Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland Oregon
936 am PDT Thu Oct 27 2016
Synopsis...a nearly stationary front remains stretched from the
okanagan Highlands of north-central Washington to the northern California coast,
producing widespread light to moderate rain southeast of a Portland to
Newport line this morning. An upper level disturbance offshore is
expected to move into western Washington this afternoon, finally pushing the
front east of the Cascades, but also bringing more showers this
afternoon. Showers should decrease this evening, with a lull in
precipitation expected Friday. The next low is expected to set up
off the coast Friday night, with another round of unsettled weather
expected through the weekend and possibly into early next week.
Update...quasi-stationary front remains stretched from the okanagan
Highlands of north-central Washington to the far northern California coast this
morning. This front continues to pound the far northern California coast with
heavy rain, with 3-5 inches over the past 24 hours in Crescent City
and Brookings. Rain is not nearly as heavy across our district, as
the deepest moisture seems to be focusing to our south. However
enough moisture is left over to produce fairly widespread rain along
the front up through much of western Oregon, primarily southeast of a
Portland to Newport line.
The steadier rain should slowly start to shift eastward as an upper
level trough off the Oregon coast pivots NE toward the Washington coast,
nudging the stalled frontal zone eastward. Cool air aloft associated
with this disturbance will keep some showers around behind the front
through this afternoon, but instability is starting to look fairly
capped around 500 mb even in the more unstable models that show
500 mb temps getting down to around -24 deg c. Decided to remove
thunder mention from the Willamette Valley and pdx Metro, but there
may still be just enough updraft closer to the coast to support stray
thunder so we decided to leave it in there for this afternoon and
early evening. Showers should decrease overnight, though a following
disturbance could continue to clip the north coast with a few showers
Otherwise, few changes were made to the forecast. Lowered the snow
level a little to around 5500-6000' for the rest of this morning, as
some of the higher elevation webcams were showing a little dusting of
snow. Impacts should be minimal below 6500' elevation.Weagle
Aviation...stalled cold front has moved little over the last
several hours but should begin to drift eastward around 19z. Have
seen pockets of MVFR 020-030 cigs as moderate rain passes over
the terminals but overall impact to operations appears limited.
12z model runs have weakened overall instability inland but
maintained a minimal threat. Still have potential for stronger
showers, and an outside threat for a thunderstorm, this afternoon
and early evening where occasional MVFR cigs will be possible.
Better convective threat will remain near the coast and over the
waters. Skies looks to somewhat clear overnight which may allow
for some 015-025 MVFR cigs to develop mainly north of ksle after
roughly 28/09z. Fog appears somewhat probable for keug and khio.
Kpdx and approaches...VFR with rain transitioning to showers
today. With unstable air mass, could see MVFR cigs under a few
heavier showers with an outside chance of a thunderstorm within 20
nm mainly 20z-03z. /Jbonk
Marine...a couple of notable changes for the morning package.
First, seas are hovering in the 8-10 ft/8-10 second range. Most of
the dominant wave periods are being driven by the winds so have
decided to combine the Small Craft Advisory wind threat with a
hazardous seas threat. As such, left timing for the central
Oregon coast waters along, but decided to extend the northern
coastal waters through early Friday afternoon when there is a more
notable break in the winds. Would then also expect the dominant seas
to revert toward the longer period swells. Previous marine
discussion follows. /Jbonk
Previous marine discussion...not a lot of change in the overall
pattern. Broad area of low pressure remains over the Gulf of
Alaska. This will continue to send fronts barreling towards the
pacnw through the weekend. Jet is such that the main lows that
spin up well offshore will continue to remain well offshore and
move to Vancouver Island.
Currently, continue to see southerly winds 20 to 25 kt on
the waters, with a south to southwest swell of 10 to 12 ft. Seas
will subside today. There will be somewhat slacker winds for
awhile this am, but winds will pick up again later today as
another weak cold front moves across the waters.
Conditions then look quieter over later Fri into the weekend with
winds generally 15 kt or less. Seas 6 to 8 ft Fri, but another
fetch of westerly swell will arrive Fri night and Sat, with
overall seas running 10 to 12 ft.Rockey.
Previous discussion... /issued 246 am PDT Thu Oct 27 2016/
Short term...(today through saturday)...National Weather Service
Doppler radar continued to show plenty of echoes and steady rain from
the central Oregon Coast Range and the south Willamette Valley
northward toward the South Washington Cascades early this morning.
This band is starting to make some slow eastward movement. The models
show an upper trough off the coast will swing northeastward through
the forecast area today, which should push the front eastward with
decreasing rain. However, this trough will spread some showers and
instability through the area by afternoon, with a small chance of
thunder especially north, and a little slower arrival than the models
The models then show the southern end of the band out near 140w will
swing northeast and brush especially the northwest half of the
forecast area with some more showers tonight. This system lifts north
of the area by Friday morning.
Friday will see a lull in precipitation, with dry conditions possible
in much of southwest Washington and northwest Oregon. There might be
a few residual showers near the coast and over the Coast Range. There
is a possibility of some fog and low clouds Friday morning as well.
The next low sets up off the coast Friday night and remains anchored
off the coast Saturday. The models show a good shot of precipitation
spreading northward overnight Friday night and Saturday morning in
the resulting southerly flow, with some moisture likely lingering
into the afternoon Saturday as well. Tolleson
Long term...no changes. Previous discussion follows...
(saturday night through wednesday)...12z deterministic models were
generally in good agreement through Sunday, bringing the next large
upper low onshore as an open wave for more cool and unsettled weather
over the weekend. Shortwave ridging may result in a brief lull in
shower activity Sun night or Mon, but models diverge quickly at that
point and beyond. One thing models do agree on for early next week is
that at least some sort of troughing will persist over the NE
Pacific, but details vary widely between the generally drier 12z
European model (ecmwf) and the wetter GFS which keeps the flow pattern more
progressive. Will hold off on committing to either solution right
now, keeping pops and temps fairly close to climo Mon-Wed but hedging
slightly toward the cool/wet side. Weagle
Pz...Small Craft Advisory for winds and hazardous seas until 1 PM
PDT Friday for coastal waters from Cape Shoalwater Washington to
Cascade Head or out 60 nm.
Small Craft Advisory for winds and hazardous seas until 6 am
PDT Friday for coastal waters from Cascade Head or to Florence
or out 60 nm.
Small Craft Advisory for rough Columbia River bar from 1 PM to
5 PM PDT this afternoon.
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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.