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fxhw60 phfo 181941 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
941 am HST Sat Nov 18 2017

Synopsis...
a cool northerly flow will increase as a weak front moves over
Kauai this afternoon, Oahu early this evening, then across the
rest of the island chain tonight and early Sunday. Showers along
the weak front will be focused mainly across northern facing
slopes. The cool northerly flow will persist through Tuesday as a
stalled cutoff low sits far northeast of the state. Trade winds
will gradually return on Wednesday and will likely become breezy
and gusty on Thanksgiving as the low is pushed away from the
region by strong high pressure.

&&

Discussion...
a rather cool northerly flow has already developed ahead of a
weak front that is nearing Kauai. A ragged band of low clouds and
showers marking the front is just north of Kauai at this time and
is advancing toward the southeast at around 15 mph. Showers will
increase during the next couple of hours along north facing areas
of Kauai and over the mountainous interior, then move onto Oahu
late in the afternoon and early evening. Ahead of the front, a few
showers can be expected over north facing slopes of Maui this
morning, with only isolated leeward showers forming over The Big
Island and Maui during the afternoon. Overnight low temps dropped
into the mid 60s at many locations near sea level, and Max temps
will be a couple of degrees below normal today as the northerly
flow holds.

A cooler northerly flow will settle over the islands tonight and
Sunday as the front moves through. The front should be clear of
Kauai by Sundown, and as the weak feature passes over Oahu then
Maui County overnight, showers will be focused across north
facing terrain, though a few showers will likely spread leeward.
The cloud band will be dissipating as it reaches The Big Island
early Sunday morning. A cool airmass will move in behind the
front, with dew points dropping into the upper 50s. As a result,
low and high temps will be running around 5 degrees below normal,
causing daytime highs to struggle to top 80. On Sunday, the GFS
and European model (ecmwf) show precipitable water plummeting to around three
quarters of an inch, which is about 60 percent of normal. Thus,
expect minimal showers along north facing slopes.

In addition, strong winds will continue across the high summits of
The Big Island through the weekend. Winds are currently hovering
around the warning threshold of 55 mph and should increase today
and tonight before trending down late Sunday.

Minor changes are expect heading into the work week. A cool
northerly flow will persist as the front's parent low, currently
sitting about 700 miles to the north-northeast of the state is
replaced by another surface low dropping into the established
longwave trough over the eastern North Pacific. We will likely see
a couple of weak and shallow moisture bands dropping over the
state and delivering light rainfall to north facing slopes, while
leeward areas remain rather dry. Temps will remain around 3 to 5
degrees below normal near sea level, meaning overnight lows in
the low to mid 60s and daytime highs around 80.

Winds will gradually shift toward a trade wind direction on
Wednesday then build on Thanksgiving. The above mentioned surface
low to the northeast of the state will lift away from the region
on Wednesday, allowing an area of high pressure move north of the
state. This will result in building northeasterly trade winds. The
high will settle closer to the islands on Thanksgiving, likely
leading to breezy and gusty trades.

&&

Aviation...
a weakening frontal boundary just north of Kauai will continue to
move south over the islands, reaching Maui tonight. The cloud
band associated with the weak front may introduce periods of MVFR
ceiling/visible as it passes, therefore the potential for airmet Sierra
for mountain obscuration can not be completely ruled out for Kauai
and Oahu later today.

North winds and a drier airmass will fill in behind the
boundary. Expect light to moderate north winds to persist through
the weekend, with land breezes forming at most terminals
overnight. Any clouds and showers will then favor northern slopes,
but minimal ceiling/visible reductions should be expected.

A jet streak and a bit of moisture above The Big Island summits
this morning prompted the issuance of airmet Tango for tempo
moderate upper level turbulence between fl200 and fl300. This will
be revisited throughout the day.

&&

Marine...
a low pressure system and its associated cold front north of the
islands will be the primary weather feature impacting the islands
over the next couple of days. Northerly wind with the system will
remain below Small Craft Advisory (sca) levels, but will result
in different areas of terrain acceleration, such as through the
Kaulakahi Channel today and tonight and the hamakua coast and near
South Point on Sunday. Moderate trades are expected to return
Tuesday and Wednesday, and may approach Small Craft Advisory levels at that time in
the windier locations.

The fetch from the low will bring rising north swell to the North
Shores beginning tonight, peaking Sunday into Monday, and then
trend down Tuesday and Wednesday. Near advisory level surf is
expected, with warning level surf likely for Kauai. Anticipate
posting a high surf warning for Kauai and high surf advisory for
remaining islands exposed to the swell later tonight for the event
starting Sunday. This swell will bring seas in excess of 10 ft to
exposed waters, so expect an Small Craft Advisory to be posted as well.

Another low in the Gulf of Alaska will provide the source for the
next northerly swell which is expected to reach the islands as
early as Wednesday night with similar heights as the swell
expected tomorrow. This swell could bring near warning level surf
for Thanksgiving before easing into the weekend.

Surf along the east facing shores will continue to trend down as
the trades are replaced by the northerly winds and swell.
Meanwhile south facing shores will continue at small levels
through the weekend in response to recent activity across the
southern Pacific.

See the latest collaborative nearshore and swell forecast for Oahu
for more details on the surf at:
www.Prh.NOAA.Gov/hnl/pages/srf.Php

&&

Hfo watches/warnings/advisories...
High Wind Warning until 6 PM HST Sunday for Big Island summits.

&&

$$

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