Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 180143
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
343 PM HST Fri Nov 17 2017

.SYNOPSIS...
Low pressure far to the north of the islands will drop south over
the next couple of days, weakening our trades. A weak cold front
will come through the islands later Saturday, accompanied by a
band of clouds and showers. After the front comes through, cool,
breezy northwesterlies are expected Saturday night through Monday.
The low to our north will move away from the islands Tuesday and
Wednesday, and trades will gradually return. A large, strong high
pressure area to our north is expected to bring windy trades for
Thanksgiving.

&&

.DISCUSSION...
A vertically stacked low about 1100 mi N of Honolulu is dropping
S at about 20 kt. The low is already weakening the local pressure
gradient over the islands, with trades down quite a bit from
yesterday`s levels. The local airmass is relatively stable and
dry with below normal precipitable water and a weak inversion
near 8500 feet.

As the low to the N drops S, the flow over the islands will
continue to weaken. The dewpoints are low enough so that even with
the lighter winds, it should not be too uncomfortable for most
folks. By tonight the low will be close enough to turn the winds
over the northern main islands to northerly. A few showers are
possible mainly for Kauai and Oahu`s mauka sections and north
shores, but land breezes are expected to dominate most everywhere
else with mainly clear skies.

The models are showing a weak cold front associated with the low
dropping S over Kauai and Oahu Saturday afternoon. This front will
have limited low level moisture to work with, but accompanied by some
mid-level support with a shot of colder air aloft. This will increase
clouds and showers somewhat there, while the other islands continue
to have lots of sunshine with only limited cloudiness. The front
will herald a modest increase in wind speeds along with lower
dewpoints, making it feel a bit chilly by Hawaii standards.

On Sunday, with the low about 600-700 miles to our NE, we should
have cool northwesterly breezes blowing statewide with sea level
dewpoints down into the 50s in many locations. Very cold air in
the mid-levels (especially over the northern main islands) should
provide enough instability to maintain scattered showers, even
with relatively scarce moisture by local standards. The ECMWF has
not really trended any colder since yesterday, but the GFS has
trended closer to the ECMWF solution, so at this point expecting
any thundery showers to remain N of the coastal waters.

On Monday the low will start to pull away off to the NE, and
strong high pressure far to the NW will begin nudging southward.
Our low level flow will remain northwesterly, but will veer to
northerly Monday night into Tuesday, and locally breezy trades
returning to most locales Tuesday night. The high will press south
a little more as we head toward Thanksgiving, with windy trades
expected for the holiday and Friday. Moisture will remain limited
during this period, so aside from a few minor bands of showers
riding in on the low level flow, the weather should fairly dry and
nice.

Winds at the summits of Mauna Loa and Mauna Kea were expected to
come up, but they increased a lot earlier and more than forecasted.
Thus, we have hoisted a High Wind Warning. The model guidance is
unanimous in showing the mid-level height gradient tightening
over the summits through Sunday as the low comes closer, so it
seems likely the strong winds up there will continue and possibly
even increase this weekend. There could be some difficult-to-
forecast diurnal fluctations in the wind, but overall warning
level winds seem likely through at least Sunday.

&&

.AVIATION...
No AIRMETs are in effect, and none are expected overnight. VFR
conditions are expected to prevail, except for brief MVFR
conditions in clouds and showers, primarily over the western
islands.

&&

.MARINE...
Light to moderate northerly flow is expected Saturday, then
becoming north northwesterly and increasing somewhat into Sunday
into Monday. Winds should remain below advisory levels through
the weekend, but the rather unusual wind directions will result in
different areas of terrain acceleration, such as through the
Kaulakahi Channel on Saturday and Saturday night and the Hamakua
Coast on Sunday. Moderate trades return Tuesday and Wednesday
next week.

Guidance is in good agreement and shows the northerly swell
filling in late Saturday night through early Sunday, then peaking
late Sunday into early Monday, before slowly easing Tuesday. Surf
along north facing shores will surpass high surf advisory levels
and near high surf warning levels around the peak late Sunday,
then remain above advisory levels Monday.

A reinforcing northerly swell is expected to fill in by midweek
and will likely translate to surf nearing the warning mark along
north facing shores once again. Elevated north swell will continue into
the following weekend.

Surf along south and west facing shores will remain small with
mainly long-period south-southwest swells expected.

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.

&&

$$
R Ballard/M Ballard/Brenchley



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