Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
305 am PST Monday Dec 22 2014
Synopsis...the frontal boundary that has been the focus of
persistent rain this weekend was in the southern part of our forecast
area this morning but was weakening...and the remaining rain will
continue to decrease today with the threat of any additional flooding
ending as an upper level ridge builds in the northeast Pacific and up
into the Gulf of Alaska...squeezing off the supply of moisture. The
remaining frontal boundary will lift north tonight as a weak front
and may produce some areas of light rain. The next cold front will
move into western Washington on Tuesday and begin to affect the far
north part of our forecast area...slowly sag farther south into our
area Tuesday night...then sweep through southwest Washington and
northwest Oregon with a good shot of rain and lowering snow levels on
Wednesday and early Wednesday night. Another disturbance will bring
some showers to the area Thursday night and Friday. Then another
decent system is likely by the weekend. Snow levels will be lowering
during the second half of the week...possibly affecting the Cascade
foothills and the Coast Range.
Short term...the frontal boundary that was the focus of heavy rain
over the weekend remained over northwest Oregon overnight and
continued to produce some decent rainfall amounts our central and
southern Cascade zones...slowing the recession of rivers in that
area. Rainfall rates appear to be finally easing early this
morning...and National Weather Service Doppler radar confirms this
trend. Have thus ended the remaining portion of the Flood Watch. The
models show 850 mb winds of around 30 to 35 knots across the southern
part of our area overnight will decrease to 20 knots or less this
morning...confirming this idea.
Though this boundary weakens...it is forecast to slowly lift north
later today and tonight as a weak warm front ahead of the next
system. The models have been persistent and coming into better
agreement with the details of some areas of light rain spreading
north through the area tonight into early Tuesday. Amounts will be
The cold front with the next system will move into western Washington
on Tuesday...brushing our far northern zones...especially north of
Portland. This front sags very slow southward on Tuesday night...not
making much southward progress. Then a trailing short wave on the
tail of the front develops into a new low along the front that then
sweeps through southwest Washington and northwest Oregon Wednesday
and early Wednesday night for a good shot of rain. Rainfall amounts
of a half inch or more are likely...with some areas around an inch.
Snow levels will finally be falling to more seasonal values on
Wednesday and Wednesday night...likely affecting the Cascade
foothills and the coastal mountains. Snow amounts could reach Snow
Advisory thresholds with this system in the Cascades. Tolleson
Long term...some showers will linger on Christmas day behind the
rather vigorous Wednesday system. The models are now all showing a
short wave dropping in from the northwest later Thursday night and
Friday for more showers. Snow levels will remain near or a bit below
seasonal values...with snow into the Cascade foothills and coastal
mountains. The models are coming into better agreement for another
cool system by early in the weekend for more rain and Cascade
snow...with snow levels remaining more seasonal. The models are now
showing more drying early next week with some east winds possible.
For now have lowered probability of precipitation for early next week. Tolleson
Hydrology...rivers are mostly receding this morning...and the Flood
Watch has been cancelled. The only rivers remaining above flood stage
are the Marys River near Philomath and the Luckiamute near Suver.
These are receding very slowly at best and will remain above flood
stage for a while today. Tolleson
Aviation...not much change this am. Mix of low VFR and MVFR across
much of the region this am. Even pockets of IFR ceilings along the coast
and peppered further inland. Will slowly trend towards VFR conditions
between 19z and 23z. As a warm front lifts north across region later
tonight...will see low VFR ceilings with areas of MVFR ceilings reform. With
light winds...threat of fog and IFR stratus will increase
tonight...especially for valleys to S of ksle.
Kpdx and approaches...generally VFR today and this evening. Expect
occasional MVFR ceilings near 2000 feet with drizzle through 16z. Will be
dry this evening...but will have patchy fog and possibly IFR stratus
late tonight into early Tuesday. Rockey.
Marine...benign pattern with winds 15 knots or less. Warm front will
lift north across the coastal waters late tonight into Tuesday am...then a
cold front will push across the coastal waters Tuesday evening. Will see
S to SW winds increase Tuesday afternoon...with gusts to 25 knots on the north
waters at that time. Seas holding around 10 to 12 feet for next few
Increasing potential for stronger winds again on the coastal waters
Sat. Models indicate decent low pressure rolling across the NE Pacific into
the Pacific northwest. Will have to monitor this week as this low could bring
gales. Seas will be on the increase by next weekend...as fetch of 20
feet swell from North Pacific arrives.
or...coastal Flood Advisory from 10 am to 4 PM today for
the north and central Oregon coast.
Washington...coastal Flood Advisory from 10 am to 4 PM today for the
South Washington coast.
Pz...Small Craft Advisory for hazardous seas today through
Tuesday on all coastal waters.
Small Craft Advisory for rough Columbia bar conditions
for today through Tuesday.
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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.