Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
845 PM PST Thursday Dec 19 2013
Synopsis...a warm front will spread up to 3 inches of snow across
the area late tonight through Friday morning. Precipitation will
change to rain during the morning or early afternoon. After a lull
between systems Friday night and Saturday...another weak warm front
will bring a little rain Saturday night and Sunday. Drier northwest
flow aloft will develop next week for a cool and dry Christmas.
Short term...the main points of the snow forecast are unchanged.
Snow will develop from the northwest after midnight and spread into
the Seattle area 10z-12z. Snow will change to rain 18z-20z. Amounts
are forecast to be in the 1-3 inch range over The Lowlands...with
the higher amounts north and east and the lower amounts south and
west. These are advisory amounts and will keep the existing advisory
in place. Temperatures Friday afternoon will reach the middle 40s and
the combination of warmth and rain should wash away the snow.
Will make one change to the headlines by extending the Winter Storm
Warning to the north Cascades in addition to the central Cascades.
Models have a widespread 1-2 inches of water falling in the form of
snow in the mountains...including the passes...in the 24 hours
ending at 10 PM Friday. That is the equivalent of 1-2 feet of snow
and meets warning criteria.
Most recent 12km MM5 run shows 0-2 inches over the southwest
interior...the coast...and near the Strait of Juan Delaware fuca. It shows
1-2 inches over Tacoma...and 2-3 inches over a broad swath along I-5
from Seattle to Bellingham. There is also an area of 4-5 inches in
western Skagit and northwestern Snohomish counties which could prove
troublesome. Will have to keep an eye on that area overnight.
A relative lull in the weather will occur Friday night and Saturday
before the next weaker warm front reaches the area late Saturday
night and Sunday. This system will be warm enough so that just rain
is expected over The Lowlands. Burke
Long term...previous discussion...an upper level ridge offshore
will maintain northwest flow over western Washington on Sunday. Models show
scattered showers...mainly affecting the Cascades. Snow levels are
around 4000 feet so the higher passes like Stevens may see a few snow
showers. The ridge will flatten Sun night into Monday as our next Pacific
system arrives. This system is wetter and the mountains will see
several inches of snow...with rain in The Lowlands. We will likely
see breezy or windy conditions in the interior as well. This system
will exit Monday evening with lingering showers in the Cascades.
Expect drier weather Tuesday through much of next week as another upper
level ridge builds over the Pacific northwest. Probability of precipitation were decreased throughout
the entire period. We may see some sunshine on Christmas day with
temperatures close to normal. 33
Hydrology...river flooding is not expected for the next 7 days.
Aviation...northwest flow aloft continues over the Pacific
northwest this evening. Precipitation...mainly in the form of
snow...will spread southeast across western Washington late this
evening and tonight ahead of a frontal system in the northwest flow.
The warm front will move through the area early Friday morning...
followed by a cold front late Friday.
VFR conditions across western Washington at 8 PM with middle and high
clouds increasing ahead of the approaching front. Conditions will
deteriorate to MVFR and areas of IFR tonight in snow. Snow will
change to rain Friday morning...initially at the coast and then
inland. Conditions will improve only marginally as the air mass
remains moist in onshore flow.
Ksea....southeast wind 5-10 knots becoming south 10-18 knots Friday
morning. Snow will begin around 10 am with conditions deteriorating
to MVFR and occasional IFR. Snow accumulations of 1 to 3 inches are
likely before snow changes to rain around 9 am. MVFR conditions with
occasional rain should continue the rest of the day.
Marine...a warm front will move into the coastal waters late
tonight and move across western Washington Friday morning. Southerly
gales are likely over the coastal waters and at the West Entrance
Strait of Juan Delaware fuca. Solid Small Craft Advisory southerlies
should occur over the other marine zones...except the central Strait.
A trailing cold front will move through the forecast area late
Friday. Strong Post-frontal westerlies will develop late Friday and
continue Friday night...then ease Saturday. Based on the latest
models...especially the uw WRF-GFS...and the fact that the flow just
aloft will be aligned perfectly with the Strait...I thing westerly
gales are a good bet in the central and eastern Strait. A Gale
Warning will be issued with the evening forecast.
A weak warm front will arrive Sunday...followed by a weak cold front
on Monday. High pressure will build over the region Tuesday and
Washington...Winter Storm Warning Cascades.
Snow Advisory all other zones.
Pz...Gale Warning starting tonight coastal zones and West Entrance
to the Strait of Juan Delaware fuca.
Small Craft Advisory tonight remaining waters except the
Gale Warning starting Friday afternoon central and east Strait
of Juan Delaware fuca.
You can see an illustrated version of the forecast discussion at