Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
315 am PDT Tuesday Sep 16 2014
Synopsis...the upper ridge that had been over the Pacific northwest
the past several days has weakened and is moving east as a
circulation along the South Washington and north Oregon coasts that
is producing a few showers or thunderstorms moves north of our
forecast area later this morning. A more substantial low is centered
well off the southern Oregon and northern California coast. Some
weak energy may spread north into our area from the low the
remainder of today and tonight...but more substantial energy will
likely spread in later Wednesday through Thursday for a better
chance of showers. The southern part of the low is heading inland a
bit to our south...so that may limit the coverage and quantitative precipitation forecast some. A
weakening system will approach the coast Friday but weaken
considerably as it tries to move onshore due to a building upper
ridge. Then the models are in much better agreement with the upper
ridge building significantly this weekend for dry and warmer
weather...and possibly approaching 90 in the inland valleys again.
Short term...the upper ridge of the past few days has weakened and
its axis has moved east of our forecast area. An upper level
circulation was along the north Oregon and South Washington coasts
early this morning...and was producing an area of showers along the
coast. There was one lightning strike so far near Cannon Beach.
There were also a couple of cells that moved north along the Cowlitz
and Skamania County borders as well. Will keep a few showers or
thunderstorms in the forecast for early this morning mostly north of
the Columbia River...except will include a small area south of the
Columbia River along the coast...before the circulation moves north
of our area near or just after sunrise.
This circulation has spread marine clouds north along the coast that
might produce patchy drizzle along the coast this morning. The low
clouds will have a chance to push into the interior this morning as
well. Nevertheless...expect the increased onshore flow should drop
temperatures inland today down around 80 or so.
A more significant upper low is located off the southern Oregon and
northern California coasts. The remainder of today some weak energy
will spread north but is mostly off the coast...and is likely
represented by the higher cloud band near the southern Lane County
border early this morning. The deeper moisture looks to stay
offshore...so will keep the rest of today on the dry side.
The low starts to make a move toward the coast later tonight and on
Wednesday as a more substantial short wave swings around the base of
the low and approaches the coast from the south Wednesday afternoon
and evening. Our forecast ramps up the probability of precipitation mainly into the chance
category as this occurs. There is a small chance of thunder in the
Oregon Cascades late Wednesday but this will be dependent on how deep
the marine air ends up being inland.
The low splits somewhat as it moves onshore and to the east on
Thursday...with the main circulation moving onshore to our south.
Chance probability of precipitation still look reasonable into Thursday.
Temperatures by Thursday may ease down to seasonal normals...which have
fallen into the middle 70s in the interior.
Smoke from the 36 pit wild fire southeast of Estacada will move off
to the north and northeast today and Wednesday...then more to the
east Thursday and Friday as the upper flow turns more westerly.
Hopefully higher humidities and lower temperatures the next few days help
fire fighters. Tolleson
Long term...Thursday night through Monday. An upper ridge begins to
build again over the northeast Pacific and causes the next system to
mostly fall apart as it approaches the Pacific northwest Friday...
allowing temperatures to begin to recover from the expected values
Thursday. The models are in excellent agreement that the upper ridge
will then move into the Pacific northwest in earnest over the
weekend...with another thermal trough building up the coast for more
substantial warming. Temperatures inland will have another chance of
approaching 90 again inland. We may see some moderation again early
next week as the flow aloft turns southwesterly and allows onshore
flow to develop...but any chance of precipitation may hold off until
the middle or latter part of next week at the earliest. Tolleson
Aviation...a passing upper level shortwave trough may trigger
isolated elevated thunderstorms through the early morning
hours...with little to no precipitation expected to reach the ground.
Otherwise...the latest observations show that S/SW onshore winds have
developed across most of the region. MVFR stratus remains along the
coast with widespread VFR over the interior. Expect the coastal
stratus to persist through much of the morning. It is possible...but
looking less likely at this point...that some patchy MVFR stratus may
also form in parts of the southern Willamette Valley during the early
Expect the stratus to scatter this afternoon...with predominantly VFR
conditions across the region into the evening. MVFR/IFR stratus may
reform along the coast during the middle to late evening...but the
interior will likely stay VFR through 12z.
Kpdx and approaches...VFR through tonight as variable high clouds
drift overhead in the S to SW flow aloft. Pyle
Marine...fairly benign conditions persist over the waters for the
next few days. Weak low pressure over the NE Pacific will maintain a S to SW
wind for next several days...but generally 15 knots or less. There will
be a mix of wave trains with varying periods...out of the northwest west and
SW. Expect overall wave heights to increase gradually today and
remain in the 5 to 7 feet range from tonight through Thursday.
High pressure will rebuild over the waters Friday and Sat...with another
thermal low pressure over northwest calif. This will bring another round of north
winds to the waters...possibly reaching near Small Craft Advisory
levels at times. Pyle
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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 as the forecast area.