Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
904 PM PST Friday Nov 28 2014
Synopsis...a well-defined cold front continues moving southeast
across the forecast area tonight...followed by a sharp drop in
temperature overnight. Well below normal temperatures will persist
through the weekend...with snow levels hovering just above the coast
and valley floors. Mixed rain and snow showers will persist into
Saturday as an upper level trough passes through...with perhaps a
slushy inch or two of snow accumulation in the higher hills. Wet
snowflakes may mix in from time to time down to the valley floors.
High pressure will settle in east of the Cascades Saturday night and
Sunday...bringing strong and chilly east winds through The Gorge and
across the Portland metropolitan area. A system coming up from the south is
likely to spread moisture over the cold air Monday...possibly
resulting in some freezing rain north of Salem during the morning...
especially in and near The Gorge.
Update...the cold front has moved south across much of the region
tonight bringing some colder air and a notable shift in winds from
south to north or northwest. Some significant rainfall has been
measured tonight with 12 hour precipitation totals of around one half
inch for the coast and interior lowlands and higher totals of around
an inch or more in the higher terrain. The front is currently located
along a line from roughly Coos Bay to Eugene to The Dalles. The
passage of the surface front has been marked by notable temperature
decrease...including a 7 degree drop in about a half hour at kpdx
this evening. However...the snow levels are a bit slower to respond.
Observations suggest that the snow level has lowered to around 4000
feet in the South Washington Cascades and cold air continues to push
south as webcam images depict some snow accumulating at Timberline
Lodge and a rain/snow mix falling at Santiam Pass. Overall...few
changes made to the forecast tonight as main frontal band shifts
south and cold advection will continue. Post frontal showers will
decrease with some subsidence overnight...but increase again
Saturday as the main upper trough rotates across the region. Cullen
Previous disc... /issued 240 PM PST Friday Nov 28 2014/
Short term...tonight through Monday...a potent frontal system
extends from about Kelso to Tillamook at the moment...pressing
southeastward at about 20-25 miles per hour. The front should be through
Portland around 4-5 PM and Eugene around 10-11 PM. Gusty south winds
continue ahead of the front...with winds generally gusting 30-40 miles per hour
both in the Willamette Valley and along the coast. Behind the front
an abrupt shift to gusty northwest winds occurs...along with a quick
5-10 degree f temperature drop. By West Coast standards...this is a
fairly well-defined cold front.
Despite the sharp drop in temperature associated with the front...a
look at wsdot webcams shows snow is only falling to about 3000 feet
at Stevens Pass roughly 3 hours after the cold front moved through.
We initially had some concern that snow levels would drop quickly
enough to provide advisory accumulations for our Cascades and
foothill zones...but based on what is happening to our north it
appears Post-frontal snow accumulations should be limited to a couple
inches or so. Temperatures have only fallen into the lower 40s behind
the front in The Lowlands around Seattle...so it appears unlikely
that our valleys will see snow from the initial shot of precipitation
associated with the front.
Cold advection will continue tonight...but showers are expected to
decrease in the subsidence behind the front. However the main upper
trough will swing through the Pacific northwest Saturday...causing a resurgence
in shower activity. With temperatures mainly in the 30s wet snowflakes will
be possible down to the valley floors...however sticking snow should
be relegated to the hills. The West Hills of Portland and the
Cascade/Coast Range foothills surrounding the Willamette Valley could
see an inch or two of accumulation...but it should mainly be on
grassy surfaces due to our recent warm spell. Above 1000-1500 feet is
where snow may start to stick on the roads and present travel
issues...though there may be some sun breaks between the showers to
mitigate this Saturday afternoon.
Cold high pressure will settle in Sat night/Sunday for drier weather.
As high pressure strengthens east of the Cascades... easterly
pressure gradients will ramp up through the Columbia Gorge. Strong
and chilly east winds will result...and the cold pool again appears
deep enough for the east winds to affect the whole Portland/Vancouver
metropolitan area. With the 12z 4km uw wrfgfs kttd-kdls pressure gradients
approaching -9 to -10 mb and koth-kgeg pressure gradients approaching
-17 to -19 mb...this east wind event is not expected to be quite as
strong as our big east wind event earlier this month but it will
still be formidable. Expect east wind gusts 60-70 miles per hour in the west end
of the Columbia Gorge and gusts 40-50 miles per hour in the greater
Portland/Vancouver metropolitan area. This would warrant a Wind
Advisory...which may be issued by subsequent shifts if models
continue to project offshore gradients of this magnitude.
The strong east winds will dry out our air mass Sunday...allowing
temperatures to fall below freezing most areas Sunday night. Models are more
or less in consensus now that some precipitation will spread
northward across our forecast area Monday morning. With the
combination of cold air in place and overrunning precipitation...
there is some concern for freezing rain in our northern valleys. The
greatest threat is in and near The Gorge where cold air will be most
stubborn. However there is still a lot of uncertainty as to how this
event will play out...but for now we increased probability of precipitation...freezing rain
chances...and potential ice accumulations in the grids Monday. Kept
ice accumulations fairly minimal at this point due to the
uncertainty...though if current models are to be believed there could
be more significant icing than presently forecast. Weagle
Long term...Monday night through Friday...extended models are in
decent agreement having the next Pacific frontal system lift a warm
front into the Pacific northwest late Monday into Tuesday morning.
12z models suggest the front will stall over the region through
midday Tuesday...with a good amount of quantitative precipitation forecast dumped over the central
part of the forecast area. Prevailing east winds will continue
through the Columbia River gorge...and as the precipitation lifts north-
northeast over the area...could see some threat for freezing rain
over the Upper Hood River Valley and The Gorge Monday night. This
will be short-lived as models drag the parent low further south
along the California coast late Tuesday. Model solutions diverge
starting Wednesday...as upper level onshore flow develops. Still low
confidence concerning the overall track of this Pacific system. It
could either push onshore into northern California and maintain
seasonably wet conditions as the 12z European model (ecmwf) suggests...or the low
could push inland over central California with much of the precipitation
staying south of our area as the 12z GFS suggests. Went more towards
climatology for the end of the week. /27
Aviation...a well defined cold front moved through kast early
this afternoon and across the remaining taf sites between 01z and
04z. Precipitation as tapered off to showers and flight conditions are
improving with VFR becoming more prevalent. Upper trough sags into
the area Sat morning and expect increased MVFR ceilings as showers
increase. May see some snow mixed in with the rain in northern taf
sites during the day at taf sites. Showers will be decreasing Sat
afternoon as colder and drier air filters across the area. Flight
conditions should be mainly VFR.
Kpdx and approaches...showers with VFR flight conditions to about
12z Sat then expect an increase in MVFR ceilings around 2000 feet as
showers increase...then trend to VFR around 21z Sat. May see some
snow mixed in the with the rain at times but no accumulation is
expected. Showers decrease during the afternoon and end by evening
with VFR flight conditions. Surfaces that remain wet Sat night may
be prone to ice as temperatures drop below the freezing
point. However developing east winds may be enough to dry paved
surfaces before freezing occurs.
Marine...gusty northwest winds behind a cold front will continue
through Sat. Winds are not quite Small Craft Advisory this
evening but winds will increase later tonight as a trough over the
Washington waters pushes against the NE Pacific high pressure. Winds
will ease Sat afternoon. Winds ease later Sat and Sat night...with
gradients becoming offshore and strengthening as high pressure builds
over Pacific northwest. Likely see gusts 20 to 30 knots in the coastal gaps on
sun and Sun night.
Northwest swell however will remain hover around 10 feet through Sat
afternoon before subsiding and remain in the 5 to 7 feet range
through middle week.
Pz...Small Craft Advisory for winds until noon PST Saturday for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 6 PM PST Saturday
for coastal waters from Cape Shoalwater Washington to Florence or out 60
Small Craft Advisory for rough Columbia River bar until
noon PST Saturday.
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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.