The weather prediction business is a humble business.
Yes, forecasting systems have gotten much better, but they are disappointing at times, especially for difficult situations like this morning’s snow event.
Modeling run on Friday shows that the Puget Sound nation is at risk of snowfall, both in terms of temperature and rainfall.
Here’s the precipitation forecast (24-hour liquid water forecast ending 4 a.m. today–Saturday. No rain in the eastern suburbs of Seattle and about 0.15 inches in the east. west of northwest Seattle.
With a typical 10 to 1 ratio for snow to rain, NW Seattle should have only been about 1.5 inches at most. A substantial portion of north Seattle has 4 inches. For wet snow, the ratio is even lower – so it’s clear that precipitation has been significantly underestimated.
Below are the observed amounts for the same period. Oh.
Much heavier-than-expected rainfall in Seattle and eastern suburbs
Models had predicted a large amount of rain on the eastern edge (above Seattle), but as shown on radar around midnight last night, a large area of rain extended across the region.
What makes this situation particularly worrisome, is that our main tool for detecting outliers of weather predictions…..the set prediction runs the model over and over again, slightly different each time–didn’t get this wet solution effectively.
The figure below shows predicted precipitation at SeaTac for multiple group members (different lines) and actual precipitation shown by purple circles. Only one out of twenty groups got it right. The mean of all populations, the population mean, shown in black, predicts less than half of the observed precipitation.
This is a rather large and unusual failure for such a short forecast. In any case, one can learn from failures and I will look at this carefully.
Finally, one fascinating aspect of this event is the extreme variability of snow, as temperatures vary with altitude.
Sites in Seattle near sea level saw only light dust, while just a few miles away saw 4-5 inches of heavy, moist snow.
Sobering up from this setback, I drove around Seattle this morning. Near sea level on Sand Point Way (NE 90th), nothing.
But when I drove to Lake City, a few hundred feet above SL, it was all white with a few inches of snow. By Aurora Avenue, about 4 inches.
Sunday Snow Event
Clouds and precipitation are starting to move northward around an area of low pressure off the coast of California (see latest infrared satellite images).
The latest model run brings some light snowfall into Oregon and southern Washington tomorrow, as illustrated by the total snow accumulation predicted through 1 a.m. Monday.
I’m starting to get tired of these snowy side situations……
