Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
327 am PST Thursday Nov 27 2014
Synopsis...a weak cold front will push slowly inland today. The
front will then stall over south-central Oregon tonight and Friday. This
front will begin to lift northward again early Friday. Another stronger
cold front will move S through the region late Friday into Sat. This
front will usher in much colder weather. Snow levels will fall
rapidly Sat...with a rain/snow mix a possibility for the interior
lowlands. High pressure over the interior northwest will turn the flow offshore
by sun for dry and cold weather. Another system coming up from the S
sometime early next week will attempt to spread some precipitation up
over the cold air mass left over from the weekend.
Short term...today through Saturday...
breezy southerly winds ahead of an approaching cold front are keeping
exceptionally mild conditions in place across southwest Oregon and
northwest Oregon this morning. In fact...the temperatures in the upper 50s to
low 60s being observed across much of the region at current are very
close to the record daily high temperatures for the last week of November.
However...the period of mild weather that we have been experiencing
over the past few days will be coming to an end very shortly.
The cold front approaching the coast this morning is not terribly
impressive. Looking at the latest infrared satellite imagery...it is
apparent that the upper level front and associated moisture has moved
out ahead of the surface frontal boundary. The surface front can be
made out on satellite just offshore around buoy 29. With only shallow
moisture remaining and little in the way of forcing aloft...the
associated precipitation that is moving onshore is definitely on the
patchier side. The rain has become a bit steadier over the past few
hours along the north coast and Coast Range...but most sites are only
picking up a few hundredths of an inch of rain per hour.
Still...expect most locations will see a decent steady rain as the
front pushes onshore this morning and then moves inland through the
day. The front will eventually stall over southern and central Oregon
later this afternoon and this evening.
The other feature of interest that we are watching this morning is a
persistent stationary baroclinic zone that is currently draped over
British Columbia and far northern Washington. An upper level trough
will drive this boundary southward Friday and Friday night as a sharp cold
front. At the same time...the stalled front over central Oregon is
forecast to push northward again Friday morning. Between these two frontal
boundaries...expect rainfall to steadily increase as the day
progresses Friday. As the sharp cold front from the north dives through
the region later Friday and Friday night...a much colder air mass aloft
will move into the region.
The parent upper level trough will swing into the region from the
north on Sat to provide some upper level support for Post frontal
shower activity...while low level onshore flow should provide
additional moisture to keep showers going through much of the day.
Based on model 850mb temperatures and 1000-500 mb thickness values...it
appears that there is the potential for a rain/snow mix to affect the
interior lowlands at times on Sat. The temperature and thickness indicators
are only marginal and there is some uncertainty amongst the models
just how cold the air mass will be...so confidence is not all that
high that the interior lowlands will see any significant flakes. At
any rate...with the mild temperatures we have seen over the past few days it
is unlikely that Road surfaces will be able to cool fast enough for
there to be major travel concerns. Pyle
Long term...Saturday night through Wednesday...most guidance
suggests the upper trough axis responsible for saturdays showers will
be moving across the Cascades Sat night... ith any lingering showers
coming to end overnight. Expect some clearing Sat night as cold high
pressure builds down from British Columbia...with low temperatures likely
getting well into the 20s for most spots. With high pressure
strengthening east of the Cascades...expect strong east winds to ramp
up through The Gorge Sunday morning and persist for much of the
period through Wednesday...with breezy conditions spilling into the
Portland/Vancouver metropolitan. Monday may be a brief break from the winds
as a system passes by to our north.
Meanwhile...a disturbance from the Aleutians is expected to cut off
from the main Pacific jet and settle off the California coast by
Sunday...where it will likely linger for a couple days. There is a
chance this system could come close enough to spread some precipitation
northward into our forecast area early next week. If this
occurs...there may be some light freezing rain in portions of the
Willamette Valley with some snow or light freezing rain in The
Gorge. Cutoff lows such as this one are notoriously difficult to
forecast...as models struggle with them in the extended forecast
period. Where this upper low GOES will dictate the weather through
midweek...but temperatures are expected to slowly moderate Tuesday and
Aviation...a mixture of IFR and low MVFR conditions have spread
onto the coast with the first incoming cold front early this
morning. Conditions at the coast will then continue in that range
into Friday as the front moves onshore but stalls and some
precipitation continues at times. Inland conditions are still VFR
early this morning but will probably see some MVFR conditions as
the front moves through today. The front stalls and weakens over
the inland areas this afternoon and tonight before the next front
approaches from the north on Friday. Model guidance suggests MVFR
ceilings or low end VFR ceilings will continue over the area behind the
front into Friday.
Kpdx and approaches...VFR conditions early this morning but expect
periodic MVFR ceilings to develop the remainder of the morning. Do not
expect much change the remainder of today and tonight as ceilings
hover between MVFR and low end VFR with some light precipitation
at times. Pt
Marine...winds have picked up as the front moved farther into the
coastal waters overnight...and Gale Warning looks good. Believe
winds in the central Oregon waters will extend an hour or two
past 4 am so have extended the gale there. The first cold front
moves onshore this morning which will allow the winds to east a
bit...but there is really not much cold air behind this front.
Then it looks like Small Craft Advisory winds from the southwest
for the remainder of today...tonight...into Friday. A secondary
cold front which has much colder air behind it will move south
through the coastal waters Friday and early Friday evening for
continued Small Craft Advisory winds...with wind directions
switching to the northwest behind this front. The northwest winds
will ease on Saturday...then offshore gap enhanced east winds will
develop Saturday night and Sunday...probably of the Small Craft
Advisory variety. These offshore winds will continue into next
Seas are running around 10 feet right now but are expected to run
in the 10 to 13 feet range today into Friday...then continue to
hover around 10 feet into Saturday...then fall below 10 feet by
Sunday. Seas today will be choppy with around 8 second periods
from the southwest. Periods will lengthen to around 14 seconds
later today and tonight...then shift and be from the northwest
with periods and 10 or 11 seconds Friday night into Saturday. Pt
Pz...Gale Warning until 4 am PST early this morning for coastal
waters from Cape Shoalwater Washington to Cascade Head or out 60 nm.
Gale Warning until 7 am PST this morning for coastal waters
from Cascade Head or to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until
6 am PST Friday.
Pz...Gale Warning until 4 am PST Thursday for coastal waters from
Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 4 PM
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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.