Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
130 PM PST Friday Mar 6 2015
Synopsis...an upper level ridge of high pressure will remain over the
region through the weekend and into early next week. This will
maintain a dry and warmer than normal weather pattern. Around middle
week next week a chance for rain returns as a Pacific system is
expected to break through the ridge.
Short term...the strong upper ridge...seen in water vapor pictures
just offshore early this afternoon...is expected the remain through
the weekend and into early next week. Models agree on turning the
low level flow around to a more northerly direction tonight and
Sat...resulting in continued dry and relatively mild conditions that
will last through sun.
A surface high centered off the coast of Washington and Vancouver Island
over the weekend is expected to sink S some Monday as a shortwave
riding over the top of the ridge flattens it. This will turn the low
level flow lightly onshore across at least the north part of the
forecast area Monday moderating temperatures a bit mainly along the coast. GFS
and NAM model soundings indicate some shallow moisture along the
coast Monday with the onshore flow...which may result in some patchy
low clouds along the coast.
Long term...Monday night through Thursday...forecast models remain
in fair agreement that the persistent upper ridge will break down
and slide eastward early next week. This will bring a transition to
more normal March weather and a return of some precipitation. While
the models continue to struggle a bit with the precise handling of
the low pressure system and associated cold front as it moves
onshore...this seems to be a high pop and low-moderate quantitative precipitation forecast event
with rainfall accumulations around a half to three quarters of an
inch at most. The system will be fairly warm so while snow levels
may fall enough to add some snowfall to the higher Cascade
locations...expect only rain below around 6000 feet or so and no
significant addition to the meager Cascade snowpack this winter.
Aviation...VFR next 24 hours. Patchy fog returns in the south
Willamette Valley after 06z Sat...primarily impacting keug. Some
local fog possible along the south or coast and along the Columbia
Kpdx and approaches...VFR prevailing next 24 hours. /27
Marine...with high pressure prevailing over the waters through
the weekend...expect benign conditions for the coastal waters.
Light southerly winds will transition to northerly for the
weekend with 15 to 20 knots gusts over the offshore waters Sat night
and sun. A closed upper low over the central Pacific Monday drifts closer
to the California and S Oregon coast Tuesday and Wednesday.
Seas holding in the 3 to 5 foot range through the 5-day forecast
period. Spectral analysis suggests there will be multiple 2 or 3
foot components through tonight...primarily from the west-northwest and south-southwest.
Guidance has come down on seas in the extended period around next
Wednesday...and now shows 10 to 11 feet which seems more
reasonable than the teens it was showing earlier. Tuesday night
and into Wednesday appears to be the next chance for any
significant winds or seas.
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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.