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Hawaiian Islands Synoptic Discussion and Guidance

624
FXHW60 PHFO 122016
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1016 AM HST Wed Dec 12 2018

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

&&

.DISCUSSION...
A col, or weakness in the subtropical ridge, about 700 miles NNE
of Honolulu has reduced the local pressure gradient, allowing
winds to back off a bit today. The 12z soundings showed stronger
inversions today, suggesting more subsidence. Rainfall amounts
and shower coverage have indeed been trending down over the past
12 to 24 hours. There are still some areas of moisture convergence
upstream riding in on the trades. One area of showery cumulus
extends northward from Maui County, and another one is farther
upstream to the E of the islands. These will bring more typical
tradewind showers today through Thu, not as active as we've seen
in recent days.

A new 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.

The overall trend should be toward fewer trade wind showers over
the next few days, as mid-level temperatures remain warm and low
level moisture decreases, especially after Thu. Fine weather with
light rainfall amounts is expected by Fri, and continue at least
through 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...
Strong high pressure northeast of the islands is driving strong
trade winds across the marine area early this morning. The trades
are expected to ease slightly today as a cold front passes by
well to the north of the state. Strong trades will then 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 trade
winds.

A Small Craft Advisory is in effect for all Hawaiian coastal
waters through Friday due to a combination of strong winds and
rough seas. Gales are possible Thursday and Thursday night for the
Alenuihaha and Pailolo Channels as well as Maalaea Bay, although
the latest model runs have trended slightly weaker with the next
high.

The current northwest swell will peak this afternoon into 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 Hawaii will maintain a
tight pressure gradient across the region today resulting in a
breezy trade wind pattern. AIRMET Tango remains in effect through
this afternoon 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 today.
This activity may bring brief MVFR conditions to these areas 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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