Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
241 PM PST Thursday Dec 25 2014
Synopsis...high pressure offshore with low pressure over The
Rockies will keep the region under a cooler northerly flow through
the end of the week. An upper jet will maintain rain and mountain
snow showers through tonight. An upper shortwave trough dropping down
the British Columbia coast will bring another round of low elevation rain and
mountain snow this weekend. Colder and drier offshore flow is
expected Sunday night through Wednesday.
Short term...today through Sunday...although scattered showers
continue this afternoon...intensity has decreased significantly over
the Cascades. Any additional snowfall there will likely be no more
than a few inches and not meet advisory criteria...so will go ahead
and cancel the Winter Weather Advisory. Shower coverage and intensity
should dissipate further this evening as a strong Upper North-northwest jet along
the West Coast weakens some and moves east...taking with it a
majority of the forcing for shower formation. North-northwest 850 mb flow will
favor northwest facing slopes of the Willapa Hills...Coast Range...and
Cascades and foothills...but a few stray showers will be possible
across the inland lowlands.
High pressure builds in Friday with waning rain chances. Lowered probability of precipitation
even a bit more Friday afternoon and evening based on good agreement
among 12z model runs. Looks like moisture transport does not increase
much at all until at least 12z Saturday ahead of the next low sliding
down the British Columbia coast and into Washington. Rain and mountain snow will
move from north to south across western Washington and northwest
Oregon Saturday and into Sunday as the shortwave moves across the
area. With the snow level remaining around 3000 feet through
Saturday...the Cascades will definitely see some snow accumulation.
Expect advisory thresholds will be met but at this time do not
anticipate Winter Storm Warning snow totals. Bowen
Long term...Sunday night through Thursday...high pressure starts to
build over the northeast Pacific Sunday...moving onshore over the
Pacific northwest on Monday. This will push showers off to the
east...but bring noticeably colder air into the region. Cold easterly
winds out of The Gorge look likely to develop Monday and persist
through Wednesday. High pressure means fog will likely develop in
valley locations...though dry easterlies out of The Gorge will help
keep the north Willamette Valley clear. -McCoy
Aviation...a few showers linger in the moist and slightly
unstable air mass across SW Washington and northwest Oregon. However
conditions are generally VFR aside from spotty MVFR in showers.
Expect this to continue through early evening...then more
widespread MVFR ceilings are expected to develop overnight tonight.
Moist low-level air mass will continue mountain obscurations on
the Coast Range and Cascades through this evening.
Kpdx and approaches...generally VFR through 02z-04z...then MVFR
ceilings developing thereafter and persisting the rest of the night.
Scattered showers will bring brief periods of -ra through
tonight and possibly 3-5sm visibilities as well. Weagle
Marine...high pressure continues to build across the NE Pacific
this afternoon...with northwest flow slowly weakening with time.
Gusts to 20 knots will be possible through this evening...but should
slacken as high pressure builds inland into WA/OR. Residual swell
from the earlier northwest flow continues to keep seas 11-13 feet in the
coastal waters...but this should subside tonight and Friday as
winds ease. Will keep the Small Craft Advisory for seas as is...expiring at 6 PM...
but there is some chance it may need to be extended a couple
hours to allow seas more time to fall below 10 feet.
After a lull tonight and Friday...it appears another cold front
will push down the coast this weekend for another surge of northwest
winds and seas. Models still suggest this system will have a
small dynamic fetch as it comes down the coast...so we bumped up
seas this weekend to 12-15 feet Sat night/Sun morning.
Strong high pressure will then dominate the Pacific northwest and
adjacent waters early next week...with light to moderate offshore
flow and seas falling to 6 feet or less for much of next week.
Pz...Small Craft Advisory for hazardous seas until 6 PM PST this
evening for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 11 PM
PST this evening.
Small Craft Advisory for rough Columbia River bar from 5 am
to 10 am PST Friday.
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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.