Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
215 am PDT sun may 24 2015
Synopsis...low level onshore flow will continue for the next few
days keeping a marine air mass in place over western Washington and Oregon.
An upper level low will move slowly from southern British Columbia across eastern
Washington and into Idaho through Wednesday. A ridge of high pressure aloft will build
over the east Pacific early in the week...then shift inland late in the
week weakening the onshore flow.
Short term...marine air mass over the forecast area Sat evening was
around 5k feet deep on 00z sle sounding. With a large scale upper
trough remaining over the western states for the next few days...a
low level onshore flow will continue. Models soundings show the
subsidence inversion at the top of the marine air mass will
gradually push down over the next few days...making the marine air
mass progressively shallower. A low pressure center aloft along the
southern British Columbia coast early Sun morning is expected to move slowly southeast
across southern British Columbia and eastern Washington today through Tuesday. Time height
cross sections of relative humidity suggest some breaks may develop in the marine
clouds in the afternoon and evening hours the next few days...which
would allow temperatures to warm a few degrees above saturdays highs.
Surface reports have shown a few light precipitation reports
overnight in the northern parts of the Cascade foothills and Coast
Range. With the marine clouds and onshore flow expect some patchy
drizzle will remain possible next few mornings...but appears most
likely Monday as the upper low sinks down into Washington while marine layer
remains sufficiently deep. Will also keep some low probability of precipitation in over the
S Washington Cascades at times in closest proximity to the upper low.
Long term...no changes. Previous discussion follows. Tuesday night
through Saturday...continued onshore flow behind the Monday system
will keep cool cloudy weather through Wednesday. Most precipitation
should stay up in the higher elevations of the Cascades. Brought
temperatures back down some through the end of the week as models are now
showing the region staying under weak onshore flow. Temperatures do
warm some Thursday-Sat as shortwave ridging moves over the region. A
thermal low to our south will set up a good pattern for dry weather
for the Willamette Valley...with clearer skies in the
afternoon...and afternoon showers and possibly thunderstorms in the
Aviation...expect a near repeat of the last 24 hours. Marine
layer will remain in some form for the next 24 hours but do stand
a better shot at notable inland cloud breaks late this
afternoon/early evening. MVFR ceilings at the coast for the day
although do see hints in models at konp breaking free for a few
hours similar to Saturday. MVFR ceilings inland to start the day and
seeing the deck lift to a VFR ceiling somewhere between 21-23z. May
actually get enough mixing to see the deck scatter out late this
afternoon or early evening for a few hours before filling back in
shortly after sunset. Will then see the ceilings fall back down to
MVFR around 025 above ground level for the rest of tomorrow night.
Kpdx and approaches...MVFR ceilings 025 through about 22z. Will see
the deck lift to between 035 and 045 perhaps opening the door to a
period of visual approaches in the 25/00 to 04z time frame. /Jbonk
Marine...no significant changes...previous discussion follows. High
pressure remains offshore with low pressure over the inland Pacific northwest for the
next several days. This will continue to bring benign conditions
to the waters with northwest winds generally below advisory criteria.
There is a chance for some advisory gusts above 21 knots Sun
afternoon/evening south of Newport...but confidence is not high
that it will come to fruition. Stronger winds off of the b.C.
Coast are producing a fresh swell out of the northwest and will continue
to bring choppy seas through at least Tuesday. /64
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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.