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Hawaiian Islands Synoptic Discussion and Guidance

085
FXHW60 PHFO 220127
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
327 PM HST Sat Apr 21 2018

.SYNOPSIS...
Stubborn high clouds from the subtropical jet stream will
continue to limit sunshine over Maui and the Big Island for the
rest of the weekend. Otherwise, trade wind showers are expected to
decrease overnight, leading to drier weather windward and mauka
for the second half of the weekend. Another area of low level
moisture will increase trade wind showers, mainly for windward
Maui and the Big Island, Monday night into Tuesday. Trade winds
will peak at locally windy speeds Monday into Tuesday as high
pressure passes far to the north, and then slowly decline later
in the week. A cold front may push down the chain with clouds and
showers by next weekend.

&&

.DISCUSSION...
Fairly typical trade wind weather is expected for the next several
days as a 1032 mb high passes about 1100 mi N of the islands on
Mon, then become nearly stationary to our NNE. The high will bump
our breezy trades back to locally windy levels Mon and continuing
through Tue. A gradual decline in trade wind speeds is expected
after that, with the trades possibly being disrupted over the
islands starting Fri as a surface low passes far to the N. The
models seem to be a bit up in the air as to whether or not we will
maintain some semblance of gentle trades on Fri, or if local
diurnal winds will take over. By Fri night and Sat, the models
agree fairly well on a late-season frontal passage coming down
the chain, with breezy trades returning.

BKN-OVC high and and mid level clouds, associated with the
subtropical jet, continue to dominate Maui County and the Big
Island, with Oahu on the N edge of this cloud area. The models
maintain this area through the remainder of the weekend, limiting
sunshine there. At lower levels, the back edge of an area of
scattered to broken clouds and showers riding in on the trades is
approaching the Big Island. This should lead to fewer windward and
mauka showers later in the night and into Sunday for most of the
state. Both the GFS and ECMWF show another trade wind shower area
arriving at windward Maui and the Big Island late Mon night or
Tue, so have adjusted PoPs upward a bit for this. Overall though,
this week looks to be fairly quiet. Shower timing and placement on
Friday will depend on how much we lose the trades. Expect an
increase in showers again Friday night into Saturday associated
with a frontal band, but this is too far out in time to give much
detail at this point.

&&

.AVIATION...
A ridge of high pressure north of the state will maintain a moderate
to breezy trade wind pattern. Clouds and showers will favor
windward and mauka areas, with occasional MVFR conditions expected.
A few showers will also spill leeward from time to time, but
predominantly VFR conditions should prevail.

AIRMET Sierra is in effect for tempo mountain obscuration over
windward sections of Oahu, Molokai, Maui and the Big Island.
Conditions are expected to remain in place overnight for at least
some of those regions.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands due to the
moderate to breezy trade winds. Expect these conditions to
continue through at least Sunday.

Models show an area of moderate to severe upper level clear air
turbulence over the islands, which is covered under SIGMET UNIFORM
series. The forecast shows this area sticking around into Sunday,
but weakening to moderate levels overnight. As this happens the
SIGMET will be able to be dropped and could prompt another AIRMET
Tango for moderate upper level turbulence over at least part of
the local area.

&&

.MARINE...
A surface high far northeast of the area will depart while another
high fills in from the northwest Sunday, resulting in a boost to
trade winds speeds beginning tonight. The Small Craft Advisory
(SCA) that was in effect for the windier areas near Maui and the
Big Island has been expanded into waters around Lanai, Molokai
and Oahu through the day Sunday. As the high passes by the north
during the first half of next week, expect a further strengthening
of the trade which may prompt more zones to be added to the SCA.

Surf is expected to remain below High Surf Advisory (HSA) thresholds
along all shores through this weekend. Surf along east facing
shores will remain rough due to the trade winds, but heights will
remain moderate. There will be some increase in surf heights
along east facing shores next week as trade winds strengthen,
possibly approaching HSA heights.

The current northwest swell will decline tonight into Sunday. A
slightly larger swell is expected to arrive Sunday evening and
peak Monday. Another moderate northwest swell is possible next
Thursday.

Generally small to moderate surf is expected along south facing
shores through next week.

See the Collaborative Surf Discussion for more information.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Maui County Leeward Waters.

&&

$$
R Ballard/M Ballard/Foster

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

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