Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
246 am PST Friday Feb 27 2015
Synopsis...an upper level trough will enhance showers across the
region today. Snow levels will lower below the Cascade passes today.
Colder air aloft associated with the trough will increase
instability Friday afternoon for a threat of small hail or
thunderstorms. Showers will end by Saturday morning as high pressure
returns. Another upper level trough may bring another round of rain
and mountain snow Monday before stronger high pressure settles in
for much of next week.
Short term...through Sunday. A cold front reached the coast just
around midnight and is now tracking across the interior lowlands and
headed for the Cascades shortly. The front is bringing a quick burst
of precipitation with rates around one to two tenths per hour. Satellite and
models indicating a following trough will move into the area during
the morning hours.
Snow levels bumped up a little overnight ahead of the front but will
slide back down with the initial front early this morning and
continue to slide as the day progresses...reaching around 3500 to
4000 feet by this afternoon and closer to 3000 feet tonight as an upper
trough moves into the region. An odot cam at Santiam Pass showed wet
roads at 34 degrees around 10 PM Thursday. At 2 am its 32 and snowing
with light accumulations on the Road. Model quantitative precipitation forecast has the main batch
of precipitation occurring with the morning systems. This will bring the
heaviest snowfall in the Cascades between 4 am and 10 am this
morning. Also the latest model quantitative precipitation forecast has more precipitation for the South
Washington Cascades than earlier forecast so will issue a Winter
Weather Advisory for areas above 5000 feet in that area for today.
Precipitation will be on the decrease later in the afternoon and tonight and
should be done by Saturday morning.
Model soundings for the coast and interior valleys suggest there may
be enough instability for a slight threat of small hail and/or
thunder today. But if it remains too cloudy it may be more of a
small hail day than thunder.
The upper trough will move further south late tonight 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 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.
There may be some patchy valley fog early Saturday morning...with
some showers over the Cascades but the rest of the day looks to be
dry. Models are in great agreement showing another Rex block setting
up over the West Coast. Dry weather holds Sunday...but clouds may
increase from the north late in the day. Weishaar /mh
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...a cold front moved onshore along the coast a little
after midnight and will move through the interior valleys by
sunrise with a period of heavy rain and low visibilities and a
decent wind shift. Thus expect a period of IFR to MVFR conditions
with the frontal passage. Behind the front a low pressure center
will move south along the coast today...and this will tend to keep
MVFR or low end VFR conditions over most of the area. Some small
hail is possible with the showers behind the front...with a
slight chance of a thunderstorm...but any thunder could be
dependent on how solid the clouds stay today. The low starts to
move south late today and especially tonight...so should see
decreasing showers and clouds tonight with increasing offshore
flow. Any fog and IFR conditions late tonight and early Saturday
will be dependent on how strong the offshore flow is. Best chance
will be away from The Gorge and south toward Eugene.
Kpdx and approaches...brief IFR conditions possible with the
passage of a front early this morning...likely moving past the
Airport by 12z. Otherwise MVFR to low VFR conditions are expected
through today into the early evening from clouds and showers in
onshore flow. There is a chance of small hail later this morning
and afternoon...and a slight chance of a thunderstorm as well...
though concerned the onshore flow will keep the clouds a bit too
solid...inhibiting daytime heating. Clouds should begin to
decrease tonight as offshore flow develops...with some east winds
possible near the Airport by early Saturday. Pt
Marine...a cold front moved onshore along the coast a little
after midnight...and we have some Small Craft Advisory winds in
its wake...including the inner waters. As a low pressure center
moves south along the coast today...the models indicate the winds
in the inner waters may ease but the outer waters may keep going
through today. Thus will just keep the inner waters advisory
through part of this morning. As the low moves south through the
waters tonight and reaches the northern California coast early
Saturday...winds will turn more northerly then northeasterly...and
still be brisk. It is possible we could see more Small Craft
Advisory northeast winds and east winds near the coastal gaps on
Saturday...decreasing from the north later in the day. Winds
remain lighter Saturday night and Sunday before increasing again
especially over the outer waters from the north Sunday night and
Monday as another moves southward through the Pacific northwest.
Wave models indicate seas will build from the west to 9 or
possibly 10 feet. Periods are forecast to drop today from 10 or 11
seconds to 8 or 9 seconds...creating choppy conditions. Have
therefore issued a Small Craft Advisory for hazardous seas as
well. Seas decrease from the north later tonight and Saturday.
Seas could get close to 10 feet again from the increasing
northerlies later Sunday night and Monday. Pt
or...Winter Weather Advisory until 3 PM PST this afternoon for
Cascades in Lane County-northern Oregon Cascades
above 5000 feet.
Washington...Winter Weather Advisory in effect until 3 PM PST this
afternoon for the South Washington Cascades above 5000 feet.
Pz...Small Craft Advisory for winds until 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.
Small Craft Advisory for hazardous seas from 7 am this
morning to midnight PST tonight 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.
Small Craft Advisory for winds until 10 am PST this morning
for coastal waters from Cape Shoalwater Washington to Cascade
Head or out 10 nm-coastal waters from Cascade Head to
Florence or out 10 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.