Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland
301 PM PDT Thursday may 28 2015
Synopsis...high pressure aloft will be over the forecast area
through Saturday. Meanwhile a closed upper level low will move into
British Columbia. Warmer than average weather will last through at
least Saturday...with high temperatures most likely peaking on
Friday. The upper ridge starts to flatten Friday and Sat. The higher
Cascades will continue to have a slim threat of late-day
thunderstorms the next couple of days. The next upper low swings
toward the Oregon coast Sunday...resulting in a little more cooling
but a better chance of Cascade thunderstorms. A much deeper marine
layer is expected over the area early next week.
Short term...tonight through Sunday...an upper level ridge is
building over the Pacific northwest today. This ridge will bring warm and dry
weather to the Pacific northwest through Sat. A closed low will move into b.C.
Over the next couple of days...which will act to flatten the ridge a
bit...but otherwise have little impact on our sensible weather.
Warm and dry weather is in control today with plenty of sunshine
across most of the region. The exception is at the coast...where
marine clouds and a cooling northwest breeze are keeping temperatures in the 50s
and 60s. Temperatures are steadily climbing over the interior...with
current observed temperatures in the upper 70s and low 80s for most of The
Lowlands. Expect highs to top out in the low to middle 80s later this
afternoon. Visible satellite imagery has shown cumulus building along
the Cascade crest since around midday. There is a slight chance that
a few of the cumulus buildups may develop into thundershowers during
the late afternoon and early evening.
Friday looks to be very similar to today. Expect a weak marine push
overnight...with clouds only able to push into the Coast Range gaps
and a little way up the Columbia River...leaving most of the interior
cloud free. Friday looks to be the warmest day of the stretch with
afternoon highs a degree or two warmer than today. A slight threat of
late afternoon/early evening thunderstorms along the Cascade crest
will continue as well.
The upper ridge axis remains over US for one last day on Sat. Expect
it will still be warm...but it appears that high clouds will be
arriving during the day ahead of the next approaching upper level
low. Although 850 mb temperatures are similar to today and Friday...expect
these clouds may knock surface temperatures down a few degrees. The flow
aloft will begin to turn southwesterly. There may again be a few
showers and thunderstorms during the afternoon...with the main threat
remaining along the Lane County Cascade crest.
Upper level low pressure will approach the coast from the SW on sun. This
will bring a further increase in cloud cover and cool temperatures a little
more. The forecast models also indicate a better chance of afternoon
thunderstorms in the Cascades as the upper level flow turns
Long term...Sunday night through Thursday...the first and strongest
upper level trough will slide north-NE into the region late Sunday and
continue to enhance shower chances through the early part of next
week. The associated front will cross the region overnight providing
for much cooler high temperatures on Monday given the abundant cloud cover
and models suggesting 800 mb temperatures decreasing around 10 degree c by Monday
morning. While models suggest some instability...particularly
Tuesday as the cold core aloft moves across the area...think much
cooler surface temperatures will largely limit thunder chances.
However...the timing of shortwave impulses may enhance this
potential at times but confidence in this scenario remains low
enough to exclude any thunderstorms in the extended period for now
with the exception of maintaining a chance for Monday late
afternoon/evening in the Cascades south of around Montana Hood.
Temperatures likely rebound a little...closer to seasonal
norms...for the second half of the week as showers end and weak
shortwave ridging providing for some afternoon sun after morning
Aviation...VFR conditions continue under dry northwesterly flow
aloft...with a slight chance of thunderstorms over the Cascades
through 04z. MVFR stratus possible between 12-18z along the
Columbia River. Marine stratus along the coast will bring a
mixture of IFR/MVFR ceilings...with IFR visibility possible after 09z.
Kpdx and approaches...VFR conditions continue through Friday.
Thunderstorms possible over the Cascades through 04z. MVFR
stratus possible between 12-18z. /64
Marine...benign conditions continue through the forecast period.
Winds will remain generally below small craft criteria. Mixed seas
around 4 to 6 feet will continue through much of next week...with
swell trains out of the west and SW. /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.