Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
248 am PDT Thursday Sep 3 2015
Synopsis...an upper low will remain centered in southwest b.C.
Today...maintaining cool onshore flow and a chance of showers and a
slight chance of thunderstorms especially over southwest Washington
and far northwest Oregon. The low will begin its move south tonight
and continue moving south Friday as energy on the west side of the
low digs south. The best chance of showers or even a few
thunderstorms will be over the coastal waters tonight into Friday
morning. However...the low will be over western Oregon on Friday...
and this should lead to a chance of showers or thunderstorms over the
Cascades and possibly even the coastal mountains in the afternoon and
evening hours that might drift over the valleys. The low will move
east Saturday for improving weather. The models are now showing a
weak system passing by to our north Sunday into Monday for a slight
chance of showers near the coast and locally in the north interior.
Drier and warmer weather may return for the middle of next week.
Short term...the models continue to say that the weather over the
Pacific northwest will be dominated the next couple of days by an
upper low that is centered in southwest b.C. This morning. The low
will not move much today...but continue to direct a few showers
onshore especially across the north part of our forecast area... from
about Salem northward. National Weather Service Doppler radar shows a
few showers early this morning. The models show a short wave moving
onshore from the west today. With some of the cooler air aloft from
the low extending into our area...we could see a few thunderstorms
again in the north this afternoon and evening.
The low moves south along the coast tonight and Friday as some energy
digs south on the west side of the low. The main instability and
showers tonight and Friday morning will be over the coastal waters.
The low will be over western Oregon on Friday afternoon and
evening...with northerly flow over the Coast Range and southerly flow
over the Cascades and central Oregon. The models indicate that we
should see some showers and possibly a few thunderstorms develop over
the Cascades Friday afternoon and evening...and possibly also over
the Coast Range. We might see a few drift out into the valleys as
well...but the best chance looks to be over the higher terrain.
The low moves off to the east Saturday with improving weather across
our forecast area. A few showers might linger over the higher
Cascades early Saturday...but the remainder of the day should see
more sunshine and warmer temperatures. Tolleson
Long term...no changes. Previous discussion follows...
Saturday night through Wednesday. Confidence again somewhat below
average for the start of next week with model guidance exhibiting
notable run to run variability. The overall pattern will have the the
longwave trough well into eastern Oregon by Saturday night. A few
weak upper troughs pass to the north of the area which may provide
for some light showers at times Sunday or Monday... primarily for the
north Oregon coast and SW Washington. Another upper trough may bring some
more widespread showers and slight cooling to the area on Tuesday.
Overall...with only weaker disturbances passing near the
area...heights gradually will rise to support temperatures closer to
seasonal normals and generally expect drier and warmer again for the
middle of the week and trended the forecast for Wednesday in this
direction in line with the 12z European model (ecmwf) solution. Cullen
Aviation...a cool showery unstable air mass will produce mainly VFR
conditions across the area through 12z Friday. The greatest chance of
IFR conditions developing will be at konp between now and 15z
Thursday. There is a chance MVFR conditions may back build from the
Cascades and impact Willamette Valley taf sites between 15z and 18z
Thursday...but do not think any deteriorations will last more than an
couple hours at most. There will be a slight chance of thunderstorm
impacting an interior taf site between 18z Thursday and 03z Friday.
Kpdx and approaches...a cool unstable showery air mass will produce
mainly VFR conditions through 12z Friday. There is a chance
conditions may deteriorate into MVFR conditions around 15z Thursday
for an hour or two...but confidence is low. There is a slight chance
of a thunderstorm impacting the terminal this afternoon and early
evening...but odds are less than 20 percent. /Neuman
Marine...low pressure aloft and surface high pressure over the
northeast Pacific will keep winds down and allow seas to subside
through the first half of the weekend. A dying front will turn winds
southerly on Sunday...but at this point the front appears weak enough
to bring few appreciable impacts to our waters. High pressure will
then return to the northeast Pacific. Strengthening thermal low
pressure over northern California should allow our more typical
Summer-like gusty northerly wind pattern to return early next week.
At which time...expect seas to become more wind dominated...
especially across the central Oregon waters. /Neuman
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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.