Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
330 PM PST Monday Dec 9 2013
Synopsis...a weak weather system will produce occasional light snow
tonight and Tuesday morning. By Tuesday afternoon there will be rain
in the mix as the air mass warms. Temperatures will rise to near
normal Wednesday. A Pacific front will bring more typical rain and
wind Thursday...followed by another Pacific front over the weekend.
Short term...a weak weather system is moving south out of British
Columbia this evening in northwesterly flow. Models generally agree
that light precipitation will come tonight...and that the whole air
mass is cold enough that it will be all snow. Beefed up the probability of precipitation to
likely for most areas and zero to one inch of snow is forecast. The
part of the east Puget Sound sound lowland zone near the Cascades might
get two inches and will issue a Winter Weather Advisory there. The
coast is likely to get mixed precipitation with rain near the
beaches but snow inland. In all areas the amounts will be light.
Do not see any sign of freezing rain in the models. Model soundings
remain below freezing from the surface up through Tuesday morning.
Then they warm from the surface upward as Tuesday progresses. Have
removed mention of freezing rain or freezing drizzle.
Snow is likely to continue Tuesday morning...with maybe another inch
of accumulation here and there. The MM5 runs have shown a pscz
signature in Snohomish Skagit and island counties late tonight and
Tuesday morning. Put a little more accumulation in those zones but
do not see the dynamics for a big pscz. The air mass will warm
during the day Tuesday and by afternoon precipitation will be mostly
a mixture of rain and snow. Also the air mass dries with models
showing mostly no precipitation at all Tuesday afternoon and Tuesday
night. Have probability of precipitation in the chance category Tuesday afternoon and night.
Warming continues Wednesday with highs forecast to be near normal...
in the low 40s. Any snow on the ground should melt as skies may be
partly sunny in the afternoon. Also the models are dry that day so
have kept the forecast dry.
A much more typical weather system arrives Thursday. Rain should
begin on the coast in the morning and spread inland in the
afternoon. Rain will continue Thursday night then change to showers
Friday. Highs both days will be in the middle 40s. Burke
Long term...both the Euro and GFS have slowed the progress of the
next system and weakened it considerably. Have delayed the onset of
likely probability of precipitation to Saturday night and Sunday. Showers follow Monday.
There is no heavy precipitation or snow in the extended forecast.
Highs will be in the middle 40s with lows 35-40. Burke
Hydrology...river flooding is not expected for the next 7 days.
Aviation...high pressure along 135 west will gradually shift eastward off
the Washington coast on Tuesday. Moderate northwest flow aloft...becoming light on
Tuesday. Moist above 1500 feet. Stable conditions. Flurries will
increase in coverage across all of western Washington tonight. Accumulations
less than 1 inch. A transition from light snow to rain/snow mix will
end by Tuesday evening.
Ksea...ceilings 1200-1800 feet. Flurries will increase to occasional
light snow this evening...continuing overnight and Tuesday morning.
IFR ceilings could occur if steadier snow develops. Snow may transition
to rain before ending Tuesday afternoon. Snow accumulation of a
dusting up to one inch through Tuesday morning. No accumulation
Marine...generally light pressure gradients with winds less than 15
knots over the coastal waters and less than 10 knots over the
interior. A stronger front Thursday will likely produce small craft
winds to portions of the waters. Onshore flow behind the front
Friday...then high pressure Saturday. Dtm
Washington...Winter Weather Advisory for east Puget Sound sound lowlands near the
Cascades tonight through Tuesday morning.
For the graphical afd...visit