Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
948 PM PDT Friday may 29 2015
Synopsis...a flat upper level ridge is extending into Oregon from
the Great Basin...keeping temperatures above normal except at the
coast. The ridge weakened slightly today...allowing the marine layer
to deepen slightly for slightly cooler temperatures than previously
expected. Onshore flow this evening may push some low clouds through
the coastal gaps and locally into the Willamette Valley...mainly
Salem south. However the upper ridge will strengthen again Saturday
and early Sunday...pushing temperatures well into the 80s inland
Sunday. West-southwest flow aloft and subsidence from the ridge
should keep thunderstorms east of the Cascade crest Saturday. The
ridge should start to break down late Sunday...allowing an increasing
chance of showers and thunderstorms Sunday night into Monday.
Cooler...showery weather is then expected early next week.
Short term...tonight through Sunday...flat upper level ridging
continues to dominate southwest Washington and northwest Oregon...
though some wrinkles have developed which make the forecast rather
challenging the next 24-48 hours. Temperatures today were generally a
few degrees cooler than expected...though still well above normal
east of the Coast Range. The reason for this is two-fold...first
there was extensive high cloudiness which limited daytime heating.
Second...a weak and disorganized shortwave appears to be moving
onshore along the Oregon coast. This shortwave appears to have
weakened the upper ridge just enough to allow the marine layer to
deepen some. This in turn caused onshore flow to be able to push
through the gaps in the Coast Range...which capped temperatures in the 70s
for Kelso...Corvallis...and McMinnville. These locations have already
dropped to 60 degrees or cooler this evening...while Portland remains
The result may be more extensive low clouds intruding into the
Willamette Valley tonight than previously expected. It is difficult
to tell how far up the Columbia the low clouds are on satellite fog
product due to the higher clouds streaming overhead. Suspect they
will at least make it to Kelso overnight...but it appears the better
chance of stratus making it into the Willamette Valley will be Salem
southward. Increased the forecast sky cover across the southern half
of the County Warning Area Saturday morning as a result.
Another interesting wrinkle associated with this shortwave is some
middle-level clouds which have recently developed around kpdx...
completely unforecast by even the 00z NAM bufr soundings. Also of
interest are some radar echoes moving onshore in southern Oregon...
from a batch of elevated but shallow convection which showed up
nicely on the last few frames of visible satellite imagery before
sunset. These appear to be virga...and with bases high enough such
that the clouds are entirely well above the freezing level and thus
unlikely to produce lightning. Will need to keep a close eye on this
but at this point it appears the chance of these producing anything
other than virga is less than 10 percent as they move across Lane
It appears the shortwave should race east of the Cascades Saturday
allowing the upper ridge to start re-amplifying across Oregon later
Saturday into Sunday. This should start to compress the marine layer
to the point where any low clouds which pushed inland during the
morning should have no problem burning off in time for it to be
another warm day inland. However it does appear SW-NE gradients will
be enough to keep temperatures somewhat in check Saturday afternoon...and
MOS guidance has been trending downward with inland highs mainly in
the upper 70s. Did not quite go this cool...but adjusted forecast temperatures
down a few degrees as a result. With the upper ridge amplifying
across the region...and flow aloft decidedly from the southwest...
any afternoon thunderstorms should be kept east of the crest Sat.
Depending on how much cloudiness there is...Sunday may actually end
up being the warmest day of this warm spell...or not. Models have
been fairly consistent on the timing of pushing the upper low now
near 35n/145w northeastward into Oregon Sunday night/Monday. The
upper low opens up into a negatively tilted trough as it approaches
Oregon from the southwest...with the upper ridge axis out ahead of it
holding off until midday Sunday before it crosses the Cascades. 00z
GFS keeps 850 mb temperatures up around +14 to +15 degree c Sunday with very
little low-level onshore flow until Sunday evening. The 12z European model (ecmwf) and
00z NAM are even warmer with 850 mb temperatures in the +15 to +17 degree c
range over the Willamette Valley Sun afternoon. If cloud cover holds
off...inland valleys should warm well into the 80s. The surface
heating and increasingly diffluent flow aloft should provide a nice
trigger for thunderstorms to develop along the Cascades Sunday
afternoon and evening. Some model runs have hinted at the idea of
enough middle-level moisture sneaking in for nocturnal thunderstorms
Sunday night...but our forecast keeps these confined to the Cascades
for the time being. Weagle
Long term...Monday night through Friday...by Monday night the base
of a negatively tilted upper level trough will be passing over the
region. The cold core of the upper low will move from west-southwest
to north-northeast across the forecast area by early Tuesday and
bring continued showers and a slight chance of thunder in the
Cascades. A shortwave trough may provide for additional showers on
Wednesday so maintained a slight chance of showers across much of
the area. Confidence is somewhat lower for second half of the week.
Expect Thursday to generally be dry for most locations but models
suggest the next upper trough will strengthen into a closed low.
While the GFS maintains this low centered over Washington and Oregon
for the end of the week...the European model (ecmwf) centers the low over California
and into the southern Great Basin...which would enable some ridging
to build over the Pacific northwest. Therefore given this lower
confidence forecast...elected to maintain the current forecast of
temperatures returning to near seasonal norms and areas of slight
chance probability of precipitation.
Aviation...dry SW flow continues aloft...with variable high
clouds streaming in over the region this evening. Expect VFR
conditions inland through Sat afternoon. A SW surge overnight
could allow marine stratus to filter into the South Valley...with
MVFR ceilings between 1000-2000 feet possible after 09z. MVFR ceilings
along the coast will become IFR after 06z as marine stratus lowers
and thickens overnight. Periods of IFR visibility and LIFR ceilings are
possible between 11-17z.
Kpdx and approaches...VFR under variable high clouds through Sat
Marine...no changes. Previous discussion follows...high pressure
over the region will allow benign conditions to prevail across the
coastal waters. Northwest to west winds will continue 15 knots or less through
middle of next week. Seas 4 to 6 feet persist through next week. /27
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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.