Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
900 PM PDT sun Aug 2 2015
Synopsis...a strong upper level ridge continues to move east as an
upper low off the California coast steers moisture and instability
over western Oregon and southwest Washington. This moisture and
instability will continue tonight into Monday morning for a threat of
showers and isolated thunderstorms. An upper level trough along the
b.C. Coast then moves southward toward the Pacific northwest later
tonight and Monday. This trough will bring cooler temperatures and
morning clouds to the area for the middle week period.
Update...moisture and limited instability continues to flow
northward around western U.S. Upper ridge. 00z sle sounding showed
most of the moisture above 700 mb...but relatively dry down low for
negligible cape and mostly trace precipitation amounts earlier
today. Not expecting substantial change in thermodynamics
overnight... although slight increase in moisture 850-700 mb along
with weak shortwave over southwestern Oregon prognosticated to reach northwestern Oregon around
09z. Hrrr develops an area of mainly light showers in a band from
central Oregon coast to eastern pdx metropolitan between 06-08z...lifting
northward thereafter. Seems plausible and given slight increase in
lower tropospheric moisture...an isolated elevated nocturnal
thunderstorm not out of the question. Current forecast covers this
well with maximum in probability of precipitation late night/early Monday morning. Then
airmass dries from the west as Pacific trough approaches coastline
18z-00z...ushering in a period of more normal weather for most of
the week. Bright
Previous discussion... /issued 253 PM PDT sun Aug 2 2015/
Short term...convective clouds have continued to increase from the
south and southeast today as a steady stream of middle and higher level
moisture spreads north from the sierras and from Nevada...being
steered by an upper low located well off the California coast. The
clouds have been thick enough to produce a few showers across the
forecast area...and were thickest over the Coast Range this
afternoon. Satellite imagery shows plenty of clouds upstream in
northern California...the sierras...and northern Nevada that will
continue to spread north overnight into Monday morning. This cloud
cover is associated with an elongated area of vorticity that is
forecast to move through our forecast area tonight and early Monday.
The models show a decent jet segment associated with this feature as
it moves through...giving support to the idea of elevated nocturnal
convection continuing tonight into Monday morning. This is how the
forecast has been trended over the last 24 to 36 hours.
The short wave or vorticity lobe and moisture area will still be over
the forecast area early Monday before moving east in the afternoon.
Despite a rather warm air mass remaining over our area...the clouds
have suppressed temperatures though the humidity has risen quite a
bit when compared to the past couple of days. Unless more sunbreaks
develop than are now expected...it looks as if we will have trouble
reaching 90 degrees today...and so will end the heat advisory.
By Monday an upper trough along the b.C. Coast will be sagging south
along the coast toward the Pacific northwest. This will help turn the
upper flow Monday afternoon to more southwesterly...and limit the
thunderstorm potential later Monday to our southern Cascade areas and
to especially near the crest. The upper trough will continue to sag
south with more energy in Washington by Tuesday... with more westerly
flow aloft over southwest Washington and northwest Oregon.
Again...the main threat of thunder for our area will be near
Willamette Pass later Tuesday...and even that is not certain.
The marine layer was again shallow and confined to near the coast
this morning. The marine layer will likely thicken some from the
south along the coast tonight as the short wave lifts through...
possibly trying to push into the coastal gaps by Monday morning...
but inland coverage will be spotty. Expect better inland coverage of
low clouds Tuesday morning and again Wednesday morning...especially
north...with the increased westerly flow aloft and the proximity of
the upper trough to our north.
We will probably see more sunshine on Monday afternoon than this
afternoon...but with a gradual cooling of the air mass. Temperatures will
tend to be in the middle 80s to lower 90s...coolest north. Temperatures will be
closer to normal Tuesday and Wednesday under the northwest onshore
Long term...models continue to show a weak/flat upper level ridge
moving through the region for the latter half of the workweek. With
decreasing onshore flow we should see temperatures climb back into
the middle 80s by Friday. Another upper level trough looks to approach
from the northwest over the weekend. However...models continue to
differ with regards to its timing and strength. Upper level flow
should begin to turn to the southwest late Friday ahead of the
trough...so will keep a slight chance of thunderstorms for the Oregon
Cascades. As the base of the trough pushes inland we will see an
increase in shower activity across the region. Based on some
lingering instability and southwesterly flow aloft have added a
slight chance of thunderstorms to the Cascades. Temperatures over the
weekend look to fall back towards seasonal normals. /64
Aviation...mostly VFR conditions this evening. The one exception
is the central Oregon coast near konp where marine stratus has
been bouncing between VFR and IFR ceilings. There is still a slight
possibility for thunderstorms tonight through Monday morning.
Confidence is low in the location and timing of any storms that
may develop. Regardless...could get some outflow gusts around 25
Low stratus will continue to plague the coast tonight and
Monday morning...but it is difficult to see how extensive it is on
satellite imagery due to the presence of middle and high clouds. The
low clouds should move further inland along the lower Columbia
River Monday morning...possibly reaching kspb. Expect clearing
and continued VFR conditions through Monday afternoon.
Kpdx and approaches...VFR conditions through Monday. Tj
Marine...a rapidscat image earlier this evening showed gusts of
25 knots are still possible over the northern waters beyond 20 nm
out...so have continued the Small Craft Advisory that GOES through
midnight tonight. Isolated thunderstorms are possible through
Northerly winds will persist through Friday...becoming gustier in
the afternoons and evenings. The southern waters will flirt with
advisory criteria Monday afternoon...but has a better chance Tuesday
afternoon. The seas will subside tonight a tad. They are mostly wind
driven and will become steep and increase slightly with the
stronger winds in the afternoons. Tj
Pz...Small Craft Advisory for winds until midnight PDT tonight for
waters from Cape Shoalwater Washington to Cascade Head or from 10
to 60 nm.
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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.