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WindSW 61 mph
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Hawaiian Islands Synoptic Discussion and Guidance

232
FXHW60 PHFO 172013
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1013 AM 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 Satuday, 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 1250 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 precipitable water below 0.9" on both soundings at 12z,
and a weak inversion near 6500-7500 feet. The flow was weak enough
to clear out the Big Island last night, as well as most interior
and leeward sections. Updated the forecast earlier for more
sunshine and lower PoPs on the Big Island today. Expecting a
pretty nice day, with just some windward clouds and a couple of
mainly light showers over windward sections of mainly the smaller
islands.

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. Land breezes are
expected to dominate elsewhere 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. 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.

&&

.AVIATION...
No AIRMETs are in effect, and none are expected today. VFR
conditions are expected to prevail, except for brief MVFR
conditions in clouds and showers that would primarily impact the
north through east sections of the smaller islands. Clear skies
over the Big Island will likely give way to some clouds later
today.

&&

.MARINE...
Guidance supports generally light to moderate northerly flow
holding through Saturday, then northwesterly on Sunday and Monday
as low pressure associated with this frontal boundary stalls
several hundred nautical miles north of the state. Winds should
remain below advisory levels through the weekend, but the
rather unusual wind directions will result in different areas of
terrain acceleration and deceleration, such as through the
Kaulakahi Channel on Saturday and Saturday night. Trades return
Tuesday and beyond.

Incoming northerly swells will likely bring the seas up to and
above advisory levels beginning Sunday, then potentially
remaining at or above advisory levels through much of the upcoming
week.

Very cold mid-level temperatures (sub-zero C at 700 mb) over the
northern offshore waters Saturday night and Sunday, associated
with the low several hundred miles to the northeast of Hawaii,
could result in a few thundershowers in that area. At this point,
these are not expected to sneak into the northern coastal waters,
but will keep an eye on model trends.

Surf along north facing shores will remain up through much of the
period due to overlapping northerly swells moving through the
local waters. The anomalous pattern across the northern Pacific
featuring storms diving southward across the eastern Pacific from
the Aleutians to near the islands due to a blocking ridge
established over the the central Pacific is projected to continue
through next week.

The latest analysis showed a strong pressure gradient between 1044
mb high pressure center near the Aleutians around 45N and 1008 mb
low centered several hundred nautical miles to its east. Recent
ASCAT passes overnight and yesterday showed a decent sized area
of gale-force northerly winds between these two synoptic features
around 1300 nm north of the islands. Altimeter passes within this
area showed seas up to 17-20 ft. Guidance remains in decent
agreement and shows the northerly swell associated with this
source filling in late Saturday night, peaking Sunday, then
holding through Monday before slowly easing Tuesday. Surf along
north facing shores will near if not exceed warning levels around
the peak Sunday, then remain above advisory levels Monday.

As the swell is peaking along north facing shores Sunday, the
progressive pattern across the northeast Pacific is projected to
continue as another gale- to storm-force low emerges over the Gulf
of Alaska near Kodiak Island and tracks southeastward toward the
Pacific NW states Monday through Tuesday. A reinforcing northerly
swell associated with this source is expected to fill in by midweek
and will likely translate to surf nearing the warning mark along
north facing shores once again. This source will hold through the
second half of the week, potentially becoming reinforced with a
similar source Thursday night into the following weekend.

Surf along south and west facing shores will remain small with
mainly long-period south-southwest swells expected. A slight
increase in surf will be possible over the weekend from 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...
None.

&&

$$
R Ballard/M Ballard/Brenchley

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