BLOG: Heavy Snow Coming, But How Much?

Good morning all,

It’s a windy and cold Thursday and we have to watch for some beefy snow showers and squalls this morning. This will be followed by subzero temperatures tonight. Stay up on the latest short term weather by checking out the social media outlets (mine and Jess’s).

This post will focus on the Sunday-Monday snow threat. We plan on looking at the latest models before putting accumulations in our official forecast this afternoon, but this post will give you a peek “behind the curtain”.

First of all, it’s important to note that the disturbance largely responsible for the snow threat is still over the Pacific Ocean. Computer models will be more trustworthy once the system is over land, since there will be more data available for them to analyze. Here’s an animation showing the system crashing into California and then coming east. Click to animate:

output_f74bhhThe colors represent the “vorticity” or spin in the atmosphere at about 18,000 feet. Spinning air parcels at that level aid in air rising and clouds/precipitation forming.

Snow will get going late Saturday night and continue at varying intensity Sunday. There will be times Sunday that it is not snowing that hard at all. We are still too far out to pinpoint when snow is likely to be lighter/heavier during Sunday.

We are pretty confident the worst of this storm will be Sunday NIGHT and early Monday. Here’s a few computer model snapshots at 1am Monday morning:

GFS: gfs

European: euro

Canadian: can

The Canadian looks a bit different than the other two; snow is lighter at this time step on the Canadian. This is an outlier at the moment.

One thing we look at when figuring out how hard precipitation will be falling is the “vertical velocity” at about 10,000 feet. It shows how quickly the air is rising. The faster it is rising, the heavier the precipitation is likely to be. Here’s the GFS vertical velocity map at 1am Monday:

vvlIt has the best upward motion south of Rt. 30, but still looks decent for most of the Valley.

How Much Snow?? That is the question.

Let’s see what the current models say. First, the GFS:

gfssnowA general 6-10″ range for the WFMJ viewing area.

The Canadian:  cansnowLooks like a range of 5-12″, with the heaviest south.

The European: eurosnowYikes! A solid 11-12″ for the region. You are likely to see this map or something similar on social media outlets today.

BUT! A word of caution. 

This is the “operational”, or “deterministic” run of the European.  There are 51 “members” of the European Ensemble Model. What’s an ensemble model? A lower resolution version of the “deterministic” model is run 51 times, with each “member” having slightly different initial conditions (pressure, temperature, humidity, etc.) This proves useful because we simply cannot model the atmosphere perfectly. Running a model 51 times, allowing for “wiggle room” in the initial conditions gives us more of a range of possibilities.

Anyway, the mean, or average snow accumulation of those 51 members is significantly lower than the operational…which makes the 11-12″ idea harder to believe. Have a look:

euro ensThe mean is 5-6″.

So, what’s the bottom line?? I think the low end of the snowfall range will be around 5″. What’s the top end? Not sure yet. Probably somewhere between 8-10″. More to come this afternoon!

Thanks for reading!