Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Portland or
200 PM PST Wednesday Nov 26 2014
Synopsis...a weak cold front off the coast Wednesday afternoon will push
slowly inland tonight and Thursday. The front will stall over western
Oregon Thursday night and Friday...then another stronger cold front will
move S through the region Friday night ushering colder weather for the
weekend. High pressure over the interior northwest will turn the flow offshore
by sun for dry but cold weather. Sun night and Monday a system coming
up from the S will attempt to spread some precipitation up over the
cold air mass left over from the weekend.
Short term...a weak cold front was offshore Wednesday afternoon...with
layered clouds spread out ahead of the front over the Pacific northwest. As
the upper ridge over the region shifts east tonight and Thursday the flow
will turn more to the SW...with the front slowly pushing inland.
Model soundings agree in showing moisture initially in themid and
high levels...above 700mb. By late tonight though moisture begins to
spread in in the lower levels. Dynamics with this front however on
the weak side...so will be slow to increase probability of precipitation overnight inland
areas...while the coast and Coast Range will benefit from an
increasing low level south-southwest flow. With clouds limiting radiational
cooling tonight and much of the area still in the warm sector...
should be another unseasonably mild night. Thursday the front appears to
stall over the region as the low offshore digs in a bit more. With
baroclinic zone stalled and low level flow a little more onshore
will take probability of precipitation back up to likely to categorical for most of the
region. Although there is only modest cooling to air mass with the
front...clouds and expected occasional light rain will limit high
temperatures to the 50s most low elevation areas.
Late Thursday night...and especially Friday better moisture arrives as a northern
stream short wave drops down through British Columbia with a true cold front
dropping down the coast ahead of it. Moist isentropic lift in the
lower levels improves ahead of the cold front as well...best
depicted in model 290k isentropic fields. As such...potential for
rain picks up considerably by Friday.
Models in relatively good agreement in depicting the western
Canadian trough digging down into the Pacific northwest states Friday night
and Sat. All models at this point showing a vigorous Continental
type cold front moving S through SW Washington and northwest Oregon Friday night.
Pattern is reminiscent of a cold front from back on 19 November 2003
that brought low elevation snow to interior north parts of the forecast
area. Keys to doing something similar rely on some of the
precipitation falling over and behind the front as well as strong
frontogenetic forcing to allow for dynamic cooling. Model soundings
at this point indicate snow level falling rapidly behind the front
Friday night suggesting at least a mix of rain and snow possible down
to fairly low levels. Whether it would be cold enough to mix snow at
lowest elevations is anybodys guess at this point...but given the
agreement in models at this point will carry a mention of mixed rain
and snow possible for the north part of the area Friday night. Sat sees a
rather cold air mass settling in over the region behind the front...
with a lingering chance for showers...potentially mixed with snow in
the north...as the upper trough moves through.
Long term...Saturday night through Wednesday...most guidance
suggests the upper trough axis responsible for saturdays mixed
rain/snow showers will be moving across the Cascades Sat night...
with any lingering snow 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 Sun night or Monday. The 12z GFS and
Canadian models are more aggressive with this than the European model (ecmwf). 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...generally VFR across the area early this afternoon
except for areas of IFR along the coast from kast to khqm. Expect
VFR to prevail through at least the evening hours. Ceilings lower
along the coast after 03z Thursday as a cold front approaches. Look for
conditions along the coast to fall into MVFR between 04z to 06z
with precipitation increasing overnight. VFR should hold over the inland
areas through at least 12z Thursday...then MVFR conditions
increase...with higher terrain becoming obscured. Considerable
low-level moisture resides over the area Thursday which will lead to
widespread MVFR or worse conditions.
Kpdx and approaches...VFR conditions to prevail through at least
12z Thursday. Rain develops over western approaches around 10z then spreads
east thereafter...with localized MVFR developing after 13z.
Increasing MVFR or worse conditions Thursday morning through the
Marine...wind speeds early this afternoon still at 15 to 20 knots
with gusts up to 25 knots. Expect speeds to ramp up later this
afternoon ahead of an approaching frontal system. Should see gale
force wind gusts over much of the waters by late this afternoon...
but more so this evening and overnight. Although not a classic
case...some model guidance is suggesting the potential for minor
coastal jet enhancement within 20 nm. Could see some gusts up to
40 knots if this occurs. High-end Small Craft Advisory speeds persist
Thursday morning. Latest model runs show 20-25 knots northwest-north wind developing
over the north waters Friday evening then spreading further S overnight
as a strong cold front drops south out of Canada. The wind turns
offshore Sat with potential gusts 20 to 30 knots near gaps in the
coastal terrain. Wind speeds relax sun.
Seas expected to build to 10 feet this evening then remain at or
above 10 feet through at least Sat morning. Wave heights drop off
Sat afternoon as offshor flow develops. Weishaar
Pz...Small Craft Advisory for winds until 4 PM PST this afternoon for
coastal waters from Cape Shoalwater Washington to Florence or out 60 nm.
Gale Warning from 4 PM this afternoon to 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 from 4 PM this
afternoon to 4 PM PST Thursday.
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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.