Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
213 PM PST Thursday Feb 26 2015
Synopsis...an upper level trough will enhance showers across the
region tonight and Friday. Snow levels will lower tonight and be near
or below the Cascade passes early Friday morning. Colder air aloft
associated with the trough will increase instability and some of the
showers may contain small hail or develop into thunderstorms Friday
afternoon. Showers will end Saturday as high pressure returns. Sunday
should be dry with possible sunshine. Another upper level
trough...weaker than the first may bring another round of rain and
mountain snow Monday before stronger high pressure settles in for
much of next week.
Short term...tonight through Sunday...water vapor imagery shows an
upper jet extending from the Gulf of Alaska to northwest Washington this
afternoon. An upper level trough...the main weather maker of interest
is just offshore the British Columbia coast. Visible satellite
imagery shows a broad area of open-celled cumulus over the NE Pacific
associated with this upper trough. These clouds indicate that the
cold air aloft accompanies the trough.
Low land rain and mountain snow across SW Washington and northwest Oregon
will increase in intensity and frequency as these two features...the
trough and upper jet...slide southward. Snow levels will start around
or just above 5000 feet...and lower late tonight to around 4000 feet
by sunrise Friday.
The precipitation will intensify between 10 PM and 3 am when the
entrance region of a 120+kt jet streak will be just offshore of
southern Oregon. This will position northwest Oregon under the left front
quadrant of the jet streak...which is favorable for enhanced lift.
This will provide a decent shot of snow for the Cascades warranting a
Winter Weather Advisory for elevations above 5000 feet where 4 to 8
inches of new snow is expected between tonight and Friday morning.
Areas between 4000 and 5000 feet may measure up to 4 inches of new
Another aspect of the cool upper trough is instability that it will
bring. So will continue with a slight chance for thunderstorms
Friday afternoon through Friday night...and expect small hail
embedded in some of the showers.
Snow levels will lower to 3000 to 3500 feet for a better chance of
snow at and below the Cascade passes Friday night. Snow fall rates
will be on the decrease by this time...but 1 to 2 inches of snow are
possible at the lower elevations. Those planning to head to the
mountains to take advantage of the recreational aspects of this new
snow should prepare for the potential of snow covered roads.
The upper trough will move further south late Friday night and form a
broad upper trough over California...Nevada...and Arizona Saturday
morning. High pressure aloft will build behind the low with showers
ending in the Pacific northwest. Colder air associated with the upper trough
combined with radiation cooling from any clearing in the clouds will
result in a chilly night Friday with minimum temperatures in the
upper 30s for the interior valleys...and down to the middle 20s for the
Cascades. Temperatures at the coast will be more moderate with
overnight lows in the low to middle 40s.
There may be some patchy valley fog early Saturday morning...with
some showers over the Cascades but the rest of the day should be dry.
Sunshine is likely Saturday afternoon as models are in great
agreement that another Rex block will try to set up over the West
Coast. Dry weather with mostly sunny skies will persist through
Sunday. However...clouds may increase Sunday afternoon and evening
from north to south as a weak upper disturbance approaches from the
Long term...another upper level shortwave trough will approach from
the north Sunday. The Middle Range models suggest that this feature will
prevent the upper Rex block from completely forming. Therefore there
is another chance for instability showers and mountain snow Monday.
This disturbance looks weaker and faster than the one we are
expecting tonight. However there is potential for lower snow
levels...around 2500 to 3000 feet with this system and although the
snow amounts may be lighter...the transportation impacts may be
greater. There is uncertainty on precipitation amounts at this point
as there is moderate model variability and run to run inconsistency
with this trough. High pressure will build in earnest behind this
system with dry weather and easterly winds expected
Tuesday...Wednesday...and possibly Thursday. Tj
Aviation...clouds and some light rain or drizzle will continue
to feed into SW Washington and northwest Oregon today and will keep
conditions generally a mixture of MVFR inland and IFR at the
coast. Additional clouds will stream across the area later today
and into tonight...maintaining MVFR conditions inland and bringing
ceilings up to MVFR at coastal sites. Showers are expected to develop
Friday morning with a chance for thunder along the coast. Although
slight...the thunder threat will spread inland Friday afternoon.
Any terminals which experience heavier showers or thunderstorms
Friday may drop briefly to IFR visible.
Kpdx and approaches...MVFR ceilings with occasional -ra or -dz.
Clouds and developing showers overnight keep MVFR ceiling with a
chance for IFR ceiling after 15z Friday. Slight chance for a heavy
shower or thunderstorm Friday afternoon.
Marine...not much wind to speak of this afternoon with buoy 29
reporting SW at 4 knots. However low pressure moving into the area
tonight will increase winds out of the west/northwest through the day
Friday. Have extended the Small Craft Advisory for winds through 09z Saturday...but
is only a brief respite before winds turn offshore and increase
again during the day Saturday. One more round of Small Craft
Advisory northerly winds are possible early next week as the next
system moves through.
Wave models indicate seas will stay below 10 feet probably through
the weekend. There is a chance they could get close to 10 feet
especially in the outer waters early next week. Bowen/pt
or...Winter Weather Advisory from 6 PM this evening to 3 PM PST
Friday for Cascades in Lane County-northern Oregon Cascades.
Pz...Small Craft Advisory for winds from 10 PM this evening to 1 am
PST Saturday for waters from Cape Shoalwater Washington to Cascade
Head or from 10 to 60 nm-waters from Cascade Head to
Florence or from 10 to 60 nm.
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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.