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Hawaiian Islands Synoptic Discussion and Guidance

738
FXHW60 PHFO 210150
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Wed Mar 20 2019

.SYNOPSIS...
Stable and dry conditions will persist across the islands tonight,
due to high pressure to the northeast. Trade winds will increase
Thursday and Friday as a cold front approaches from the northwest.
This front will be close enough to usher in increased clouds and
showers across the area Saturday and Sunday.

&&

.DISCUSSION...
Have made some tweaks to the PoPs/Sky/Wx grids for tonight and
tomorrow morning based on radar and satellite data that show
little in the way of clouds and showers upstream of the islands.

Forecast remains generally on track. A dry and stable airmass
continues over the islands with afternoon soundings showing an
inversion around 6000 feet near Lihue, and closer to 8000 feet near
Hilo. In the short term, pressure gradient should strengthen over
the next day or so, which will increase our trade winds. The
increasing pressure gradient is due in part to the cold front to
the northwest of the islands moving a bit closer to the islands.
The front will continue to move towards the islands through the
end of the week, and then dissipate and/or move to the east of the
islands.

While the models continue to agree that we will not see a frontal
passage with this system, there is agreement that the front will
help to bring increasing winds to the region ahead of the front.
And that moisture will increase as the front comes closest to the
islands Saturday and Sunday. With winds expected to remain out of
a trade wind direction, the higher rainfall totals are expected to
be focused over the windward and mauka areas. The winds are
expected to increase, so some of the showers will reach leeward
areas.

As we head into Monday, the ECMWF and GFS both agree with the
depiction of a high pressure system approaching from the
northwest, that will move north of the islands Monday
night/Tuesday. This will maintain the trade wind flow, and with
leftover front moisture in the region, expect a wet trade wind
pattern.

&&

.AVIATION...
The high pressure ridge north of the islands will weaken through
24 hours. Light trade wind flow will continue to steer low topped
stratocumulus towards windward slopes and coasts. Meanwhile, sea
breeze driven cumulus development over island interiors will
taper off after sunset. Partial clearing is to be expected along
leeward coasts after midnight.

A new cold front will approach the islands from the northwest on
Thursday. There will be an increased chance of rain on across the
smaller islands starting Thursday afternoon.

Isolated MVFR conditions are possible in passing showers,
otherwise VFR conditions will prevail through 24 hours and beyond.
No AIRMETs are currently in effect or anticipated.

&&

.MARINE...
A brief period with light to moderate trades will continue into
tonight as the ridge remains just north of the area. A return of
fresh to strong trades is expected by Thursday afternoon through
as high pressure builds north of the area through the weekend. A
combination of overlapping north to northwest swells and wind
waves rising from the strengthening trades will lead to Small
Craft Advisory (SCA) conditions over exposed waters late Thursday
through early next week.

The new long-period northwest swell will peak this evening into
tonight and will be reinforced by a mid-period northwest swell on
Thursday. Surf is expected to stay elevated at least through
Thursday night and possibly through Saturday as another
reinforcing north-northwest swell builds Friday night into
Saturday. A High Surf Advisory (HSA) has been posted for north
and west facing shores of Niihau, Kauai, Oahu, Molokai, and north
facing shores of Maui through early Friday morning. But with
another reinforcing swell expected late Friday, the HSA may need
to be extended through Saturday. Surf is expected to gradually
trend down Sunday into Monday with a small northerly swell
possible late Monday into Tuesday. Another low is expected to
develop off the coast of Japan on Saturday and may bring moderate
surf towards the middle of next week.

Surf along east facing shores will remain small through Thursday,
then rise Friday through the weekend as fresh to strong trades
return.

Surf along south facing shores will remain small each day with
mainly background southerly pulses moving through.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

&&

$$

DISCUSSION...M Ballard
AVIATION...ABEDAL
MARINE...KINO

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

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RH		6 %
Wind		W 3 mph
Road		Open (4x4)