Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
911 am PST Friday Nov 27 2015
Synopsis...a blocking pattern continues over the Pacific northwest
with an upper high along the southern b.C. Coast that will move
inland into southern b.C. Today through Saturday...and an upper low
over the northern Great Basin that will wobble around before moving
east early next week. This will result in dry offshore flow through
much of the Thanksgiving Holiday weekend. The dry pattern is looking
like it may start to break down during the first part of next week
and become more active during the second half of next week...though
confidence is still not high on the timing nor details.
Short term...an upper high was along the southern b.C. Coast this
morning...with an upper low over remaining over the Great Basin. This
blocking pattern is transitioning into a Rex block as the upper high
moves onshore into southern b.C. Today and Saturday...with little
movement of the Great Basin low. This is maintaining the offshore
pattern over southwest Washington and northwest Oregon... with decent
upper level northeast winds and increasing offshore surface
The only fog or low stratus this morning was near and north of Kelso
and south of Eugene along the Cascade foothills...much like
Thanksgiving morning. Gradients through The Gorge have reached 7 mb
this morning...and expect then to reach at least 8 mb today into
Saturday. Thus look for wind gusts near Rooster Rock and Corbett of
50 to 60 miles per hour...and more at Crown Point. Up valley gradients in the
Willamette Valley reached 3 mb late Thursday...and the models
indicated we should see 2 or 3 mb again today for a near repeat of
the breezes of Wednesday and Thursday.
The pattern does not change much Saturday...so expect similar
conditions to those today. The Willamette Valley gradient may ease a
little...but there still looks to be enough gradient for some
afternoon breeze. Morning fog or stratus should change much...
perhaps a slight increase but not a lot...with it mainly confined to
the south Willamette Valley and up near Kelso.
MOS keeps dropping temperatures today and Saturday versus yesterday... but
as long as enough wind continues this drop off looks too fast.
The Great Basin upper low begins moving eastward Sunday...and we lose
the upper level support for the offshore flow. The surface gradient
through The Gorge continues Sunday...so it looks like the offshore
wind event becomes more of a gap flow by Sunday with less wind over
the higher terrain or through the Willamette Valley. This may allow a
increase in coverage of fog and freezing fog Sunday morning.
Still not any low clouds or fog near Hood River or The Dalles this
morning. There might be some Saturday morning...but suspect better
coverage is possibly Sunday morning.
The low level air mass remains cold and dry...so should still see
several more mornings of temperatures well down in the 20s away from windy
areas...and a few upper teens remain possible. Tolleson
Long term...no changes. Previous discussion follows...
Sunday night through Thursday...the next system trying to impact our
block approaches around Monday...but the 12z GFS and Gem models have
gone toward the recent runs of the European model (ecmwf) taking it south of our
area...versus previous runs of the GFS that had tried to push it
through the upper ridge and bring some precipitation to our area.
While the 18z GFS reverted back to this previous wetter
solution...the 00z GFS/NAM are more in line with the 12z cycle. The
dry solution looks more reasonable given the current blocking
pattern...and have continued to trend forecasts that way. The western
Pacific is looking energetic though...and the models are trending
toward really bringing a system into the Pacific northwest around the
middle of next week. Overall...while the overall confidence in the
precise timing remains lower than average...the evolution toward a
more active pattern and renewed precipitation chances seems
increasingly likely for much of next week after around Tuesday.
Aviation...there are two slivers of fog/stratus with IFR conditions
this morning. One along the lower Columbia River valley in Washington
bordering Cowlitz and Columbia counties. The other is in the south
Willamette Valley...south of keug...impacting Cottage Grove and
Springfield.These pockets of IFR should clear by 18z. Otherwise...VFR
conditions. East winds will continue today with gusts around 35 knots
around kttd. North winds expected through the Willamette Valley this
afternoon...except for the North Valley where east winds should
Kpdx and approaches...VFR conditions next 24 hours. East winds
today...with gusts up to 20 knots possible this afternoon. East winds
with gusts around 35 knots likely near the entrance of the Columbia
River gorge. Tj
Marine...offshore winds are still gusty this morning...with
localized gusts up to 25 knots near the coastal gaps...and will
gradually decrease this afternoon through Saturday. Models have come
into better agreement on the frontal systems expected early in the
week...but am skeptical that the Rex block will break down as
smoothly and quickly as they suggest. Suspect that the first front
will be a tad weaker and slower then the current solutions which
suggest the cold front will produce 15 to 25 knots south winds Monday
morning. The second front...a warm front will bring another chance
for small craft winds Tuesday afternoon or night.
A long period swell around 4 feet will subside through Saturday.
Another long-period northwest swell will build Sunday with seas possibly
exceeding 10 feet Sunday afternoon. Tj
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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.