“Sneak Attack Windstorm,” This Was NOT!

Should we really be relying on one source for our news?  Relating that question to last Wednesday’s wind event, should we be relying on one type of weather model forecast?

Far too many in our community were caught by total surprise with Wednesday’s major windstorm that hit the Pacific Northwest. Puget Sound Energy Emergency Coordination Center even called it a Sneak Attack Windstorm.   Damaging southerly winds early Wednesday exceeded 60 mph in many Puget Sound locations, including the Snoqualmie Valley.

Top Wind Gusts – Snoqualmie Ridge (2020/21):

The problem is we’re too often fed one baseline forecast that is all encompassed by the mucky law of averages with little awareness of the standard deviation of possibilities from the mean.  Unless your confidence interval is extremely high, only forecasting a wind gust range of say 30-35 mph when there are other models (and very good ones at that) saying 60+ mph, clearly this isn’t enough information for the public and municipalities to be adequately prepared.  Forecasting a mean range often suffices for the more predictable gap winds in my experience, but not for a nascent developing Low off the coast, seemingly headed for an ideal storm track that would actually generate the first major PNW windstorm we’ve seen in some time. 

As we saw Wednesday, an ideal storm track resulted in damaging winds, but if off by a hundred miles or so, it could have easily resulted in just a measly breeze.  In essence, we were advised (not warned) about wind gusts derived from a simple average hiding the more extreme potential, leaving the public caught off-guard.

I’ve always thought weather forecast probabilities should be more bifurcated, akin to something more like a Las Vegas odds book, perhaps. 

For example, what are the chances each of several different scenarios play out?  Something like the following..

Windstorm 1/13 – wind gust probability for Puget Sound:

> 30 mph 60% chance – gusty

>40 mph 50% chance – Very windy, broken tree limbs, isolated power outages

>50 mph 30% chance – Major windstorm, downed trees, wide spread power outages

>60 mph 20% chance – Wide spread property damage and power outages, potentially for >24 hrs

>70 mph 10% chance – Wide spread property damage and power outages, potentially 1-7 days

I think the above could more or less be considered a fair shake at what the odds profile for last Tuesday reflected without the benefit of hindsight of what actually went down subsequent. The point is to give people that better transparency to assess the risk.  For me personally, making a run to get gas and stocking up on other essentials might trigger when there’s a forecasted greater than 25% chance of 50 mph gusts; for you, that level of preparation may kick in at a different threshold.

Maybe we’re putting too much pressure on our forecasters to get so accurate to the point they fear the appearance of being wrong if you tell folks to be warned, even if you disclose the more dire scenarios may only have, say, a 20 percent chance of occurring. And perhaps there’s a bit of a US vs. Europe pride thing going on too. Specifically, I’m referring to the more consistently accurate, higher resolution European Centre for Medium-Range Weather Forecast (ECMWF). The model’s baseline ensembles had this windstorm as a real threat as far back as Saturday (9th).

I get it. In years past, funding restrictions at higher government levels have left the U.S. well behind in the technological development of numerical data assimilation. This basically means that even though American and European models are fed the same inputs from the global observation system, the Euro model excels in its ability to use this data received in real-time.

From the ECMWF:
“The purpose of data assimilation is to determine a best possible atmospheric state using observations and short range forecasts. Data assimilation is typically a sequential time-stepping procedure, in which a previous model forecast is compared with newly received observations, the model state is then updated to reflect the observations, a new forecast is initiated, and so on.”

While I’m no modeling expert (as I’m only a weather enthusiast, I mostly enjoy handicapping the models), I know from experience in recent years, particularly with wind speed forecasts in the PNW, that I’ll never be completely ignoring the Euro model. That’s not to say it’s always the model that wins out, but I always want to know what it’s saying and will probably always assign some weight to its baseline forecast.

But enough on that long-winded recap of Wednesday’s major windstorm.

The week ahead:

As you may have heard, a now progressing pattern shift is parking dominant high pressure off the coast, making the PNW more prone to north winds, which pull in colder air and potentially the development of Lows from the colder N/W track.

High pressure is already building rapidly off the coast Sunday, and that should keep out any material precipitation for the first part of the week. Showers may eke in Thursday, but I would still expect mild enough temperatures to see light rain at that point. If we look to Friday (below chart), you can see a pattern of northerly flow and probably some Fraser river basin outflow. Models are mixed whether some light precipitation (potential snow) will move in over the top of the colder air for the coming weekend. Still, it could very well remain dry as high pressure looks set to continue dominating.

We should be clear; this is not a super cold arctic express.  Fringe snow events look more likely if anything. But models do suggest this threat extends at least into early the following week!

As we move closer to any potential snow threat here at Snoqualmie weather, we’ll try to break out the probabilities, similar to how we forecasted the Dec 21st snow when 1-3″ of snow blanketed the Snoqualmie Valley area.

Have a great week!

[Originally published at Snoqualmie Weather blog During active weather, follow more frequent updates @snoqualmiewx or on Facebook]