Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Spokane Washington
930 PM PST Thursday Dec 18 2014

a more active weather pattern will continue into early next week.
Another weather system will bring snow to the mountains with
mainly rain for the valleys tonight into Friday. A stronger storm
Saturday into Sunday will bring mainly rain for the
well as mountain snow with rising snow levels. Temperatures are
expected to remain above average through early next week...before
dropping down towards normal values by Christmas day.


tonight and Friday...there is good model agreement of an occluded
front tracking into central and eastern Washington
tonight...reaching north Idaho Friday morning. There is good model
agreement of this system bringing widespread precipitation to the
region due to a combination of isentropic already moist
low level air well as upper jet support with a south-
north jet oriented parallel to the front. Isentropic ascent will
be maximized in the Cascades and northern mountains with low level
upslope flow into these areas as well. However this is a quick
moving front...with models generating anywhere between a tenth to
a quarter inch of liquid with locally up to a half inch over the
high terrain of the northern mountains. Based on current wet bulb
temperatures and model snow levels with this system...most valley
areas will see rain with snow in the mountains. Exception is the
Methow Valley where expected low temps near 32f should allow for
2-3 inches of snow. Republic may also see light accumulations with
expected snow levels in the Okanogan Highlands between 2000-2500
feet. The mountains will pick up 3 to 6 inches of snow with this
system with winter driving conditions likely over the mountain
passes. Jw

..major storm expected to dump heavy precipitation and heavy
mountain snows to portions of the inland northwest this weekend...

Saturday through Monday...confidence is still very good that a
very impressive atmospheric river will impact the region through
much of this period. As of 2pm...the leading edge of the river was
located just west of 160w and moving steadily eastward. Most model
solutions take the leading edge of this moisture onto the
Washington coast early Saturday morning and into the Cascades
before midday. It will then cross the remainder of the forecast
area during the day with the passage of a strong warm front. The
atmospheric river will generally remain locked over the region
through Sunday. Strong isentropic ascent associated with the warm
front will spell measurable precipitation for all locations on
Saturday. Then the mid-level warm air advection will either cease
or weaken sometime late Saturday evening resulting in a developing
rain shadow which will either end the threat of precipitation in
the Lee of the Cascades or at least significantly lower the
chances. This would affect Wenatchee, the Columbia Basin, Okanogan
valleys and to a lesser extent the Palouse and Spokane area.
Meanwhile moderate orographic ascent from low-level west-southwest
winds will keep persistent precipitation going over most of the
Idaho Panhandle and near the crest. Whether or not the
precipitation rates pick up again on Sunday or Monday in the Lee
of the Cascades is questionable. The NAM shows a strong enough
shortwave trough to lift the atmospheric river northward on Sunday
resulting in another round of precipitation, albeit significantly
lighter than that which falls on Saturday. By Monday the plume of
moisture is expected to sag south of the Oregon/Washington border
with precipitation rates falling off over most areas...however
residual precipitation will likely continue mainly over the
mountains...especially over southeast Washington the southern Cascades
and the southern Idaho Panhandle. So from an impact standpoint
here's what we are expecting.

*Precipitation type and amounts...this is likely the most
difficult part of this forecast. Our confidence is quite high
that significant precipitation will fall. Total precipitation
amounts near the Cascade crest will likely range from 1.5-3.0
inches and anywhere from 0.75-1.50 inches over the Panhandle
mountains. Meanwhile totals across the remainder of the forecast
area will generally range from 0.40-1.00 inches. If the
atmospheric river decided to remain fixed over our area...instead
of slowly sagging south these values would undoubtedly need to be
raised. Instead it looks like the brunt of the river will focus
on points south of our forecast area...but not by much. As for
precipitation type that is easily the toughest part of this
forecast. Not necessarily for the mountains, but for the valleys,
mainly in the Cascades. Initially snow levels will be close to
the valley floors near the Cascades but they should steadily rise
through the weekend. By Sunday we will be looking at snow levels
ranging from nearly 4k feet near the Canadian border to almost 7k
feet near the Oregon border. The models have been too cool across
most of the forecast area lately and not sure this trend will end
anytime soon. The models have unquestionable snow for most of the
Cascades at the onset of the event on Saturday morning as they
have surface temps in the lower to middle 20s. We think that's
too cold but is it 10 degrees too cold? That is not likely the
case, however even if they are 5 degrees too cold it won't take
much warming to make accumulating snow tough to come by. If the
precipitation rates are high enough that can be overcome though.
So locations such as the Methow Valley could very well see
moderate to heavy snows through at least early Saturday evening
with moderate amounts possible. Farther south at Leavenworth and
plain there would be a much tougher time as the temperatures
should be a little bit warmer. The critical thing could be how
cool will temperatures get on Friday night/early Saturday. When
we know that answer we could issue the appropriate weather
highlights, if any. For now we will address via weather stories.
Meanwhile our confidence is growing that locations that have an
easier time of mixing with a dynamic warm fronts, such as the
Waterville Plateau or Okanogan Valley, will see much less snow.
Even valleys near the Canadian border may not see much snow. So
this looks like it will mainly be a mountain event...with some of
the Cascade elevations above 4000 feet seeing up to 2 feet of
snow by Sunday afternoon. We expect to initially see snow over
the major passes on Saturday but there should be a changeover
sometime during the day. Again it looks like snow and
precipitation rates will steadily decline on Sunday night/Monday.

*Winds and temperatures...this will be a very strong system with
850 mb winds climbing into the 40-50 kt range. Whether or not
these fully mix down to the ground isn't certain. Cold fronts are
much more efficient at this downward mixing. Meanwhile the
surface pressure gradient isn't all that impressive on Sunday or
Monday, which is when we'd expect the strongest winds. So this
leads US to think it could be breezy on Sunday and Monday but
nothing extraordinary. Meanwhile temperatures will be unusually
warm once again. Highs on Sunday will be in the mid 40s to mid
50s over much of the region...with cooler temperatures near the
Canadian border and Cascades. Monday will be a little
cooler...but not by much. So whatever snow falls on Saturday over
valley locations will have a good chance of melting, at least
part of it especially considering nighttime lows will be above
freezing as well. Fx

Monday night through thursday: a change in the large scale pattern
weather pattern is almost certain during this time-frame. For
several days, models continue to indicate a transition from mild
and wet weather toward drier and cooler. How fast this transition
occurs carries moderate to high uncertainty. Consequently, this
will have moderate ramifications on precipitation chances,
amounts, and type largely within the Monday night to Wednesday
time-frame. The atmospheric river or plume of subtropical moisture
starts off over the region Monday night night and sags southward
Tuesday into Tuesday night as yet another trof of low pressure
suppresses the large scale ridge in place. Generally, speaking
most valleys will continue to experience rain with mid to High
Mountain snows given the origins of the moisture tap and
accompanying 850mb temperatures between -1 and 4c. Some models
indicate a weaker wave ahead of the main trof...and these features
will bring some fluctuations to mountain snow levels and potential
for wintry impacts on the mountain passes. The potential for any
valley snow will arrive during the Tuesday night into Wednesday
time-frame (christmas eve) as the ridge finally gives way to
northwest flow and a stronger shortwave trof. This will allow much
cooler air (not Arctic by any means) to move back into the region
accompanied by 850mb temps of -1 to -4c. Generally speaking, the
deeper moisture will be shoved southward and the cooler air will
be drier however, there is a good chance that the cooler air
clashes with the moisture and leads to a period of lower elevation
snowfall. Model trends continue to push this heavier band south
and south each run suggesting the best chances near the blue mtns,
Camas Prairie, lower Idaho Panhandle, and perhaps the Palouse...but I
think there is enough uncertainty at this time to completely rule
out the potential for some smaller scale bands to pass through
just about any valley location as the air masses change. What is a
strong possibility is snow on the mountain passes, a period of
gusty winds, and transition back to seasonal temperatures. We will
continue to update you on the potential for valley snow as
confidence increases during this busy Holiday period. /Sb


06z tafs: a weather system is spreading -ra across the taf sites.
Widespread IFR/MVFR cigs expected through at least 12z. -Ra will
end aft 12z starting across the west and spread east through the
day. Aft 20z drier air will move in to provide some temporary
clearing...which may lead to fog formation after sunset.
Confindence isn't real high, so didn't hit it hard in the tafs.


Preliminary point temps/pops...
Spokane 36 41 33 40 38 45 / 70 100 10 90 90 60
Coeur D'Alene 35 40 33 38 35 43 / 50 100 20 90 90 70
Pullman 37 43 34 40 39 48 / 70 100 10 90 90 80
Lewiston 38 47 34 44 42 51 / 40 70 10 80 100 70
Colville 35 40 33 37 35 41 / 100 100 20 100 90 70
Sandpoint 33 38 32 37 34 39 / 50 100 30 80 90 80
Kellogg 33 37 31 37 34 40 / 40 100 40 70 100 100
Moses Lake 37 45 34 40 39 47 / 90 50 10 90 60 30
Wenatchee 35 43 32 36 36 43 / 80 30 10 90 70 40
Omak 35 39 30 34 34 38 / 100 80 10 90 80 30


Otx watches/warnings/advisories...
Washington...air stagnation advisory until noon PST Friday for east slopes
northern Cascades-Moses Lake area-northeast mountains-
Okanogan Highlands-Okanogan Valley-upper Columbia Basin-
Waterville Plateau-Wenatchee area.


National Weather Service Glossary of Abbreviations