Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Portland or
250 PM PST sun Dec 28 2014

Synopsis...a cool and showery weather pattern will continue through
tonight. An additional 3 to 6 inches of snow will fall in the
Cascades. Before the last of the showers dissipate Monday...a few wet
snow flakes may mix down to the valley floor in heavier showers.
However...any snow accumulations will likely remain less than an inch
and will be primarily confined to elevations above 750 to 1000 feet.
The coldest air mass of the winter season will then spread into the
region Monday night into Tuesday...and linger through at least
midweek. Dangerous wind chills appear probable across the higher
elevations of the Cascades on Tuesday morning. A weak storm system
may bring a chance for winter precipitation towards the end of the
work week...but at this appears weak and uncertainty
remains high. An additional storm system may impact the area over
the weekend.


Short term...tonight through Wednesday...a cold and unstable airmass
under deep northwesterly flow has spread across the region. Snow
levels are currently hovering between 2k to 3k feet across most of the
forecast area. 6 to 15 inches of snow have fallen so far across the
Cascades. Snow amounts have been highest across the South Washington
Cascades...Montana Hood and Santiam Pass. The lowest totals have been
towards the Lane County Cascades.

Showers have been fairly widespread today with most Willamette Valley
locations measuring a couple hundredths of an inch
of precipitation over the six hours. Shower intensity has increased a
touch this afternoon. Dual pol radar and spotter reports indicate
small hail/graupel will be possible for the remainder of the
afternoon with the strongest showers.

The 12z ksle sounding and subsequent kpdx aircraft soundings today
indicate a subsidence inversion continues near 700mb. Models
generally agree the subsidence inversion will hover around 700mb...if
not rise slightly through late tonight. Given low level flow will
remain northwesterly and low level instability will continue for much
of tonight...expect off and on showers to continue...even across the
Willamette Valley. Probability of precipitation were generally left in the likely to
categorical range.

Surface observations show modified Arctic air continuing to spill
southwestward through interior British Columbia. As this air pushes
up against the east slopes of the Cascades on Monday...some light
snow will likely fall across the central Columbia River gorge and
Upper Hood River Valley. There is still some uncertainty in surface
temperatures and quantitative precipitation forecast amounts...but a Winter Weather Advisory may be
needed. Will let the evening shift examine this in more detail...but
even a half inch of snow may create impacts given temperatures will
plummet well below freezing late Monday.

As colder air spills into northwestern Oregon and southwestern
Washington there is a chance some of the last remaining showers
could mix with snow down to the Willamette Valley floor. Suspect
this would be most likely to occur along the east slopes of the
Coast Range as low level east winds upslope and release any
lingering low level instability Monday morning. The NAM is most
aggressive with this idea...while the GFS suggests this is less
likely. Continued chance to low end likely probability of precipitation given the
uncertainty. Any slushy snow accumulations will likely be light and
largely confined to elevations above 750 to 1000 feet.

Offshore pressure gradients will increase Monday into Monday night.
The kgeg to koth gradient looks to peak out between 15 and 20mb
depending on the model of preference. The large scale offshore
pressure gradient and easterly flow of 40 to 50 kts in the 850mb to
700mb layer will bring breezy winds to much of the area on Tuesday
and make conditions feel considerably colder than reality. In
fact...wind chill readings may dip into the -15f to -25f range late
Monday night into Tuesday morning across the high Cascades where
winds and temperatures in the single digits will bring dangerous wind
chill readings. As a result...will continue the wind chill watch for
elevations above 3000 feet. Expect wind chill values in the teens and
upper single digits in the Willamette Valley even though
temperatures will only dip into the high teens and twenties on
Monday night.

As the upper level trough slides south of the area and the subsidence
inversion east of the Cascades lowers and strengthens...expect winds
near the west end of the Columbia River gorge to increase Tuesday
night into Wednesday even though winds elsewhere across northwest
Oregon and southwest Washington will weaken considerably. Given the
dry air that will be in place and northerly pressure gradients still
continuing...removed the mention of fog/freezing fog Tuesday night
into Wednesday morning. As winds pressure gradients weaken further
on Wednesday and the lower atmosphere begins to moisten...expect
valley fog development to become more prevalent Wednesday night.

Long term...Wednesday night through Sunday...easterly winds start
to weaken Wednesday night into Thursday...allowing temperatures to
warm slightly during the day Thursday. A shift in the weather
pattern occurs late this a cold front moves through
bringing rain on Thursday and Friday. The European model (ecmwf) and GFS are a little
slower bringing the system through today...bringing it through on
Friday morning as opposed to Thursday night as forecast yesterday.
The timing could easily still change. There may be a period of
wintery mix all the way to the valley floor as the front moves
through on Friday morning until daytime heating and onshore winds
change over precipitation to all rain below 3000 feet. More
seasonal...rainy weather continues through the weekend. -McCoy

&& northwest flow aloft...with widespread VFR conditions
and scattered showers...though as snow above 2500 feet. Will
continue to see pockets of MVFR ceilings. As weak low drop into
region from north later tonight...will see areas of MVFR ceilings
increasing after 03z...mainly inland. Colder air will push into
the region Monday am...with snow down to 1000 feet or possibly lower at
times through 20z. Mountains will be frequently obscured in clouds.
Clouds will break up and clearing as offshore winds increase later
Monday am into Monday afternoon.

Kpdx and approaches...VFR under cool showery air mass...with
occasional MVFR ceilings through 02z. Then will see primary ceilings
lower back to 2500 to 3000 feet. Not much change through 16z except
that may have mix of snow with the showers after 12z. But only
brief slushy accumulation on grasses and trees...if any at all.
Does look as threat of showers done by 18z as colder air and east
winds increase at that time. Rockey.


Marine...gusty northwest winds continue on the waters tonight... but
strongest winds continue over north Oregon waters. Bit less wind as
go along central Oregon coast. Not much change into this
evening... but winds will ease back a tad as another low drops S
over the coastal waters later tonight. Seas mostly 13 to 15 feet
today...but will drop off slowly as winds drop later today and
tonight. Still...seas will stay above 10 feet tonight.

Once low drops S of the region late tonight into Monday am...winds
will begin to turn north to NE. This front will also increase of
offshore flow over the interior. End result will be arrival of east
to NE winds by late Monday am into the early afternoon. Likely to see
gusts 20 to 25 knots on the waters Monday afternoon through Tuesday am. Seas
will drop back under 10 feet as the typical west to northwest fetch decreases.
Even through will be 6 to 8 feet...the seas will be a bit choppy
due to waves being driven by winds for Monday night through Wednesday.


Pqr watches/warnings/advisories...
or...wind chill watch from Monday evening through Tuesday morning
for northern Oregon Cascades.

Winter Weather Advisory until noon PST Monday for Cascades in
Lane County-northern Oregon Cascades.

Washington...Winter Weather Advisory until 7 am PST Monday for South
Washington Cascades.

Wind chill watch from Monday evening through Tuesday morning
for South Washington Cascades.

Pz...Small Craft Advisory for winds tonight and Monday on S wash and
north Oregon coastal waters...or from Cape Shoalwater to Cascade Head
and out 60 miles.
Small Craft Advisory for winds tonight on central Oregon
coastal waters...or from Cascade Head to Florence and out 60 miles.
Small Craft Advisory for hazardous seas tonight onall coastal
Small Craft Advisory for rough Columbia bar conditionsthrough
3 am Monday.



Interact with US via social media

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.

National Weather Service Glossary of Abbreviations