Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Portland or
225 am PST Friday Dec 6 2013
Synopsis...an upper level disturbance will continue to drop south
along the Oregon coast today. This will bring some snow to many areas
today...especially the central coast east to the central Cascades.
Much colder air settles over the forecast area tonight and lingers
through the weekend. Strong offshore low-level flow today through
Saturday will create windy conditions...primarily in the Columbia
Gorge and in the Cascades. A slow moderation begins to occur Sunday
and continues into early next week...followed by a transition to
milder and wetter conditions the latter half of next week. This
transition could start icy for many inland areas later Tuesday into
Wednesday with cold air in place.
Short term...today through Sunday...National Weather Service Portland dual-pol Doppler
radar showing some light returns along the coast...Oregon Coast Range
and portions of SW Washington as of 0930z. Had a few reports of light snow in
Florence...in elevated terrain of Clark County in SW Washington and in the
north and central Oregon Coast Range. Water vapor imagery shows the
short-wave just west of the S Washington coast at 0930z. Not much change to the
overall snow forecast. Latest models still pinpoint the central
Oregon coast...central Oregon Coast Range and S Willamette Valley as
the prime areas. Latest GFS and NAM are a little more generous with
quantitative precipitation forecast...spreading it a bit more north and east compared to the past
couple of runs. However...4km WRF-GFS...and to an extent the 06z
NAM...keep the pdx metropolitan area in the dry slot. Kttd-kdls gradient of
almost -6 mb creating gusts to 30 miles per hour at kpdx. The east wind will
keep the low levels rather dry...making it difficult for any
accumulating snow to occur in the metropolitan area. The NAM did indicate
some frontogenesis from middle-morning through mid-afternoon. Would not
be surprised if areas such as Forest Grove...banks...Vernonia and
other places close to the east slopes of the Coast Range get a little
dusting. Current advisories and warnings look to be on track and will
leave them as is. Will need to watch the central Willamette Valley
for possible warning-level snow amounts. System swings through fairly
quickly...with the bulk of the dynamics headed into medford's area
this afternoon and early evening.
Much colder air settles over the forecast area behind the departing
system. By 12z Sat the GFS has -14c over kpdx while the European model (ecmwf) is -14
to -16c. Coupled with the strong offshore low-level surface
gradients...bitter wind chill conditions will develop this evening
and tonight in the Cascades. Have opted to go with a wind chill
warning for the Oregon and Washington Cascades from this evening
through noon Saturday. Although criteria is -20f...it is a rare
occurance in our forecast area and think in this case it is Worth
highlighting. Of course the windiest spot will be the pinch-Point of
the Columbia Gorge...but surface temperatures will not be quite cold
enough to create widespread dangerous wind chill conditions. Also
decided to include the Cascade foothills and west end of The Gorge in
a Special Weather Statement to highlight sub-zero wind chills.
Will maintain the Wind Advisory for the Portland and Vancouver metropolitan
through Saturday morning. Strong high pressure associated with a
building cold pool east of the Cascades will couple with the low
offshore to produce strong and gusty east to northeast wind
throughout the pdx/vuo metropolitan area. Also assisting this wind will be
downslope acceleration due to the dense cold air from the east side
spilling westward over the Cascade crest through the broader gorge
gap between Mount Adams and Mount Hood. Since this air will be more
dense than its environment...it will tend to sink and accelerate down
the west slopes of the Cascades...surfacing across portions of the
metropolitan area. This effect shows up nicely on NAM cross-sections with a
strong downward Omega field to the Lee of the Cascades.
Models hold on to some precipiation across the S and southeastern portion of
the forecast area this evening...and upped probability of precipitation slightly in those
areas. Saturday night looks to be one of the coldest nights in
several years. Areas with snow-cover...minimal wind and clear skies
will achieve exceptional radiational cooling. The central and south
Willamette Valley likely to see overnight temperatures into the
single-digits. Sheltered areas could drop below zero.
It sitll appears a slow moderation begins Sunday. GFS shows 850 mb
temperatures of -6 to -8c by 00z Monday. Another new wrinkle is that the latest
NAM is showing another weak short-wave diving down the coast Sunday.
Should this occur...the coast could get another light dusting. GFS
does not have any quantitative precipitation forecast sun. Weishaar
Long term...no changes. Previous discussion follows...Sunday night
through Thursday. Forecast models indicate that the cold Arctic air
mass will remain in place over the region through early next week.
12z European model (ecmwf)/GFS depict an area of light precipitation late Monday into
early Tuesday associated with a weak impulse dropping south along the
West Coast. Given the strength of the cold air firmly in place...this
could bring some freezing rain...snow...rain...or some mix of the
above. Confidence is somewhat low and amounts look to be fairly small
at this time...so kept probability of precipitation in chance north to slight chance south
The next system looks to move into the area for Wednesday or Thursday with some
more significant moisture content. As this moist and mild system
approaches the Pacific northwest...some heavy rainfall looks possible
as well as some moderating temperatures back closer to /but still
below/ seasonal norms. Given the lower level cold air in place...this
may begin as freezing rain or ice for some locations. This is
especially true for areas in the Columbia River gorge and valleys in
the Cascade foothills. However...the particular track of the
developing low and timing differences remain to be sorted out between
the forecast models...and these factors will influence where snow
levels may be along with the potential for mixed frozen precipitation versus
just rain for lowland locations. Cullen
Aviation...low pressure is now off the Oregon coast. Expect clouds to
to remain VFR until daybreak...then deteriorate this morning. With
ceilings higher now...expect snow to generally be flurries
overnight...then expect accumulations at airports to S of a ktmk
to ksle line...with over 1 inch at airports S of a konp to ks12
line. Likely MVFR ceilings with the snow with IFR visibilities along
the coast and into keug where snow bands will likely be more
intense. Should remain VFR as move north and east towards kpdx and kttd.
East winds will increase in the morning around kpdx and
kttd...with increasing east to NE winds spreading west to coast by
afternoon. Expect improving conditions after 0z with some
lingering snow showers and MVFR from sle south...otherwise
improving to VFR late tonight in cold north winds except near The
Gorge where it will be gusty east wind.
Kpdx and approaches...offshore flow should produce VFR conditions
at the terminal through the period..though maybe getting ceilings
close to 5000 feet later this morning.Will have occasional snow
flurries between 12z and 20z...but no accumulation expected. Main
weather will be increasing east winds after 18z. Kmd
Marine...gusty east to NE winds continue to strengthen this
morning...and continue through Friday night. A Small Craft Advisory for
winds remains in effect for the outer waters...and the inner waters
near gaps in coastal terrain. Seas remain at 2 to 4 feet.
Benign weather for Saturday through Tuesday. Then stronger low
pressure will move into the region...with increasing potential for
southerly gales. Kmd/Rockey.
or...Winter Weather Advisory until 4 PM PST this afternoon for
Cascade foothills in Lane County-central Coast Range of
western Oregon-central Willamette Valley-northern Oregon
Wind Advisory from 8 am this morning to 6 am PST Saturday for
greater Portland metropolitan area.
Winter Storm Warning until 4 PM PST this afternoon for central
Winter Storm Warning until 4 PM PST this afternoon for south
Winter Weather Advisory until 2 PM PST this afternoon for Coast
Range of northwest Oregon-north Oregon coast.
Wind chill warning from 7 PM PST this evening until noon
Saturday for north Oregon and Lane County Cascades.
Washington...Wind Advisory from 8 am this morning to 6 am PST Saturday for
Greater Vancouver area.
Winter Weather Advisory until 10 am PST this morning for South
Wind chill warning from 7 PM PST this evening to noon
Saturday for South Washington Cascades.
Pz...Small Craft Advisory for winds until 4 am PST Saturday for
coastal waters from Cape Shoalwater Washington to Florence 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. The area is commonly
referred to as the forecast area.