Was Yesterday's Forecast a Success?

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Was Yesterday's Forecast a Success?

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on - Sat at 3:35 AM -

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Time for a little "inside baseball" on weather forecasting, particularly regarding yesterday's wind event.

There were a few comments about the forecast not being as dramatic as "predicted". I will let you be the judge after we examine the model forecasts and what actually occurred.

Early uncertainty.

I refrained from discussing this event until the prior day (Wednesday).

Why? Because there was substantial uncertainty regarding the track and intensity of the key low center.

The main tool meteorologists use to examine uncertainty is evaluating ensembles of many forecasts.

To illustrate, below is the ensemble of sustained wind forecasts on Tuesday for Seattle Tacoma Airport based on many forecast runs (UW high-resolution WRF model). The purple circles show observed winds and each line is a different forecast. The black line is the average of the many forecasts.

The event occurred around 18Z on the 26th (18Z is 1800 UTC or GMT).

HUGE uncertainty. Forecasts ranged from high winds (35 kt) to less than 5 kt.

Seeing this I held off.

As time progressed, agreement among the various model forecasts increased, and by the night before (initialized at 00Z 26 December, 4 PM PST 25 December), most forecasts were going for a significant event. As shown by the purple dots (observed winds), the ensemble mean (black line) UNDERPLAYED the winds if anything.

So how did the forecast work out more generally? Based on the model forecasts, I highlighted two areas in my blog--the south Sound and the easterly slopes of the Cascades.

On the eastern slopes of the Cascades, several locations had wind speeds exceeding 90 mph, with one location above Ellensburg hitting 112 mph. Ridge tops within the Columbia Basin reached 60-75 mph. So the forecasts for strong winds on the Cascade eastern slopes were fairly good.

Strongest wind gusts on Thursday

Now considered Puget Sound (blow up below of observed max winds on Thursday) As predicted, the strong winds were mainly south of Seattle, with gusts reaching 40-50 mph (see below). Even stronger near the Columbia River.

Thousands of customers lost power over the South Sound (see below), but the damage was mitigated by several previous events that already tested weak branches.

Looking forward, another minor wind event is expected tomorrow (Saturday): see the Seattle City Light windwatch site predicting below. Plus more precipitation.

Yes, this is late December in Seattle.

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