Heavy Snow in Oregon, December 22 to 23, 2017

Area Forecast Discussion
National Weather Service Portland OR
235 PM PST Fri Dec 22 2017

.SYNOPSIS...A weak warm front will continue to slide southeast over
the area tonight through Saturday morning. Offshore flow develops in
the Columbia Gorge this evening and then strengthens overnight and
Saturday. The offshore flow continues Saturday night and Sunday,
which will allow cold air to funnel through the Gorge. Another
weather system approaches the area Sunday and moves through Sunday
night and early Monday. This will bring another threat of mixed
precipitation to low elevations, especially near the Gorge.
Confidence in the details of these two events is still not high.
Subsequent systems next week will probably bring mainly rain to the
area with snow in the mountains.
&&

.SHORT TERM...Tonight through Monday...Early afternoon water vapor
imagery indicated a sharp 500 mb ridge axis between 135W and 140W
with cold north flow dropping down across British Columbia. The flow
becomes more NW over Washington and Oregon. An obvious short-wave was
noted near the North Washington coast. KRTX dual-pol doppler radar at
21Z showed a large area of light precip returns across SW Washington
and far NW Oregon. QPF amounts so far have been rather meager, with
most reports under one-tenth of an inch. Temperatures have continued
to moderate with interior lowlands in the upper 30s to lower 40s.

Models in general agreement regarding the system dropping south from
Vancouver Island. This baroclinic boundary slowly sags south through
tonight and then appears to stall out Sat morning over Northwest
Oregon and into SW Washington close to the Columbia River. There will
be snow accumulation in the Cascade foothills, especially the South
Washington and North Oregon sections. Will maintain the current
advisory, although the prime period for snow looks to be late this
afternoon through early Sat morning. Not expecting advisory level
amounts for the higher Cascades.

Offshore low-level flow starts to develop around 06Z Sat. 18Z NAM
develops a KTTD-KDLS gradient around -7 mb by 18Z Sat. The NAM
continues to be much colder than other models showing around -12C 850
mb temps banked up against the S WA and N Oregon Cascades Sat
morning. The GFS has 850 mb temps -6 to -8C for the same time period.
Will lean with the ECMWF compared to the NAM or GFS as there is
little to no snow cover in the Columbia Basin. Current temps near the
Canadian border are in generally in the 20s. Will just have to wait
and see how cold the air is when it pushes into the Washington
Columbia Basin. There remains the concern for some frozen precip at
low elevations Sat morning with lingering moisture from the decaying
baroclinic band. However, a look at MM5 NAM sounding for KTTD would
indicate just cold rain. The 18Z NAM sounding for KTTD valid Sat
morning shows a colder surface layer suggesting a snow profile. Think
the NAM is too cold and will maintain a rain/snow mix to low
elevations and limit any -FZRA threat to colder valley locations in
the North and Central Willamette Valley.

The models are coming into better agreement with the system
approaching Sunday and moving through the forecast area Sunday night
into Monday. However, there are still some fairly large model
differences with the surface low track Sun and Sun night. The 18Z NAM
provides a little more reassurance as it shows a track somewhere
between the 12Z ECMWF and GFS. The offshore gradient peaks at -8 to
-10 mb 06Z-12Z Sun. This will allow colder air to funnel through the
Gorge. The major question will be how cold this air will be. The 12Z
ECMWF and 18Z NAM show the surface low moving to near or just south
of KAST 06Z Mon. A slight deviation north or south of this location
will result in a big difference in precip type for areas in vicinity
of the low. The UW MM5-NAM soundings late Sun through Sun night
provide a favorable snow profile for the Central Gorge and Upper Hood
River Valley and more of a -FZRA signature for the West Gorge. Cannot
completely rule out the possibility of -FZRA for the east metro area
and the normally colder pockets in the North Willamette Valley. Areas
south of about Woodburn are likely to see just rain. However, if the
surface low tracks south of KAST snow chances increase for the North
Willamette Valley and certainly for the SW Washington interior
lowlands.

Forecast QPF would imply at least advisory snow accumulations Sun
afternoon through early Mon morning for the Central Gorge and Upper
Hood River Valley. The UW MM5 NAM puts the surface low near KPDX 12Z
Mon. Should this occur, there is the possibility of a short period of
snow and/or -FZRA. The MM5 NAM 3hr snowfall product valid 12Z Mon
does show a half inch or so near KPDX, closer to KSPB, Forest Grove
and Banks. All in all, this continues to be a very tricky forecast
with potential large swings in forecast thought over the next 24-36
hours. The ECMWF holds on to offshore low-level flow through Mon
morning, followed by a change to neutral or very weak onshore flow by
the afternoon. Weishaar

.LONG TERM...Monday Night through Friday...Models in general
consensus that Tues will be a dry day across the region, then a
Pacific front brings a return to valley rain and mountain snow on
Wed. Model continue to exhibit differences beyond Wed with GFS being
more amplified, taking frontal systems to the north, while EC has a
more zonal, progressive pattern into Fri. Have opted to take a blend
of the GFS and EC which maintains valley rain and mountain snow
through Friday. tw
&&

.AVIATION...A frontal boundary will continue to sag slowly south
this afternoon and tonight. The boundary is currently over SW
Washington and will push into NW Oregon this evening. Conditions
are a mix of MVFR/IFR for the northern half of the CWA, roughly
for areas north of KSLE. These locations will likely trend
toward more IFR during the late afternoon and early evening as
rainfall intensity begins to increase. Conditions will also
gradually deteriorate for KSLE later this afternoon, then toward
KEUG later tonight as the front continues to push southward. The
northern half of the CWA should see conditions improve behind
the front late tonight. MVFR/IFR will likely persist for KSLE to
KEUG well into Sat AM, but they should also improve toward
midday as the stalled front dissipates. Dry offshore flow
develops through the day Sat, which should lead to widespread VFR
conditions Sat afternoon and night.

PDX AND APPROACHES...Cigs around 2000 ft at current will
gradually lower later this afternoon and this evening as a front
approaches and rain increases. There will likely be a period of
IFR conditions during the evening. Rain continues overnight, then
drops off between 12Z-15Z Sat. Conditions should gradually
improve to VFR Sat morning and continue through the rest of the
day. Gusty offshore flow will gradually strengthen through the
day, with gusts 30 to 35 kt likely out closer to the gorge. Pyle

&&

.MARINE...Offshore flow will begin to develop tonight, then will
strengthen Sat as high pres develops over the interior Pac NW.
The offshore gradient will be strong enough to produce 20 to 25
kt easterly gusts later tonight through Sat night over the
northern waters. A small craft advisory for winds is in effect
starting at midnight. A cold front will move through the waters
Sun and Sun night. A secondary surface low is modeled to form
along the front and is expected to push onshore somewhere along
the WA or OR coast. Confidence in details is not high at this
time, but there will be the potential for solid small craft
winds to low-end gales on the SW side of the low. The low moves
onshore overnight Sun, bringing NW winds on Mon.

Seas generally holding 4 to 6 ft through Sat. Then a longer
period westerly swell is expected to arrive Sun, pushing seas
into the low to mid teens. Seas then settle back to around 6 to 8
ft for most of next week. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 7 AM PST Saturday for Northern
     Oregon Cascade Foothills.

WA...Winter Weather Advisory until 4 AM PST Saturday for South
     Washington Cascade Foothills.

PZ...Small Craft Advisory for winds from midnight tonight to 6 AM
     PST Sunday for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 NM.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA, or forecast area.