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Hawaiian Islands Synoptic Discussion and Guidance

643
FXHW60 PHFO 222002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 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...
While winds are elevated, the grids were a little higher than
recent observations. Have nudged the wind grids down just a touch
to be closer to current observations through this evening, while
continuing the upward trend that is observed in the models.
Currently the grids reflect winds remaining just below advisory
levels today, but exceeding advisory criteria tonight in some
locations. Will be taking a closer look over the next couple of
hours, but leaning towards the need for a Wind Advisory for some
locations for tonight. The areas covered by any such advisory will
likely need to be expanded over the weekend as winds increase.

The overnight surface analysis depicts a 1035 mb high to the
northeast of the islands, and a series of troughs and fronts to
the northwest. The ridge associated with the high is expected to
nudge closer to the islands tonight, tightening the pressure
gradient over the island, which should cause an uptick in winds.
As the troughs and fronts to the northwest move to the east over
the weekend, and a low potentially develops along these
boundaries, we can expect the wind direction to become a little
more southeasterly by early next week. At the same time, the high
to the northeast will be reinforced over the weekend and strength,
causing the winds to increase further.

While today still looks to be a little on the drier side, an
uptick in trade wind showers is expected through the weekend, and
a further increase in activity from Sunday night into Wednesday.

A broad mid to upper level low to the south of the state has
brought some upper level clouds to the southern end of the state
recently, but the latest satellite imagery shows most of that has
moved to the south and east of the Big Island. 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 extending as
high as 9 to 10 thousand feet. Combined with the stronger trade
winds mentioned above, orographic forcing is helping to generate
showers over and immediately downstream of the terrain. Meanwhile,
many leeward communities remain mostly dry.

As the winds become more southeasterly, a moisture convergence
band extending southeast from the low creeps towards the islands
from the south. The latest guidance indicates this band will move
over portions of the islands from Monday into Wednesday, bringing
precipitable water values 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 west of the area. High pressure to the distant
northeast will continue to support locally strong trade winds.


&&

.AVIATION...
A strong surface high is in place far northeast of the main
Hawaiian Islands today and will remain somewhat stationary
through the next few days. This will keep breezy to locally strong
easterly trade winds persistent throughout the forecast period.
Showers will favor eastern sections of the islands, particularly
during the night and early morning hours. As a result, VFR
conditions are expected to prevail across the Hawaiian Islands
throughout the period, with the exception of brief MVFR conditions
in any heavier showers that do occur.

AIRMET Tango remains in effect for moderate low-level turbulence
in areas southwest to west of the mountains of all islands. The
persistence of breezy to strong easterly trade winds across the
islands will likely allow for this low-level turbulence to
continue into the weekend. AIRMET Tango also remains in effect
moderate turbulence between FL250 and FL300.

&&

.MARINE...
A surface high far northeast of the area is generating moderate
to locally strong trade winds. The high will strengthen Saturday,
increasing trade winds across the entire area through the first
part of next week. A Small Craft Advisory is in effect for all
coastal water for strong to near gale force trade winds as well as
building seas from the trade wind swell and a large west-
northwest swell.

A small northwest swell will linger today. Long-period forerunners
from a larger west-northwest (310 degrees) swell will arrive 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 small bump is south-southwest surf 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...M Ballard
AVIATION...Donaldson
MARINE...Almanza

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



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