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Hawaiian Islands Synoptic Discussion and Guidance

294
FXHW60 PHFO 240632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sat Mar 23 2019

.SYNOPSIS...
Moderate to locally breezy trade winds will continue through Monday
as a surface high pressure stays north of the islands. Modest
windward rainfall will prevail through the rest of the weekend,
followed by an increase in precipitation on Monday. Trade winds
steadily will drop on Tuesday and Wednesday, as a front from the
northwest will affect portions of the state on Thursday.

&&

.DISCUSSION...
Locally breezy trade wind weather continues across the state this
evening, as a high pressure stays far north of the area, providing
the winds. Moisture carried by the winds will reach the islands at
times, bringing showers to mainly windward areas. There are also
high clouds streaming over the state, keeping skies a little more
cloudy.

Little change in the winds is expected through the rest of the
weekend into Monday as the surface high pretty much remains far
north of the islands. Low clouds embedded in the trade winds will
reach the area at times, with showers focusing along windward areas.
The atmosphere will remain somewhat stable and with the locally
breezy trade winds prevailing, expect rainfall totals to be limited.

An increase in shower activity is expected Monday as an upper level
trough is forecast to approach the islands from the west.
Furthermore, an area of moisture currently north through northeast
of Kauai will sag southwest, spreading across the area. Rainfall
will still be focused over the windward slopes, though some showers
may reach the lee areas of the smaller islands.

Trade winds will steadily decline on Tuesday and will be disrupted
on Wednesday as a front approaches the area from the northwest. As
the front advances closer to the area , it will push a surface ridge
to the island vicinity, resulting in winds weakening significantly
on Wednesday. Expect a land and sea breeze weather regime to take
over, with afternoon clouds and showers developing over interior
sections. Although not much increase in low level moisture is
expected, an approaching mid to upper level trough will create some
instability. Hence, briefly heavy showers are possible on Wednesday,
specially during the afternoon and evening hours.

Looks like this pending front will reach the state on Thursday, with
shower bands associated with the front affecting the northern
islands. Forecast models still differ upon how far south this front
will advance, with the old ECMWF having the frontal band stalling
over the northern islands while the latest GFS has it reaching the
Big Island late Thursday. Regardless, do expect a return of the
north to northeasterly winds to much of the islands by late
Thursday, while Kauai and Oahu will see much of the wet weather
associated with this system. Both models do hint lingering moisture
from the dissipated front will continue to affect the area by the
next weekend.

&&

.AVIATION...
A high pressure center far north of the Hawaiian Islands will keep
moderate trade winds in the forecast for the next few days. Showers
moving along with the trade winds will bring periods of showers with
some MVFR cloud ceilings over the windward mountain slopes of each
island.

Low cloud ceilings associated with a remnant frontal band are
drifting across the island of Kauai this evening and AIRMET Sierra
remains in effect for tempo mountain obscuration through 10Z
tonight. Low cloud ceilings over Kauai will decrease after 10Z with
the AIRMET Sierra likely dropping before sunrise on Sunday morning.

&&

.MARINE...
High pressure north of the state will keep a moderate to strong
trade wind flow in place through Monday night. The trades will ease
Tuesday and Tuesday night as a front approaches from the northwest,
with winds becoming light and variable by Wednesday. There are some
differences with respect to the progressiveness of the front moving
through the island chain Wednesday night and Thursday and this will
have an impact on wind speeds and directions. The models then come
back in line by late in the week and into next weekend, showing
moderate trade winds redeveloping across the marine area.

A Small Craft Advisory (SCA) remains in effect for most waters
tonight due to winds, elevated seas or a combination of the two. The
current north-northwest swell should drop sufficiently by late
tonight that the SCA can be cancelled in most areas. Strong winds
will continue across the typically windy waters around Maui and the
Big Island Sunday through Monday, and the SCA will likely need to be
extended for these areas later tonight.

The current north-northwest swell will continue to decline through
the rest of the weekend. A small reinforcement out of a more
northerly direction is expected Monday night through Wednesday.
Overlapping long-period northwest swells will move through during
the mid to late week time frame, with surf possibly approaching low
end advisory levels Thursday through Friday along north and west
facing shores. Otherwise, surf should remain below advisory
thresholds through the next week.

Surf along east facing shores will rise tonight through Monday due
to the building trades. A downward trend is expected Monday night
through Wednesday as the trades weaken.

Surf along south facing shores will remain small most days with
mainly background long period southerly pulses and short-period
southeast energy expected. There could be a bit of a bump Wednesday
through Friday as a small long period south swell arrives from the
southern hemisphere.

&&

.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 until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Big Island Windward Waters.

&&

$$

DISCUSSION...Hui
AVIATION...Bohlin
MARINE...Jelsema

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



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