Current Conditions
Temp0.3 C
RH10 %
WindNE 41 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

151
FXHW60 PHFO 120214
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
414 PM HST Tue Dec 11 2018

.SYNOPSIS...
Windy trades will continue through midweek as a series of highs
pass to the north of the islands. The trades will slowly ease
late in the week into the weekend. A surface trough developing
near the islands early next week may give us light winds by
Monday. In the meantime, the trades will be wet for some spots, as
areas of moisture moving over the islands enhance rainfall,
especially for Oahu and Maui. Trade winds are expected to
gradually trend drier for the latter half of the week.

&&

.DISCUSSION...
An earlier ASCAT pass showed that the background winds around the
islands have come down a bit, but still remain locally windy.
These trades are due to a strong subtropical ridge extending
roughly W-to-E about 700 miles N of Honolulu. Although the trades
may ease a little further overnight as the ridge weakens slightly,
we could still see some localized spots reaching Wind Advisory
criteria. Big Island summit winds are still solidly at Wind
Advisory levels as well, due to a tightening pressure gradient
between a large mid-level anticyclone to the NW of the islands,
and a mid-level low far to the E of the Big Island. Models
indicate the summit winds should start to come down by daybreak
Wednesday.

Another in a series of strong migratory surface highs will pass
well N of the islands Thu into Thu night, with another peak in the
blustery trades. There's some chance we will need another Wind
Advisory for some areas for that time period, though this high
looks slightly weaker and a little farther away than the latest
high. After that, the winds back off a bit, back to breezy-to-
locally windy levels as the ridge weakens and yet another weaker
high passes N of the islands about Saturday. Models show the winds
weakening even more through the remainder of the weekend and into
early next week as the subtropical ridge weakens and pushes S
closer to the islands. By Monday, there's decent agreement in the
models that a weak surface trough will develop near the islands,
although the precise location of this feature is in question. If
this holds, we'll see more areas with gentle land- and sea-
breezes setting up early next week in a locally breezy ESE
background flow across the windward waters. Models are in good
agreement that the next cold front starts to come down the chain
on Tuesday with lower dewpoints and brisk N or NNE winds.

Moisture is rather limited near the islands, with CIMSS MIMIC-TPW
showing values near and upstream at or below 1.2 inches.
Nevertheless, areas of moisture convergence interacting with our
local terrain, along with a relatively weak subsidence inversion,
have been wringing out impressive rainfall in some localized
spots. Open-celled showery cumulus should continue to affect Kauai
and Oahu tonight, while other windward areas should be quite a bit
drier. A gradual trend toward somewhat drier trades statewide
appears likely over the next few days as the 700 mb temperatures
warm (suggesting a stronger subsidence inversion) and upstream
moisture decreases further (PW dropping below 1" for most areas by
Thu night). The front on Tuesday does not appear to be too wet at
this point, with very limited moisture to work with.

&&

.MARINE...
Strong high pressure is currently centered far north of the area.
This places the islands in a rather strong trade wind environment.
Winds are expected to drop off slightly on Wednesday, but
increase once again on Thursday as a new strong high builds in to
our north. Winds will gradually drop off Friday through Sunday as
the high to our north weakens.

A Gale Warning is currently in effect for the Alenuihaha and
Pailolo Channels as well as Maalaea Bay through tonight. A Small
Craft Advisory (SCA) in effect for the rest of the Hawaiian
Coastal Waters due to a combination of strong winds and elevated
seas. Another round of Gales is possible Thursday and Thursday
night for the Alenuihaha and Pailolo Channels as well as Maalaea
Bay as the new high builds in.

The current northwest swell will continue to lower tonight but a
new moderate size northwest swell will gradually fill in overnight.
This swell is expected to peak on Wednesday with surf heights
expected to be just below High Surf Advisory (HSA) levels. This
swell will lower gradually Thursday and Thursday night. Another
smaller northwest swell is expected during the Thursday night
through Saturday time period. Another moderate size northwest
swell is expected to arrive late Saturday and Saturday night, peak
on Sunday, then lower gradually into early next week. Advisory
level surf is expected with this swell. An even larger northwest
swell is expected Monday night and Tuesday with surf heights
possibly reaching warning levels.

A HSA is posted along east facing shores due to short period
choppy surf produced by the strong trades. These conditions are
expected to continue through the remainder of the week.

&&

.AVIATION...
A strong surface high north northeast of Hawaii will maintain a
tight pressure gradient across the region late this afternoon and
into tonight, fueling a breezy to windy trade wind pattern.
AIRMET Tango remains in effect for all areas below 9000 feet
south and west of the mountain ranges due to expected TEMPO
moderate turbulence. For Lanai, the Alenuihaha Channel, and water
south of the Big Island, surface winds greater than 30 KT are
forecast through this afternoon and will likely continue tonight.

Moisture caught up in the trade wind flow will bring scattered
showers to mainly the windward slopes and mountain areas over the
next 24 hours. Expect brief MVFR conditions associated with the
passing showers, but mainly for airports on the windward coasts.

&&

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

Wind Advisory until 6 AM HST Wednesday for Lanai-Kahoolawe-Maui
Central Valley-South Big Island-Big Island North and East-Kohala-
Big Island Interior-Big Island Summits.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

Gale Warning until 6 AM HST Wednesday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel.

&&

$$
R Ballard/Burke/JT

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