Update on Tuesday’s snow event

We can still expect to see snowfall in the lowlands tomorrow, and as the time draws nearer, I have powerful additional forecasting tools to apply.

But before doing so, let me note that there was a beautiful display of the aurora last night, mostly behind the clouds and in some gaps, the sky lit up.

This is an image from North Kitsap, courtesy of Skunk Bay Weather forecaster Greg Johnson. Amazing.

Back to the snow.

This is the last season of snow in western Washington/Oregon and low temperatures. But with enough rainfall, melting and evaporation can push snow levels to the surface. Result: wet snow.

The latest projections are converging at about 2 inches near sea level, but as we will see, there will be significant variation.

The feature of interest is a low center (and related fronts) centrally located off the Oregon/Washington coast right now (see image). Right now some weak bands of rain are on land. Light drizzle and sleet.

That low center will drift northwest for the next 24 hours, relative to an approaching upper-level depression. The sea level pressure map at 4am tomorrow (Tuesday) is shown for illustration.

Now, let’s check the output of the predictive model.

Total snowfall (not snow depth) until 4 p.m. Tuesday from the high-resolution UW model is shown below. Maybe 2 inches in Sound Center, less in warmer waters. Same amount for Bellingham and San Juans. Less south of Seattle.

The Central Europe model, with a much lower resolution, has a dimmer but similar forecast.

If I have tried to convey one idea in this blog, it is that forecasting has uncertainty and meteorologists must identify this uncertainty and communicate it.

If only politicians, Covid forecasters and some local newspapers would do the same.😆

An important tool for determining forecast uncertainty uses a set of multiple but reasonable forecasting simulations. UW runs one of the best such groups in the country and you can enjoy the fruits of this technology.

For example, here’s the total snow forecast for Seattle. The lines show different forecasts, with the black line being the average of all of them.

Snow returns after 22:00 tonight (June 28), with considerable uncertainty. The average total snowfall is about 2 inches.

Olympic? It snowed this morning and it will be about 3 inches thick tomorrow afternoon.

Portland will get about 1.5 inches.

Central Europe has a lower resolution population. For Seattle, it suggests about 3-4 inches.

So plan for the snow to fall overnight through tomorrow morning. And with temperatures above freezing, there’s no tape on the surface. And it will warm up tomorrow afternoon with inshore currents after the lows.