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Hawaiian Islands Synoptic Discussion and Guidance

063
FXHW60 PHFO 200706
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
906 PM HST Tue Sep 19 2017

.SYNOPSIS...
A moderate to locally strong trade wind flow will continue for the
rest of the work week as high pressure remains to the NE of the
islands. The trades will become a tad lighter over the weekend
as the high drifts further away from the islands, and the
associated surface ridge approaches the islands from the NW.
Showers will continue to favor windward and mauka areas, with a
stray shower reaching leeward areas from time to time. Showers
will be most prevalent during the night and morning hours.

&&

.DISCUSSION...
A 1031 mb surface high centered 1330 miles NNE of the islands is
generating a mainly moderate trade wind flow across the main
Hawaiian Islands this evening. Infrared satellite imagery shows
a patch of clouds clearing Kauai, while a much larger patch is
gradually spreading over Maui and approaching Molokai. The
southern fringe of this cloud mass is brushing up against windward
side of the Big Island. There is a small break, followed by a
smaller cloud patch that will likely affect the Puna and Hilo
districts mainly after midnight. Oahu is sitting under an area of
fair skies now, but should this patch of clouds over Maui County
continues to move west with the trades, it will begin affecting
the island a few hours after midnight. The forecast was updated
to reflect this change with higher POPS for Maui County now,
spreading to Oahu after midnight. The Wednesday's morning commute
on Oahu in particular, could be showery should this cloud mass
holds together. Although some erosion to this cloud mass is
expected before reaching Kauai after sunrise Wednesday, it'll
still provide some showery weather for the Garden Isle interior
and windward sections.

Otherwise, it is trades for the foreseeable future. The strength of
the trades, currently moderate to locally strong, will drop
off to light to moderate on Friday, and hold at this strength through
the upcoming weekend. As noted above, the surface high producing
the trades, is expected to ease east, further away from the
islands. The high's associated ridge that extend westward from the
high, is expect to move south to a point 625 miles NNW of Kauai
Friday evening. The ridge will nudge another hundred miles closer
to Kauai Sunday morning, close enough to maintain a gentler trade
wind flow across the island chain.

At the upper levels, we have a low 930 miles ENE of the Big
Island. The low is slated to drift W to a point 540 miles NE of
the Big Island by Thursday afternoon. The marine layer or inversion
slopes from 7 to 8k feet to 14k over the Big Island this evening.
The GFS solution shows no big changes through Wednesday afternoon.
On Thursday afternoon, the inversion lowers over Kauai and Oahu
to 5k feet, but remains the same across Maui and the Big Island at
10k feet over Maui and to 14 or 15k feet over the Big Island. In
essence, no real drastic change from what was experienced today.
So, in brief, the trade wind regime continues, although the
forecast may need adjustments due to large areas of clouds and
embedded showers like to one affecting part of the island chain
this evening.

&&

.AVIATION...
High pressure north-northeast of the state will keep a moderate to
breezy trade wind flow in place through Wednesday. Some MVFR
cigs/vsbys will be possible in windward and mauka areas overnight
through mid morning Wednesday, with PHTO, PHMK and PHLI the most
likely TAF sites to see conditions drop below VFR levels.
Otherwise, prevailing VFR conditions are expected through 06Z
Thursday.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. The AIRMET will likely
remain in effect through Wednesday.

AIRMET Sierra may be needed for mountain obscuration across
windward sections of some islands later tonight.

&&

.MARINE...
Small craft advisory conditions are forecast to continue through
Thursday due to strong trade winds across the Pailolo and
Alenuihaha Channels, Maalaea Bay and the waters south of the Big
Island. Conditions are near advisory levels across the Kaiwi
Channel and Oahu leeward waters and may need to be added in later
packages. Trades are expected to drop below small craft criteria
Thursday night into Friday as high pressure north-northeast of
the state shifts eastward and weakens. Light to moderate trades
are expected over the upcoming weekend, with the exception being
across the typically windier channel waters where fresh breezes
will remain possible.

A combination of moderate to fresh breezes locally and upstream of
the islands south of high pressure will continue to generate rough
surf along east facing shores into Friday. Surf along east facing
shores will trend down over the upcoming weekend as the trades
weaken.

Surf along south facing shores will remain small through the
weekend with mainly background southeast and south-southwest
swell energy expected. For the extended, strong- to near gale-
force southwest winds associated with a low that is currently
tracking across the Tasman Sea may lead to another small south-
southwest (210-220 deg) swell early next week.

A small northwest (310 deg) swell associated with broad low pressure
centered over the central Pacific just east of the Date Line is
expected to fill in locally Thursday and hold into the weekend
before easing. A recent ASCAT pass showed a decent sized batch of 25
to 35 kt winds focused toward the islands within the 310 deg band.
Peak seas from an altimeter pass within this batch of northwest
winds ranged from 15 to 18 ft, which lines up well with the latest
wave model guidance. Small surf will be expected from this source
Thursday through Saturday, before dropping Sunday along north and
west facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay,
the Pailolo and Alenuihaha Channels, and waters south of the
Big Island.

&&

$$

DISCUSSION...Lau
AVIATION...Jelsema
MARINE...Gibbs

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

Current Conditions
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RH		16 %
Wind		SSW 21 mph
Road		Open (4x4)