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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220637

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Thu Jan 21 2021

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


Windy and locally wet trade wind weather will continue through the
weekend and into next week. The strong winds will shift slightly to
the SE early next week, with increasing moisture potentially leading
to more widespread clouds and showers.

A broad mid- and upper-level low is nearly stationary to the S of
the area, with some of the associated layered clouds aloft over the
islands from Molokai eastward. 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, island slopes are efficiently
creating rainfall, even when upstream skies contain little. A
perfect example is the near non-stop light rainfall over windward
portions of E Maui and Oahu's Koolau mountains over the past couple
of hours, as depicted by radar. Some of these showers have been
briefly heavy due to their increased vertical depth and the
efficiency of warm rain processes. Meanwhile, many leeward
communities remain dry, not having received any recent rainfall.

While some drying of the upstream flow tonight and Friday may lead
to some reduction in windward rainfall, it is not expected to last
long, and little overall change to the current regime is anticipated
through 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. A Wind Advisory may be
required for parts of the state over the next couple of days,
potentially as early as Friday.

The high to the NE 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. This is expected to cause the
winds to veer to the SE, while a moisture convergence band extending
SE from the distant low creeps towards the islands from the S.
Latest guidance indicates the potential for this band to move over
portions of the islands from Monday into Wednesday, bringing
widespread layered clouds and rainfall. 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


Breezy to locally strong trades have developed and 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 mod mechanical lee turbulence below
9kft as well as for mod turbulence between FL250 and FL300.


Surface high pressure is centered far to the north-northeast of the
area, resulting in moderate to locally strong trade winds. A Small
Craft Advisory (SCA) is currently posted for the typically windy
waters around Oahu, Maui County and the Big Island. The high will
get a reenforcement this weekend into early next week,causing trade
winds to increase across the entire area. This, combined with higher
seas from both a large northwest swell and the trade wind seas, will
result in SCA conditions statewide.

A small northwest swell will linger through Friday. Long-period
forerunners from a larger northwest (310 degrees) swell will fill in
late Friday and Friday night. This swell is expected to peak on
Saturday with an 18-second period and high-end advisory or low-end
warning level surf, then lower gradually Sunday and Monday. Surf is
then forecast to 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.

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 on Monday, peak Monday
night and Tuesday, then lower Wednesday through Thursday.


Small Craft Advisory until 6 PM HST Sunday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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