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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 150150

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Sat Dec 14 2019

Trade winds will diminish somewhat tonight and Sunday, but will
increase Monday and Tuesday before easing Wednesday. The trade winds
will continue through the week, delivering low clouds and showers to
windward areas through the week, but their coverage and intensity is
expected to be limited Monday and Tuesday as the island atmosphere
becomes very stable.


Trade winds (at times strong) dominate the forecast through the
upcoming week, supported by a trio of high pressure cells moving
rapidly eastward across the N Pacific. The first high is centered
far NE of the islands and moving steadily E, with some easing of
today's breezy trade winds expected tonight and Sunday as a weakness
in the ridge passes N of the islands. Supporting this notion is the
fact that surface pressures across the islands are down ~2 mb over
the past 24 hours. A second high will build N of the islands Sunday
night/Monday, leading to increased trade winds until another
weakness in the ridge passes to the N around Wednesday. A third high
is expected to pass N of the islands Thursday/Friday, likely
bringing another period of locally strong trade winds heading into
next weekend.

A low aloft centered several hundred miles W of the islands is
embedded within a NE-SW oriented trough, with the trough axis
located several hundred miles N of Kauai. SW flow aloft is bringing
diminishing high clouds over the islands, although they have been
especially thick over Oahu today. As the low weakens and the trough
moves S over the next 12 to 24 hours, high clouds will diminish
further, lingering over the Big Island Sunday before finally moving
E of all islands as a strong ridge aloft builds in from the W.

Although the low and trough aloft do not appear to be significantly
destabilizing the island atmosphere (with afternoon soundings
depicting a subsidence inversion based near 8000 feet), visible
satellite images show an abundance of open-celled showery low clouds
approaching from the E in the trade wind flow. It looks as though
most windward areas should expect some rainfall overnight, with a
few showers spreading leeward on the smaller islands. Forecast
models indicate showery low clouds will continue to move over the
islands with the trade flow through the forecast period, but they
are expected to diminish in coverage and intensity as the ridge
aloft builds over the area Monday and Tuesday. Current forecast
grids feature reduced PoPs / light showers for this time period due
to the increased stability, reverting back to a climo forecast
thereafter as the strong ridge aloft weakens somewhat. A rather wet
solution for next weekend is noted with the 12Z/18Z GFS as a
shortwave trough aloft passes overhead, but 12Z ECMWF guidance keeps
the trough W of the islands. Both models portend a front approaching
from the NW at some point next weekend however.


A 1028 mb surface high far northeast of the state will allow
moderate to occasionally breezy trades to persist through through
tonight. This could lead to moderate turbulence on the lee side of
the local mountains. Thus, isolated to scattered showers will affect
mainly windward areas. This activity has the potential to bring
periodic MVFR conditions and mountain obscurations to the windward
side of the islands.

AIRMET TANGO remains in effect over and immediately south thru west
of mountains on all islands below 9000 feet due to tempo moderate


Current northwest and northeast swells will lower through tonight. A
long period northwest swell is expected to build late Sunday,
peaking Monday and Monday night bringing surf around warning levels
along exposed north and west facing shores. This swell should slowly
decline Tuesday night into Wednesday. Another northwest swell will
build Wednesday night into Thursday, then decline Friday into the

High pressure north of the state will keep strong trade winds over
the area tonight with a gradual taper off expected into Sunday. A
Small Craft Advisory is posted for all coastal waters through
tonight and may need to be extended for some areas into Sunday.
Trades will boost back to strong breezes by Monday through at least
midweek. Seas will also increase early next week due to the
combination of swell and winds.


Small Craft Advisory until 6 AM HST Sunday for all Hawaiian



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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