Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
834 PM PDT Wednesday Sep 17 2014
Synopsis...low pressure off the north calif coast will slowly weaken is it
moves into north calif through Thursday evening. Southerly flow over the
region will maintain clouds and scattered showers. Drier weather for
Friday as high pressure builds...with much warmer and drier conditions
this weekend. Cooler unsettled weather returns next week.
Short term...no changes in overall forecasts and trends. Most
showers this evening continue to be over SW Oregon and along the
coast. Decent rainfall over SW Washington and Willapa Hills where
0.25 inch fell at Huckleberry RAWS in hills above Longview...and
about 0.15 inch near Hoquiam. Not much elsewhere...with only a few
hundredths along the Oregon coast and trace amounts over the inland
valleys. Overall...showers will continue to stream northward
tonight...but again best threat of actual rainfall will remain over
coastal areas down into SW Oregon.
Will maintain minor threat for a thunderstorm or two this evening
over Lane County Cascades as weak anomaly and the left exit region
of a 90kt jet will brush that area tonight as it moves across
For Thursday...models continue to hint that low level warm air
advection just offshore and a subtle low to middle level boundary will
concentrate shower activity across far northwest Oregon and southwest
Washington late tonight into Thursday morning. When coupled with
increasing westerly flow at 850mb...measurable rainfall or more than
0.01 inch looks likely on the coast...Coast Range and Cascades.
Wetting rains which are rainfall of at least 0.25 inch still do not
appear all that promising...but there is certainly a chance
especially near the coast.
For Friday...building 500mb heights Thursday night and Friday should
result in conditions drying out considerably across most of the area.
The lone exception to this will be across extreme northwest Oregon
and southwest Washington where a dying front pushing into the region
from the northwest may bring a shower or two Friday.
For Saturday...thermal low pressure over northern California will
build northward up the coast...bringing increasing offshore flow and
a return to much warmer temperatures. Areas of smoke will once again
become a concern over the Willamette Valley during this period.
Long term...Sunday through Wednesday...Sunday warm temperatures
will continue due to continued easterly flow. Sunday afternoon winds
will shift to more southwesterly with a marine surge along the coast
bringing clouds back to the coast. Southwesterly winds and marine-
sourced air will cool temperatures for Monday and Tuesday ahead of
our next weather system...an upper-level trough approaching the West
Coast on Monday. The GFS...ECMWF...and dgex are in good agreement
that while the longwave trough stalls in the Gulf of Alaska...a
series of shortwaves sourcing from the parent low off the West Coast
will bring several chances for rain across our area Tuesday and
Wednesday. The models recently have tended to generate a lot of
moisture further in the forecast period...then back off as the event
approaches. For this reason...I am hesitant to commit to this being
a productive rainfall event. -McCoy
Aviation...middle and upper level clouds continued to stream north over
western Oregon this evening. Conditions were primarily VFR across
the region...except for some spotty MVFR conditions in light rain
and fog along the coast. VFR conditions expected to continue most
areas through 18z...although some areas of MVFR conditions are
likely to develop...primarily near the coast after 09z.
Kpdx and approaches...VFR likely to continue through 18z.
Marine...winds expected to remain below small craft criteria
through Friday morning. A trough of low pressure developing north up the
coast late Friday and Friday night will strengthen north winds...potentially
up to small craft criteria Friday night into early Sat. A return
to quieter conditions is likely to return for the rest of the
Seas expected to remain in the 5 to 7 feet range for the next few
days...but there are some indications a westerly swell arriving
Sat could drive seas up close to 10 feet.
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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.