Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Portland or
840 PM PST Friday Feb 27 2015

Synopsis...showers will continue through this evening an upper level
trough pushes southward across Oregon...then taper off from north to
south tonight. Snow levels are near the Cascade passes... and a few
more inches are possible in the Lane County Cascades before snow
showers taper off later tonight. High pressure and increasing
offshore flow should yield a mostly sunny and dry weekend with chilly
nights and mild days. The next upper level through forecast to dig
down the British Columbia coast will bring another shot of relatively
cool air and possibly more valley rain and mountain snow showers
Sunday night and Monday. Strong high pressure will then build back
for drier weather for much of next week.

&&

Short term...water vapor pictures suggested the upper low was
slowly moving S over western Oregon...probably in the vicinity of
Linn County. Radar echoes...while still scattered across much of
oreogn and western Washington...have shown some decrease in coverage and
intensity in the past few hours over northwest Oregon and western Washington. As
the low continues S overnight and with drying kicking in with a
return to a more northerly low level flow expect to see showers
decrease overnight from north to S. Snowfall rates at the snowtel sites
in the Cascades this afternoon and evening have been rather
modest...on the order of 1 to 2 inches per 6 hours...and generally
limited to 5k feet and above. As such...will keep probability of precipitation high across the
S part of the forecast area for the rest of the night...but drop the
advisory for snow in Lane County.

Remainder of short term discussion unchanged...
models show good agreement that any lingering showers will taper off
quickly Saturday morning as our upper trough moves into northern California
and flow turns offshore. High pressure will build into Washington
Saturday...with the resulting dry north-NE flow clearing US out quickly.
By midday expect plenty of sunshine for most of the district but with
some blustery northerly flow and temperatures stuck in the lower 50s. The
north-NE flow and dry air mass may make it difficult for any night and
morning fog to form this weekend except in the most sheltered
valleys. Not much change expected for Sunday...except perhaps less
wind away from The Gorge and a few more high clouds in advance of our
next disturbance.

Northwest flow aloft will guide the next disturbance into the Pacific northwest
Sun night/early Monday. Models continue to struggle a bit with the finer
details...but the broad consensus keeps two distinct systems during
this event. One will dig southeast along The Spine of The Rockies...
followed the next surge of cold air to push into the northern plains
states. The second disturbance remains weaker and tracks down the British Columbia
coast. 12z/18z GFS/NAM/ECMWF all have enough overwater trajectory for
at least some showers Sun night/Mon...while the 12z Gem keeps our
system too weak to produce much in the way of shower activity. Held
probability of precipitation generally in the chance category late Sun night/early Monday. If we
do get precipitation...the Cascades could get another dusting of snow but it
probably will not be much. Weagle

Long term...no changes. Previous discussion follows. Tuesday
through Friday...the mean upper ridge position returns to where it
has been much of this winter Tue/Wed...over the NE Pacific.
Therefore expect at least a couple days of dry weather
Tue/Wed...probably longer. It appears the bulk of the cold air
behind the Sun night/Monday system will stay east of The Rockies...with
only a small piece of it pushing into the middle-Columbia Basin. Thus
expect near normal temperatures and dry weather through
midweek...clear and chilly nights with sunny days and afternoon
temperatures in the 50s for The Lowlands. Offshore flow should limit fog
formation. Held probability of precipitation below climatology toward the end of the week as
models continue to show the mean ridge position anchored near the
West Coast. Weagle

&&

Aviation...a low pressure moving southward along the Oregon coast
will result in increasing offshore flow and flight conditions
trending towards predominantly VFR tonight where they will persist
through 06z Sunday. However...moisture will linger longest at
konp...keug and ksle so MVFR conditions and possibly even LIFR/IFR
conditions may develop temporarily in the short term and possibly
last through approximately 15z Saturday. 10 to 20 knots north to east
winds Saturday afternoon will ensure all locations turn VFR.

Kpdx and approaches...increasing offshore flow and drier air will
bring VFR conditions to the taf site through 06z Sunday. /Neuman

&&

Marine...a low pressure will slowly drop southward along the coast
tonight and Saturday. This will result in gusty northeasterly winds
spreading from north to south across the waters tonight. At this
point...it appears gusts of 25 knots will be most likely across the
outer waters...but there is a chance the inner waters could
experience these higher gusts Saturday. Opted to leave them out of
the Small Craft Advisory for now...but they may need to be
added...particularly the central Oregon coast inner waters.

Expect winds to relax Saturday evening and Sunday before increasing
again across the outer waters Sunday evening and Sunday night as
pressure gradients temporarily increase again. Another Small Craft
Advisory for winds may be necessary. Otherwise...high pressure looks
to become increasingly dominant over the northeast Pacific next
week...resulting in relatively benign winds and seas. /Neuman

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory for winds until 3 PM PST Saturday for
waters from Cape Shoalwater Washington to Cascade Head or from 10 to
60 nm.

Small Craft Advisory for winds until 9 PM PST Saturday for
waters from Cascade Head to Florence or from 10 to 60 nm.

&&



$$

Interact with US via social media
www.Facebook.Com/nwsportland
www.Twitter.Com/nwsportland



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.

National Weather Service Glossary of Abbreviations