Wildfire in Oregon, January 1 to 6, 2021

Area Forecast Discussion...Updated
National Weather Service Portland OR
827 PM PST Thu Dec 31 2020

Updated Aviation and Marine discussion

.SYNOPSIS...Rather mild over next several days, as region stays under
a mild southwest flow aloft. But, will be rather wet, as a series of
strong fronts push across the region. First arrives later Friday,
with a much stronger front later Saturday. Plenty of valley rain and
mountain snow, and breezy south winds, especially the front later
Saturday. Wet and mild weather continues well into next week.

&&

.SHORT TERM...Tonight through Sunday night...Rather benign pattern50
with weak ridge over the Pac NW. Weak warm front offshore will buckle
northward tonight, with some spotty light rain. Think this rain will
be along the far northwest corner of Oregon later, and mainly across
west/northwest Washington overnight. Still, will keep higher PoPs for
rain for areas north of Tillamook tonight. As the warm front lifts
farther north, so will the rain potential. Most areas will keep a low
PoP for tonight, with 10 to 20 pct.

Rather active pattern across the north Pacific, with next front
taking shape out near 140W. This front will push towards the Pac NW
on Fri, with rain spreading to the coast by midday, and inland during
the afternoon. Models fairly consistent on keeping the interior
mostly dry until early to mid afternoon. Will adjust forecasts to
reflect that trend. Will be mild, with temperatures over much of the
region into the lower 50s, perhaps even a tad higher for areas to the
south of Salem.

As the front shifts into the region later Friday, will see rain.
Fairly decent lift with the front, so would not be out of the
question to see a narrow band of moderate to locally heavy rain just
ahead of the front. Current timing would put the front on the coast
around 4 pm Fri, moving to the Cascades by 8 or 9 pm. Rain will
quickly transition to showers behind the front. With the unstable air
mass behind the front, some of the convective allowing models (CAMs)
are suggesting enough instability to support a minor thunderstorm50
potential. So, in coordination with Seattle forecast office, will put
a slight chance of thunderstorms along the coast and over the coastal
waters for the evening hours.

Snow levels still around 4000 feet, with lingering light snow showers
over the Cascades into this evening. But, little if any new
accumulation is expected. Snow levels will be on the rise tonight, up
to around 6000 ft by daybreak Fri, and holding around 6500 Fri into
early Fri evening. Snow levels do drop back to 4500 feet later Fri
night, but may bulk of moisture has passes on by. So, only few inches
of snow for the passes, with more over the few higher peaks.

Somewhat of a break for Saturday morning, but next system is fast
approaching. Looks like will see rain spread across the region again
by afternoon. System will have tapped rather moist sub-tropical plume
as it approaches. Integrated water transport (IVT) showing main
plume of moisture-rich air pushing over our area late Sat afternoon
into the evening. WIth in mind, will push QPF up a bit more, with 2
to 4 inches of rain along the coast into the coastal mtns, with
heavier amounts over the Willapa Hills and far northwest Oregon Coast
Range. Such rainfall will push rivers upwards, but at moment not
flooding expected. This likely due to the duration, as is much
shorter than that seen with last atmospheric river back on the
19th-20th of December. Will see gusty south winds increasing ahead of
the cold front, with gusts 45 to 55 mph along the coastal communities
away from the immediate coast. But, gusts 60 to 70 mph are likely for
the headlands and open-exposed areas. No high wind watch issued yet,
as would like to get a few more model runs in order for better
confidence.

Farther inland, still wet and breezy for later Sat and Sat evening.
But, would not expect many problems aside from areas of standing
water in areas with poor drainage.

Higher up in the Cascades, snow will be piling up again. Looks like
snow level will hover between 4000 and 5000 feet on Sat, then lower
again behind the front later Sat night. So, with anticipated snow
levels and QPF, looks reasonable to see 4 to 8 inches of new snow on
Saturday, with most of that from Mount Hood northward into
Washington. Heaviest snow will be Sat night, with  8 to 16 inches of
snow possible over the south Washington Cascades into North Oregon
Cascades, and 6 to 10 inches farther south over Lane County Cascades.
Again, heaviest snowfall will be above 5000 feet.

Showers decreasing late Sat night into Sunday morning. But, will
maintain higher PoPs with snow showers for the Cascades. The cold
front that pushes across the region Sat night will stall over far
southern Oregon by Sun morning, then slowly lift back north across
Oregon into south Washington as a warm front later Sun afternoon and
Sun night. Rain will increase from south to north later Sunday.

Long story short, expect plenty of rain and mountain snow this
weekend, with breezy periods.			      /Rockey

.LONG TERM...Sunday through Wednesday...The active weather pattern
is expected to continue into next week as a series of additional
systems are very likely to impact the Pacific Northwest. Extended
ensemble guidance shows a mean trough across the northwest with a
strong Pacific jet around 40N. An upper level low over the Gulf of
Alaska will eject several shortwaves through the long term period
approximately every 24-36 hours. Deterministic models are still
having difficulty resolving the low pressure system Sunday-into-
Monday, while another strong front may be possible Tuesday-
Wednesday. Confidence remains low in the timing, strength and
placement of these systems at this time. Overall freezing levels are
relatively low next week, oscillating between 3000 to 6000 feet as 50

these systems traverse the area. Generally expect wet and possibly
windy conditions, along with Cascade snow, to persist through at
least the middle of next week, if not later.

&&

.AVIATION...A warm front continues to push light rain into the
the area tonight and will result in ceilings lowering towards
MVFR. Precipitation has already started along the coast and is
expect to start for most inland areas between 06Z and 12Z Friday.
Expect gusty southerly winds along the coast starting around 08Z
Friday as the front moves closer to the coast with gusts up to
20 kt. Expect improving conditions towards low end VFR to high
end MVFR after 12Z Friday.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...VFR conditions will slowly deteriorate
towards MVFR between 06Z Friday and 12Z Friday as a warm front
brings light rain to the terminal. Expect improvement towards
low end VFR to high end MVFR after 12Z Friday. /42

&&

.MARINE...Winds have weakened across the waters as a transient
ridge moves across the region. Seas will continue to be in the
mid teens through the night as the northwesterly swell from the
passing low continues to move across the waters. Winds will
quickly ramp back up tonight as the next storm system approaches.

Current satellite shows a deepening 975mb low near 47N, 152W
moving northeast into the Gulf of Alaska. Southerly winds will
beginning to ramp up tonight as the warm front shifts inland.
Expect winds to reach 35-40 kt late tonight into Friday morning.
Winds will weaken behind the cold front which is expected to
cross the waters around 4 PM Friday. There may be a couple of
hours post- frontal that Gale gusts will occur but winds should
weaken fairly quickly by late Friday evening. Expect southerly
wind waves to reach 10-12 ft early Friday morning and into the
afternoon. This storm system will send a westerly swell across
the waters that will start out around 15 ft Friday and then build
into 20-22 ft by Saturday morning.

A short wave within the parent low will bring a potent cold
front into the region Saturday afternoon into Saturday evening.
This cold front appears likely to end up noticeably stronger.
Storm Force wind gusts of 50 kt appear increasingly likely for
late Saturday afternoon into Saturday evening across the waters.
This should drive seas into the 25-30 ft range at least briefly
Saturday evening. Seas will diminish slightly but continue to be
around 20 ft into early next week. This is because the Gulf of
Alaska low will work together with an even stronger a 920mb low
near the Aleutian Islands to keep seas high into early next week.
These two storm systems have setup a dynamic fetch environment
from the dateline to 140W.

There will be a brief window of weaker winds Sunday morning but
the next storm system will move in from the southwest bringing
another round of high end Gales to low end Storm Force winds.
Expect this active pattern of high winds and big seas to
continue into early next week.

There remains an elevated threat for sneaker waves through at
least Sunday. Current guidance suggests that this elevated
threat will continue through the start of next week as seas are
likely to remain over 20 feet. -BPhillips/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 10 PM PST this evening for coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 60
     NM.

     Gale Warning until 7 PM PST Friday for coastal waters from Cape
     Shoalwater WA to Cascade Head OR out 60 NM.

     Small Craft Advisory until 4 AM PST Friday for coastal waters
     from Cascade Head OR to Florence OR out 60 NM.

     Gale Warning from 4 AM to 7 PM PST Friday for coastal waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory until 7 PM PST Friday for Columbia River
     Bar.

&&

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