Heavy Snow to End the Cold

It's rather common for arctic cold snaps to end with snow in this region.  The reason?  Well, east of the Rockies, air masses like this one move around fairly freely, since the topography is mainly flat (except for the Appalachian Mountains).  But in the West, our mountain ranges trap and hold on to this cold, dense air.  Eventually, a warmer Pacific storm comes along and tries to push out the cold air.  And usually it often wins.  But not without first creating some snow for us.  And in this case, that has the potential to be a lot of snow.

First, we'll start with temperatures.  As I said in the previous blog, we'll continue to gradually warm up, reaching above the 32F mark by the weekend.

Our storm will arrive Tuesday night and snow will fall all day Wednesday.  Temperatures during this storm will be in the mid-teens to lower-20s.  Very cold snow, which typically means drier, fluffier snow.  But there have been some instances in these situations where we get more "sugar snow" than the "flakey snow".  Another weaker storm will bring a little more snow Thursday night and Friday.

You can see that we also will be looking at more rain than snow starting this weekend and into next week.  Notice something else from the graph above.  After the snow event(s), the purple line around Friday shows that freezing rain is a possibility.  Again, not unusual.  Everything (houses, roads, cards) are really cold right now.  So as we transition from snow to rain, there could be an icy period in there.

So how much snow are we looking at?  Most all of the computer models are agreeing on a fairly heavy event.  Here's the ECMWF forecast for total snowfall by Thursday morning.  About 8-10" for Spokane and points north and east.

 And the U.S. GFS model is similar, with about 10".

The Canadian model also has a similar forecast of 10-12".

A newer ICON model is not very different, forecasting about 10-13".

So as you can see, a fairly good agreement among 4 models.  As I have said in the past, there are actually over 100 different computer weather forecasts out there.  So when you average them all together, you get a slightly more conservative forecast of about 7" for Deer Park.

But when you have over 100 forecasts, you can calculate some probabilities of different snowfall amounts, rather than just average them all together.  The graph below shows that 100% of the forecasts have at least 4" for Deer Park, so that's a done deal.  93% of them have at least 6" of snow, 76% say we'll get at least 8", and 21% (i.e. 1 out of 5) predict 12" or more.

For those who plan to do their plowing on Thursday, temperatures will be in the upper teens to lower 20s.  For those who wait until Friday, it will be a little warmer with highs in the mid to upper 20s.

I've said before that snowfall forecasting is one of the toughest aspects of weather forecasting.  One factor is the ratio of snow to liquid.  If it's a 10:1 ratio, we'll get about 6" from this storm.  If it's a 20:1 ratio, we'll get 12" of snow.  Since temperatures are cold, higher ratios are expected, but never guaranteed.