVFR
within any heavier showers.

AIRMET Tango is in effect for mechanical lee turbulence below 9kft
as well as for mod turbulence between FL250 and FL300.

&&

.MARINE...
Surface high pressure is centered far to the north-northeast of the
area, resulting in moderate to locally strong trade winds. The high
will get a reenforcement beginning Saturday, causing trade winds to
increase across the entire area through the first part of next week.
This, combined with higher seas from both a large northwest swell
and the trade wind seas, has prompted expansion of the current Small
Craft Advisory to include all coastal waters.

A small northwest swell will linger today. Long-period forerunners
from a larger west-northwest (310 degrees) swell will fill in late
today and tonight. This swell is expected to peak Saturday with an
18 second period and high-end advisory or low-end warning level surf
for most north and west facing shores of the smaller islands. The
more westerly direction of this swell will allow west facing shores
of the Big Island to experience advisory level surf, likely later in
the day Saturday. Swell and surf will lower gradually Sunday and
Monday, then become rather small for this time of year on Tuesday
and Wednesday.

As the trades strengthen this weekend, rough and short-period surf
will gradually rise and a High Surf Advisory is likely along east
facing shores for several days, beginning Saturday afternoon.

Small, mainly background, southerly swells are expected over the
next several days. A slightly larger, south-southwest swell from the
Tasman sea is expected to gradually fill in Monday, peak Monday
night and Tuesday, then lower Wednesday through Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for all Hawaiian waters.

&&

$$

DISCUSSION...Birchard
AVIATION...JVC
MARINE...TS

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



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