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Area forecast discussion
National Weather Service Portland Oregon
357 am PST Wed Dec 8 2016

Synopsis...a significant winter storm is headed for northwest Oregon and SW
Washington Thursday, which will likely have substantial impacts even
for the lowest elevations. Precipitation will spread north across the
area Thursday morning, starting as a period of snow for most but
quickly changing over to freezing rain or rain for the coast and
areas south of Salem. Snow is expected last a few more hours in the
Portland Metro area before changing to freezing rain Thursday
afternoon and eventually rain Thursday night. The Columbia Gorge and
Cascades will see primarily snow, as cold air remains entrenched east
of the Cascades. Temperatures should rise above freezing elsewhere
from south to north as relatively milder Pacific air moves in, first
along the coast and near Eugene Thursday midday or afternoon...then
eventually Portland and inland SW Washington Thursday night. East
winds will remain strong in and near The Gorge through Thursday
before slowly easing Thursday night and Friday. Beyond this system,
cool and showery weather will continue into early next week.

&&

Short term...today through Friday...not much change from the
evening shift with the next several hours going to be a combination
of a little blind Faith in the models, some assumptions, and some
facts.

First the facts: temperatures along the coast and in inland pockets,
are holding a couple degrees above freezing. These areas are under
the influence of gusty east winds. The elongated and sharp trough
axis is combining with a strong cold pool and associated high
pressure east of the Cascades to bring the winds. These same winds,
however, are bringing significantly dry air west of the Cascades.
Those dew points in the upper teens to 20s will easily result in
temperatures below freezing should moderate rates of precipitation
develop. Next, radar returns are beginning to advance northward over
the last couple hours with Roseburg and Medford having recently
measured. Other areas to the south have somewhat steadily begun to
show rain falling with several hundredths measured in and south of
the Siskiyous over the last few hours.

Next, the blind faith: raw weather models, in general, appear to
have a fairly good handle on this event so far. Multiple models
continue to show periods where moderate precipitation will occur,
however, with widely varying points of focus. Models may have been a
bit too aggressive with the quantitative precipitation forecast forecasts, but really not by much.
The surface reflection of the front appears to have a couple weak
circulation centers and a couple occlusions associated with the
frontal boundary. Feel this may have slightly weakened the overall
structure of the system. However, since they all show the idea of
frontogenesis developing somewhere from Albany northward, will not
drop warnings just yet as there will be a fine line between the
haves and have not as muchs. Have not seen much satellite evidence
of frontogenesis either, but that may need to develop more as the
frontal boundary starts to interact with the stronger offshore flow
cutting through the various central and north Oregon Cascade passes.
Timing does appear to be a little slower than from the afternoon
package and will go with the following timing of notable precip
onset. Looks like the main band would reach a Florence-Eugene line
around 7 am, a Lincoln City-Salem line near 10 am, Portland around
noon, and an Astoria-Kelso/Longview line near 1 PM. Have not seen
much evidence of a large scale warm nose yet per surface
observations or special 06z soundings. Not terribly surprised though
as the updated timing wouldn't bring it in until much later today.
Would think it should appear on an upcoming 18z sounding maybe at
Salem but more likely from Medford. So the blind Faith comes in with
believing there will be precip enhancement and there will be the
warm nose advancing northward to melt the falling snow before it
falls back into the shallower trapped cold air near the surface.

Finally the assumptions: temperatures are plenty cool enough and
paired with dry enough dew points to result in sub-freezing values
from wet-bulb evaporative effects. This will take moderate precip
rates to cut drag the temperatures down...similar to what we saw for
a short periods on Monday where snow did reach the low elevations.
As such, am also continuing to have the Faith the models are
accurate with the frontogenesis.

All said, confidence in the current warning amounts and timing is
perhaps a little lower than a few hours ago. Will be making some
minor adjustments to the statements. Main concern is for the
impacts we've advertised for the coastal zones. Like the idea of
keeping the advisory for the central coast, but will be adjusting
the emphasis on the north Oregon/South Washington coasts statement
to focus on the ice impacts further inland. The immediate coast (e.G.
Beaches and adjoining communities) will likely see only rain and
only becoming snow under moderate rates. Would expect the freezing
rain to be more than a couple miles inland. The north Oregon/South
Washington coast zones do extend quite a ways inland and could
experience significant ice accumulations.

Now for Friday. Expect showers with moderate onshore flow, with up
to a foot of new snow in the Cascades. Low elevations will be above
freezing meaning all rain for these areas, except the chance The
Gorge and Hood River Valley may still be below freezing in the
morning hours./Jbonk



Long term...Friday night through Wednesday (previous discussion
follows)...onshore flow aloft will continue Friday night into the
weekend, which will support continued showers across the region.
Snow levels will rise to between 2,000 and 2,500 feet, which will
support continued snow in the Cascades, as well as in the higher
Cascade foothills and the highest peaks of the Coast Range. A few
impulses are modeled to move through the region early next week, but
some timing and strength differences remain in how these are
resolved in the various models. While the latest GFS would favor a
dry period early next week, the European model (ecmwf) continues to be more
aggressive with the movement and development of an energetic and
moisture-rich system as a surface low pressure system develops.
Meanwhile, the GFS keeps the bulk of this initial energy south
before bringing a surface low to the region by mid-week.
Have generally utilized a blend of the previous forecast with the
ECMWF, which results in a solution also generally supported by a
number of gefs ensemble members. Thus, this appeared a reasonable
compromise for this time and will hope to refine this forecast in the
coming days. Depending on the precise development of this system,
there could possibly be yet another transition period of snow/rain
around this time as snow levels again lower closer to the valley
floor, particularly across the northern portions of the forecast area
and through The Gorge. Cullen

&&

Aviation...rather challenging forecast for todays tafs, as to
precipitation type. Warm front now south and southwest of Oregon
will lift north today, with precipitation falling through a chilly
air mass. Will keep with trend of sleet/freezing rain/snow
spreading north to a konp to keug line 13z-14z, then spreading
slowly northward to a kast-kpdx line by 18-20z. Transition to
freezing rain depends on when warm air aloft arrives. Current
though is for transition to above freezing with precip as rain at
keug by 20z, and around 22z at ksle. But this could be delayed.
But cold air will keep precip as snow farther north until around
02z, with a mix of snow, sleet and/or freezing rain at that time.
Coastal areas will see precip at snow initially then change to
rain at konp by 17z, with transition to kast by 22z.

Kpdx and approaches...dry this am, with increasing mid/high
clouds. Light snow will increase over the area 18z to 20z, and
continue through 00z to 03z. Snow will continue until 00z with 1
to 3 inches expected, then will see the transition to a mix of
snow and sleet and/or freezing rain. After 04z, should just see
occasional light rain, possibly as light freezing rain for several
hours longer. Rockey.
&&

Marine...overall, not a great day to be on the waters. Chilly
easterly winds will gradually turn more southeasterly by early
afternoon, and more southerly later today. But with rather strong
gradients, so will continue to see gusts 30 to 35 kt today into
tonight. Gradients will relax a bit tonight, with winds easing
back under 30 kt later tonight into Monday. Will keep Gale Warning
on all waters through this evening. Seas running 6 to 8 ft.

Bit of a break later tonight into Fri am, but then winds increase
again as the main low well offshore shifts towards the pac NW, and
inland into southwest British Columbia early Sat. Will see south
to southwest winds increasing later Fri, with 25 to 35 kt for late
afternoon through Fri night. Seas will also be building up to
around 20 ft, mostly due to a 16 to 18 ft fetch of southwest
swell. Winds and seas will subside later Sat.

Active weather continues into next week. Rockey.

&&

Pqr watches/warnings/advisories...
or...Winter Storm Warning 8 am today through Fri afternoon for...
Columbia Gorge
central Columbia Gorge
Upper Hood River Valley.

Winter Storm Warning 8 am today through this evening...
Coast Range of northwest Oregon
foothills of north Oregon Cascades
lower Columbia
north Willamette Valley including greater Portland Metro.

Winter Storm Warning 7 am today to 4 am Friday for...
northern Oregon Cascades.

Winter Storm Warning 6 am to 4 PM today for..
central Willamette Valley including Salem area.

Winter Storm Warning until 2 PM today for...
foothills of Lane County Cascades
Coast Range of west central Oregon
S Willamette Valley including Eugene/Springfield area.

Winter Storm Warning until 6 PM today for...
Cascades of Lane County.

Winter Storm Warning 8 am to 8 PM today for...
north Oregon coast including Astoria.

Winter Weather Advisory until 1 PM today...
central Oregon coast including Newport.

Wind Advisory until 4 PM today for...
north Willamette Valley including greater Portland Metro.

Washington...Winter Storm Warning 8 am today to 4 PM Friday for...
central Columbia River gorge
South Washington Cascades
western Columbia River gorge.

Winter Storm Warning 8 am today through this evening for...
Clark County including the Greater Vancouver area
I-5 corridor in Cowlitz County
foothills of South Washington Cascades
Willapa Hills.

Winter Storm Warning 8 am to 8 PM today for...
South Washington coast.

Wind Advisory until 4 PM PST this afternoon for...
Clark County including Greater Vancouver area.

Pz...Gale Warning today and this evening on all waters.

&&

$$

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