Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PST sun Dec 8 2013
Synopsis...westerly flow will develop Monday...and Pacific moisture
will overrun the cold air mass over western Washington. This will
produce some light snow and possibly freezing drizzle...with a
transition to light rain Tuesday afternoon. Temperatures will rise
to near normal Wednesday. A Pacific front will bring more typical
rain and winds Thursday...followed by another Pacific front Saturday.
Short term...northerly flow aloft continues over the Pacific
northwest this evening...ahead of an upper ridge well offshore along
140w. A cold dry air mass remains entrenched over western
Washington...but the low level northeast flow has become very weak.
Temperatures at 8 PM were in the upper teens and 20s under clear to
partly cloudy skies. On average the lows tonight will be a couple
degrees warmer than last night...ranging from the middle teens to
around 30 degrees.
A transition from the cold weather to more typically seasonal
weather will occur over the next few days. The northerly flow in the
lower 8000 feet or so of the atmosphere will back to westerly on
Monday...and Pacific moisture will move into western Washington and
overrun the surface-based cold air. This will lead to areas of light
precipitation. It will start at the coast Monday morning...then
eventually spread inland across western Washington to the Cascades
as westerly flow continues Monday night and Tuesday.
At the same time...the warmer maritime air mass will gradually erode
the cold air for a return to widespread temperatures above freezing.
This will also begin at the coast and...generally speaking...spread
inland. Prior to the transition...the light precipitation will fall
as snow or freezing drizzle or more likely periods of each. By
midday Tuesday...the air mass should be warm enough for
precipitation to fall as rain over most of the western Washington
Total precipitation in the western Washington lowlands Monday
through Tuesday will mostly be 0.20 inch or less. The exception is
the east Puget Sound sound lowlands near the Cascades -- places like
Enumclaw...North Bend...Gold Bar...Darrington...etc -- where up to
0.40 inch precipitation could fall. We are most likely to get
snowfall accumulations of an inch or two from around Everett
northward...and areas of 2 to 4 inches could fall in the east Puget Sound
Most of the Seattle area...the south Puget Sound sound region...the south
interior...and the coast are likely to get an inch of snow or less.
However those areas could be more prone to freezing drizzle late
Monday evening through Tuesday morning. The afternoon forecast
discussion had a nice explanation of the process that produces
freezing drizzle...and it is included in the paragraph below.
It is possible to get liquid falling from the sky...even when the
entire atmospheric sounding is below freezing. The key is that
precipitation has to be generated in a layer that is -5c or warmer...in
which case supercooled water droplets are formed... rather than
snowflakes. When these contact a surface that is below freezing...it
will freeze upon contact and be observed as freezing drizzle or
freezing rain. This becomes a concern in a setup like Monday and
Tuesday...where precipitation is being generated at fairly low and relatively
warm layers of the atmosphere...even if those layers are a little
Onshore flow will weaken Tuesday night and Wednesday...and the
offshore upper ridge will progress inland. This should be a mostly
dry period...and temperatures will return to near normal for middle
Long term...here is the long term section from the afternoon
forecast discussion -- extended models show a warm front spreading
across western Washington on Thursday...with a return to more ordinarily rainy and
breezy weather. A lull in the weather will take place Friday. A Pacific
cold front will bring another round of rain...wind and mountain snow
on Sat...followed by Post-frontal showers next Sunday. Haner
Hydrology...river flooding is not expected for the next 7 days.
Aviation...north flow aloft will become northwest Monday while in the
lower levels light southerly flow will develop. Areas of clouds are
developing over the coastal waters and a patch of clouds is forming
in the Strait of Georgia and in the San Juans at 8pm. The time
height cross section for Seattle clearly shows an increase in relative humidity
below about 7000 feet Monday and skies should become cloudy across
much of western Washington around daybreak Monday with flurries increasing
through Monday night.
Ksea...mostly clear skies tonight should give way to clouds Monday
morning...the NAM time height cross section suggests that a ceiling
ought to develop around 3000 feet around daybreak Monday and then
descend Monday evening as occasional light flurries develop.
Marine...gradients are pretty light and will remain light through
Monday as high pressure dissipates over the region. There is a weak
trough over the coastal waters in the evening sea level pressure
analysis...and there is lower pressure over b.C. With high pressure
over eastern Oregon. All this adds up to a more southerly flow developing
through Monday. Rather weak systems will brush the area for the
early part of the week and then the weather pattern will become more
active around the middle of the week.
You can see an illustrated version of the forecast discussion at