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Hawaiian Islands Synoptic Discussion and Guidance

166
FXHW60 PHFO 130113
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
313 PM HST Wed Dec 12 2018

.SYNOPSIS...
The trades will increase once again on Thursday as the next in a
series of surface highs passes north of the islands. The trades
will ease slowly late in the week and into the weekend. A surface
trough developing near the islands early next week will give us
light winds by Monday. Starting Thursday, the trades will trend
less showery, with fine weather expected for most areas over the
weekend.

&&

.DISCUSSION...
A subtropical ridge is maintaining breezy to locally windy trade
winds over the main Hawaiian Islands. This ridge connects a high
far to our NE, and another high approaching from the NW. The
afternoon soundings show the inversions continue to gradually
strengthen and lower, as they've done over the past 36 hours.
There are still a couple of areas of clouds and showers riding in
on the trades. One recently brushed Maui county and is now
affecting Oahu, while another is approaching the windward side of
the Big Island. However, with a lowering and strengthening
inversion, these shower areas are not quite as active as we've
seen over the past couple of days. Note that the radar echoes near
Kauai appear to be non- meteorological chaff.

The next surface high will zip by well N of the islands on Thu.
The trades will respond by ramping back up to windy levels. Models
have been trending weaker with this high over time, thus Thu
doesn't look quite as windy as it did earlier. After the high goes
by, a very large, strong low pressure area over the far N Pacific
will shunt the subtropical ridge southward toward us. Our local
winds will respond by decreasing a bit, back to breezy-to-locally
windy levels Fri-Sun. On Mon, winds will weaken even more as the
subtropical ridge to the N weakens and a weak surface trough
develops over the islands. Local island circulations will become
more dominant. At least for now, the models are in remarkably good
agreement with the timing of the next cold front, expected to
reach Kauai and Oahu on Tuesday, which will likely be accompanied
by blustery, cool N or NNE winds.

Trade wind showers tonight will initially favor Oahu and spread to
Kauai. Later in the evening, the windward side of the Big Island
should see an increase in showers as well. These areas should be
pushing W of the islands on Thu, with the models advertising a
drying trend to start afterward. Fine weather is expected by
Friday with light rainfall amounts, and this should continue
through at least the upcoming weekend. A band or two of showers
is expected with the frontal passage on Kauai and Oahu on Tuesday.
For now, the models do not show a lot of upper support or
moisture with this front, but will keep an eye on model trends for
any changes.

&&

.MARINE...
A cold front passing by well to the north of the state has
weakened trade winds slightly today. Strong trades will return
Thursday and Thursday night as a new strong high builds north of
the area. The high will weaken as it settles slowly southward
Friday through early next week, gradually weakening the trades. A
Small Craft Advisory (SCA) remains in effect for all Hawaiian
coastal waters through Friday due to a combination of strong winds
and rough seas. Gales are possible for a short time Thursday for
the Alenuihaha Channel.

The current northwest swell is slightly late in its arrival, but
is slowly coming up at the nearshore buoys and is expected to
peak tonight just below the High Surf Advisory (HSA) level. A
smaller, northwest swell is expected Thursday night through
Saturday. A new, large, northwest swell will build late Saturday
and peak late Saturday night and Sunday, likely producing warning
level surf along north and west facing shores. Another, even
larger, northwest swell is expected to build late Monday or Monday
night, bringing another round of warning level surf to north and
west facing shores Tuesday through the middle of next week.

An HSA is in effect through Friday for east facing shores of
Kauai, Oahu, Molokai, Maui, and the Big Island due to short-
period choppy surf produced by the strong trades. Surf should
lower below advisory levels over the weekend as the trades
diminish.

&&

.AVIATION...
Elongated surface high pressure north of the Hawaiian islands
will maintain a tight pressure gradient across the region tonight
resulting in a breezy trade wind pattern. AIRMET Tango remains in
effect through tonight for all areas below 9000 feet MSL south and
west of the mountain ranges due to expected TEMPO moderate
turbulence.

Moisture caught up in the trade wind flow will bring scattered
showers to mainly the windward slopes and mountain areas tonight.
This activity may bring brief MVFR conditions due to lowered vis
and mountain obscuration.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Friday for east facing shores.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters-

$$
R Ballard/TS/JT

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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