Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
900 PM PST Friday Dec 6 2013
Synopsis...cold and dry weather will continue tonight through early
next week...with some of the coldest temperatures of the year
tonight and Saturday night. Temperatures will begin to moderate
across the region next Tuesday and Wednesday as the low level flow
becomes more onshore. Snow or mixed rain and snow is possible Monday
through Tuesday until the precipitation becomes all rain Wednesday.
Short term...northeast flow aloft over western Washington this
evening in the wake of the weak system that is now well to our
south. Northeast surface pressure gradients increased behind
that system as well today...which is bringing a colder air mass into
the forecast area. There is still a mix of cloud cover over the
area. At 8 PM temperatures were in the 20s...with northeast winds
gusting into the 20 to 30 miles per hour range in spots lowering the wind chill
to the middle teens. Bellingham is both the cold spot at 20 degrees and
about the windiest...with winds gusting to 36 miles per hour last hour and a
wind chill of just 3 degrees above zero.
The upper level pattern will change little tonight and Saturday.
Windy conditions near the region of Fraser outflow -- ie mainly in
the north interior -- will continue tonight and then ease Saturday.
Cloud cover will gradually clear tonight. Lows tonight will be in
the middle teens to lower 20s...and highs Saturday will remain below
freezing for probably the entire forecast area under sunny skies.
Saturday night will be even colder with overnight lows ranging from
the single digits to the lower 20s.
Models show a weak system approaching western Washington Sunday in
the northerly flow aloft. It looks weak in all models...but they
differ somewhat on its strength. Our forecast leans toward the NAM
solution...which is stronger than the others and brings a chance of
snow to the coast. Skies should be partly sunny inland. Highs will
rise a few degrees from Saturday but will still mostly be below
freezing. The cloud cover will keep Sunday night a little milder
with lows in the upper teens to upper 20s.
There is another chance of light precipitation over the area
Monday...as another weak feature arrives. Again the models differ on
the strength of this system...with the European model showing the
best threat of snow. If we do get some snow...the amounts should be
less than an inch. Mcdonnal
Long term...here is the long term section from the afternoon
issuance of the forecast discussion -- Tuesday looks like the
transition day. A weak system will move inland bringing a little
precipitation. Thicknesses rise above 1300m over most of the area
except for the far north. Typically the Hood Canal area also does
not warm in these scenarios. In any case precipitation is again
pretty light. If the transition does occur the day could begin with
a little snow and end with a little rain or mixed precipitation. The
Hood Canal area and far north would remain in snow and perhaps get a
few inches. Still pretty far out to be making these bold
Wednesday looks fairly dry but kept chance probability of precipitation in the forecast with
likely probability of precipitation on the coast as the next system moves toward Washington.
The air mass should be warm enough for just rain by Wednesday. The
system arrives Thursday and Friday looks showery. Temperatures are
forecast to be back to normal by then. Burke
Hydrology...river flooding is not expected for the next 7 days.
Aviation...north-northeast flow surface and aloft...with Fraser outflow. A
1039mb high is centered over the interior of British Columbia with a
1012mb low near the southern Oregon border. Mostly clear skies except as
of 8pm there was a layer of clouds around 7kft over the Olympics and
from about Olympia south...and there were some clouds south of about
Snoqualmie Pass. These areas of clouds will likely scatter out by
daybreak as the system over Oregon shifts southeast.
Ksea...NE breeze and mostly clear skies. No frost to speak
of...single digit dewpoints so the air is Bone dry.
Marine...bli-ywl gradient is around -13.5mb this evening with gale
warnings up for the strong NE flow as well as a freezing spray
advisory up in the cold windy Fraser outflow over the water. At 8pm
this evening there is a 1039mb high centered over b.C. With a 1012mb
low over the southern Oregon border...giving strong dry offshore flow.
Washington...Wind Advisory remains in effect until 4 am Saturday for the far
north interior and western Strait of Juan Delaware fuca.
Pz...Gale Warning and freezing spray advisory in effect until 10 am
Saturday for the northern inland waters.
Gale warnings remain in effect for the coastal waters from Cape
Flattery to James Island out to 60 nm and the West Entrance to the
Strait of Juan Delaware fuca.
Small craft advisories remain in effect for the remaining
waters except the Admiralty Inlet.
You can see an illustrated version of the forecast discussion at