Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
300 am PDT Tuesday Jun 2 2015
Synopsis...a broad area of low pressure remains over the northeast
Pacific and the Pacific northwest this morning...and will linger over
the area into Wednesday as several embedded circulation centers
affect the forecast area for some continuing showers. Strong onshore
flow will keep conditions cloudy and cool as well. A strong upper
level ridge of high pressure is expected to build over the Pacific
northwest later this week and continue through the weekend...likely
bringing the warmest temperatures so far this year to much of
southwest Washington and northwest Oregon.
Short term...a broad area of low pressure will continue over the
northeast Pacific and the Pacific northwest today...gradually moving
mostly onshore over Washington and northern Oregon Wednesday...
maintaining the deep onshore flow and the cool temperatures. The low
will move to near the Idaho border by Thursday as the next upstream
system digs south into California...gradually improving our weather.
Initially...several circulation centers will rotate about the low and
modulate the showers over southwest Washington and northwest Oregon
today and Wednesday.
One circulation center was north in western Washington...the feature
that brought all the showers and thunderstorms to our area Monday...
but today was causing some showers to wrap around from the west
across the northern part of our forecast area this morning.
Then there is another circulation center about to move onto the
central and southern Oregon coast best seen in the water vapor
imagery...that had been the main offshore low center a day or two
ago. This feature will also reinforce the onshore flow and keep some
While there is a significant marine inversion that will be a cap to
prevent any thunderstorms from developing today...we do expect an
increase in showers this afternoon over the forecast area from the
daytime heating...such as it is...that will help to overturn the
boundary layer air mass.
There is one last circulation center moving through tonight and
Wednesday morning as the low lingers not too far from our area
through Wednesday. Clouds may try to thin a bit Wednesday afternoon
but conditions will likely stay on the cloudy side with some
afternoon increase in the showers again from daytime heating.
The next upstream system is forecast to dive south toward California
Wednesday night and Thursday...mostly missing our forecast area.
After some morning clouds...the models indicate and the surface
pressure pattern with its surface ridge axis and inflection Point
North of the Columbia River in the afternoon suggest some afternoon
clearing finally. Temperatures inland should rebound into the middle 70s
Long term...no changes. Previous discussion follows...
Thursday night through Monday...12z GFS and European model (ecmwf) both show the deep
upper level trough getting pinched off and becoming a cut-off low
over northern California late Thursday. As this cut-off low digs
further south into California...an upper level ridge develops over
the east Pacific...attempting to set up a Rex block over western
Continental U.S. Friday and through the weekend. Expect gusty northerly winds
along the coast...periods of morning clouds for coastal locations and
along the lower Columbia River...and temperatures likely warming well
above average towards the weekend. Models are showing 850 mb
temperatures hovering at or just above 20 degree celsius for Saturday
and Sunday...which would allow surface temperatures to flirt at or
just above 90 degree fahrenheit. /27
Aviation...coastal sites continue to have MVFR/IFR ceilings today...
with MVFR visible possibly dropping to IFR with and area of showers
moving through this morning. Inland...generally VFR early this
morning but a shortwave moving through this morning will bring
showers and increased areas of MVFR ceilings through about 20z-22z Tuesday
before gradually returning to VFR. Reduced visible possible with heavier
Kpdx and approaches...primarily VFR through about 15z Tuesday then
increasing showers likely to bring occasional MVFR ceilings around
2000-2500 feet possibly down to 1500 feet with heavier showers.
Conditions are expected to trend toward VFR after 20z./26
Marine...a surface trough will be moving through the waters this
morning bringing increased south to southwest winds during the
morning hours. The 06z ascat wind pass matches well with models. So
if model trend of further developing the trough holds...wind gusts
may peak near 20 knots for the waters south of Garibaldi this morning.
Later today and tonight high pressure starts to build over the
waters. Models are suggesting that winds may get up above 20 kts by
Thursday afternoon/evening...with seas building to around 6 to 7 feet.
A Small Craft Advisory for winds may be needed Thursday afternoon
through the weekend...especially over the central or waters as winds
are forecast to stay above 20 knots. During this period there may be
short breaks during the morning hours with weaker winds...before
winds ramp back up during the afternoon hours. Gusts up to 25 to 30
kts will be possible during the afternoon hours...especially over the
central or waters. /26
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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.