Inkling for Some Fluffy Stuff?

Around the Northwest, the mere mention of the "s" word can bring about a panic only one can experience to understand. Well this time of year, snow becomes a popular topic around communities as people become anxious for cooler weather and just the possibility of not having to go to the mountains to play in the terrific white fluff. In addition, snow becomes a popular topic among meteorologists because weather models begin to turn from the summer time calmness (around the Pacific Northwest) to the stormy fall weather and the hint of colder winter temperatures. Sometimes, the two even come together. ;)

The issue with predicting snow around the Northwest, is that it's hard to get snow. The Pacific Ocean strongly mediates the air temperature here near the coast so it takes VERY cold air temperatures to overcome that heating and get snow.

The next hardest part about forecasting snow, arguably the hardest part, is that forecast models ARE NOT PERFECT! As meteorologists we learn about the atmosphere and it's motions, and these models predict those motions. Well that's a pretty tall task to ask of the models. I learned from my professor at the University of Washington, Cliff Mass, a good picture of how to understand model variability. It looks something like this:

Imagine a ski resort. You have 5 runs that all start at the top of the mountain and go down the mountain in different directions. Now think of each run as a forecast and going down the mountain as going farther out in time for the forecast (this will stretch your thinking a little). Here is a rough picture of my illustration

Now at the bottom of the mountain, the forecasts (runs) are all at different spots. As you go farther up the mountain, closer to the forecast time, the forecasts become closer and closer together until you get to the top, where they become relatively similar and agreeable.

Well in forecasting snow, sometimes models will paint a snow forecast 7 days out, lets say the Green line. You notice how the green line doesn't start and end at the same point along a vertical axis, so it may say snow 7 days out and rain the day before the forecast time. It's a very risky business to trust models 7 days out from a forecast time.

Models have gotten better over the years though. 2 decades these models were just getting started and you might not have been able to trust a forecast 2 or 3 days out. Now, 5 days is reasonable for trusting, although exact details (amount of snow, rain, winds, etc.) shouldn't be taken from a 5 day forecast.

Now to what I really wanted to talk about, but I felt I should give some grounds for my feelings about this. The model run by UW this morning painted a "favorable" picture for snow next Thursday and Friday. It would be a classic Pac NW scenario for snow, which I will get back to in a minute. Here are a couple images that the model outputs

I'll start with the upper level pattern of the atmosphere, at 500hpa pressure. Those pink colors, those are very cold temperatures associated with a trough of low pressure digging south from the Canadian continent. This is an important factor because air coming off the ocean is not very cold, but from the Canadian land, it would be VERY cold.

The next thing that is needed is a low pressure developing at the surface, which is clearly visible here. The low is centered right off Vancouver Island, bringing moisture and "pulling" the cold air at the surface towards it. Air tends to accelerate from High to Low pressure. This forecast is for 147hrs (from this morning at 4am). Here is the forecast for 171 hrs.

The low is centered right over Washington, pulling cold air right into Western Washington. The white color indicates temperatures near 0 degrees celsius.

Here is estimated snowfall for a 3 hour period ending Friday afternoon. This shows some snowfall around South Puget sound and the Eastside. Another 3 hours later...

 Some snow in Northern Western Washington. And here is the 24hr accumulated snowfall totals...

As expected the Eastern foothills would see more accumulation, and only a slight amount around the lowlands.

However, all of this forecast for the end of next week should be taken as a grain of salt, as I noted previously. It's just TOO EARLY to tell.

I felt this would be a good opportunity to explain this aspect of forecasting too far out, while also arousing your interest in some possible snow :) Because I'm sure excited

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I will post updates to this story as I get them :)

Cheers!