Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
933 am PDT Wednesday Jun 3 2015
Synopsis...an upper level low pressure area will remain over the
Pacific northwest today for continued rather cloudy conditions and a
few showers. The upper low will move east and weaken tonight and
Thursday as the next upstream system dives south off the coast and
develops into a closed low over California on Thursday...remaining
mostly to our south through the weekend. This will allow a strong
upper level ridge to move into the Pacific northwest from the west
that will cause a transition to much much warmer and drier weather
for late in the week and this weekend. The warmest weather of the
year so far is likely this weekend...possibly continuing into Monday.
We may start to cool things off as we get into the middle of next
Update...visible satellite imagery shows cloudy skies across most of
the forecast area this morning with a growing patch of fewer clouds
in the western valley from about Salem south. Expect even this area
to mostly fill back in later this morning though...but could see a
few additional sun breaks middle to late afternoon. Have updated sky
grids to reflect this trend. Latest radar shows light rain showers
particularly across northern parts of the forecast area. With an
upper low moving off to the northeast into BC/Alberta...lowered
morning probability of precipitation for southern sections of the forecast area but afternoon
probability of precipitation look reasonable. Bowen
Short term (previous discussion)...the models continue to indicate
that an upper low will continue to remain over Washington today as it
also affects a good part of northern Oregon. Infrared satellite
imagery shows wraparound cloudiness on the back side of the low off
the Washington coast continuing to spread into northwest Oregon with
a few showers. The forcing today looks a bit weaker though and there
is still a weak area of vorticity in the clouds along the coast that
will slowly move inland today. So expect fairly cloudy conditions to
continue today...possibly a few breaks in the clouds in the
south...with a few showers mainly across the north and over the
higher terrain. The showers should be less numerous today as well.
Temperatures will remain cool...but may rise a few degrees into the middle 60s
The next upstream system currently approaching 130w continues to be
forecast to dig south off the coast and move into California as an
upper low later tonight and Thursday. In its wake an upper ridge will
start to nose into the Pacific northwest that should bring some
clearing Thursday afternoon after areas of morning clouds. 850 mb
temperatures suggest highs inland on Thursday will push up into the middle
70s...a bit above seasonal normals.
The upper ridge will continue building into the Pacific northwest
Thursday night and Friday...with 500 mb heights rising to nearly 580
dm...approaching summertime values. After brief morning clouds...
look for abundant sunshine on Friday with temperatures rising to between 80
and 85 degrees inland in the afternoon. Tolleson
Long term...Friday night through Tuesday...the strong upper ridge
will be over the Pacific northwest in force over the weekend...with
500 mb heights in the 580s and 850 mb temperatures exceeding 20 degrees c.
Thus it looks as though temperatures should reach 90 over the weekend...
possibly the middle 90s on Sunday. The upper low over California is
forecast to stay south...but we may need to watch whether any
thunderstorm moisture spreads back north close to our Lane County
Cascades. With tropical activity beginning to pick up in the Pacific
basin...model consistency is not the best with how long the hot spell
will last into next week and for when the next trough will approach
and cool US down. Current trends suggest that Monday will stay quite
warm especially inland...and have trended the forecast that way. We
may finally get some more onshore flow around the middle of next
week...but the timing is uncertain as the European model (ecmwf) and GFS models start
to deviate then. Tolleson
Aviation...weak upper trough remains over region with some
enhancement of showers moving over SW Washington and Portland
metropolitan terminals associated with an embedded upper low. General mix
of VFR and MVFR ceilings across the region with lower MVFR to IFR ceilings
in the metropolitan area with higher showers. Expect MVFR ceilings to prevail
along the coast today with occasional lowering to IFR possible.
Relatively deep moisture inland will provide for ceilings remaining
across much of the interior...particularly north of ksle in the
2500 to 3500 feet range...trending towards the higher end by late
morning through the afternoon. Satellite imagery indicates some
clearing in the central and South Valley as of 16z but suspect
ceilings will return at times...but remain VFR. Widely scattered
showers likely continue through today.
Kpdx and approaches...lower ceilings around 1000 feet continue this am.
With abundant moisture through the lower atmosphere this may
linger for another few hours along with a few passing showers.
Expect these lowest ceilings to scatter out 17-18z becoming VFR with
ceilings generally 3500 feet this afternoon and into the evening.
Surface winds will be northwesterly and generally below 10 knots. Cullen
Marine...high pressure is building over the NE Pacific and over
the coastal waters today with light northwest winds developing.
This high pressure system will continue to strengthen and persist
over the waters through this weekend into early next week bringing
persistent gusty northerly winds over the waters...weakening a bit
during the morning hours and ramping back up during the afternoon
and evening hours each day. The strongest winds are expected over
the central Oregon waters...with Small Craft Advisory gusts up to
30 kts starting Thursday afternoon and likely persisting through
at least Sunday. The northern waters could stay below advisory
thresholds until Friday afternoon...with gusts over the weekend
likely staying a bit lower and only reaching around 25 kts. There
may be periods during the morning hours over the northern waters
that winds drop below criteria...but there remains some uncertainty
with respect to how much winds may ease and how long these periods
Seas will remain 3 to 5 feet today with a mixed swell...a long
period SW swell and a shorter period west swell. As winds start to
increase out of the north...expect seas to become increasingly
wind wave dominated...building to around 6 to 8 feet by Thursday
afternoon...and remaining in this range through the weekend.
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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.