December 03, 2022

A Forecast Failure and the Next Snow Event

 The weather prediction business is a humbling one.     

Yes, forecasting systems have gotten far better, but occasionally they disappoint, particularly for difficult situations like this morning's snow event.

The model runs on Friday indicated that Puget Sound country was on the edge for sn ow, both in terms of temperature and precipitation.

Below is the precipitation forecast (amount of liquid water predicted for the 24h ending 4 AM today--Saturday.   No precipitation over Seattle's eastern suburbs and around 0.15 inches over the western side of northwest Seattle.   

With the typical ratio of 10 to 1 for snow to rain amounts, NW Seattle should have had only had about 1.5 inches at most.  Substantial portions of north Seattle had 4 inches.  For wet snow, the ratio is even lower--so there was clearly a substantial underestimate of precipitation.

Here are the observed amounts for the same period.  Ouch.

Much more precipitation than predicted over Seattle and the eastern suburbs

The models had predicted a sharp eastern edge of the rainfall (over Seattle), but as shown by the radar last night around midnight, a broad area of precipitation extended over the region.


What made this situation particularly disturbing, was that our primary tool for spotting weather prediction outliers.....ensemble prediction running the model many times, each slightly differently--did not effectively pick up on this wet solution.  

The figure below shows the predicted precipitation at SeaTac for the many ensemle members (the various lines) and actual precipitation is shown by the purple circles.  Only one of two dozen ensembles got it right.   The average of all the ensembles, the ensemble mean, shown in black, predicted 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 be examing this one carefully.

Finally, a fascinating aspect of this event was the extreme variability of the snow, driven by temperature variations with height.

 Locations in Seattle near sea level had only a light dusting, while just a few miles away there was 4-5 inches of heavy, wet snow.   

Sobered by this failure I drove around Seattle this morning.   Near sea level on Sand Point Way (NE 90th), there was nothing.  


But as a drove to Lake City , a few hundred ft above SL, it was all white with a few inches of slush.  By  Aurora avenue, around 4 inches.


Sunday's Snow Event

Clouds and precipitation are starting to swing northward around the low-pressure area off of California (see latest infrared satellite image).


The latest model runs brings some light snow into Oregon and southern Washington tomorrow, as illustrated by the total predicted snow accumulation through 1 AM Monday.


I am starting to get tired of these marginal snow situations......



30 comments:

  1. I too am tired of the marginal snow events and I expressed this exact same thought yesterday.
    Nevertheless, some snow excitement is better than nothing. Hopefully, we get the real deal near xmas this year!

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  2. Parts of east King County did get more snow, such as union Hill which got about 5 in last night.

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  3. The snow line was almost perfect right in the convergence zone area... but it was "tehnically" not a convergence zone event.

    Or, maybe it was was, that the geography worked the weather into another manifestation in the same zone.

    Pretty neat tho none the less.

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  4. In our region, it seems we need forecasts for every neighborhood!...I got at least two more inches of snow in my S. Everett neighborhood...but two miles away, there was half that amount. The upside this am Sunday, is that much of the snow has melted away, even though at this moment, the temperature is 31degrees, with a full moon shining down at 4am.

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  5. I was alittle skeptical that the storm could drop so much snow on the Olympics and nothing only a 10 or 20 miles further east but it is very hard to predict the path of these cut off lows. I also skeptical that it would rain when dewpoints were so low although it did almost rain Seatac only got 2" of snow with .5" of water. This is very wet snow on the verge of rain if it were alittle colder we would have gotten 4 to 5" of snow. Overall I'm not that impressed with the high resolution models because although they got it mostly right during these marginal snow events they were flip flopping alot a day or two before the storm I rember seeing 4 or 5 in of snow forecast for Seatac the next day nothing.

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  6. Thanks for your work, it is much appreciated!

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  7. Fairlure of the models is actually kind of awesome, without the failures we wouldn't be able to build a better system. I am a bit nervous this morning, we are suppose to get rain, but we are still at 33 degrees and I fear we may get more snow, ack!

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  8. Does anyone remember when their grandfathers sat near the wood heater at the local mercantile, played checkers, and talked about the weather? They got it wrong occasionally, too.😒

    Seeing the ensemble model results with the actual is a nice touch. Thanks.

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  9. Didn't the Thursday UW model pretty much nail the Saturday snow event? If I recall correctly, the maps on your Thursday blog post had roughly 3-5 inches for much of North King / South Snohomish with less closer to Lake Washington. If so, it's interesting that the Thursday model was so much more accurate than the Friday model.

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  10. Uncertainty is always a feature of prediction. What amazes me is when any prediction is mostly correct. Keep up the hard work. Admittingly 'failure' is almost always the one path to improvement: no (noticed or admitted errors), there's no possibility of error correction. Or to paraphrase Terry Pratchett, 'good judgement comes from experience, which comes from prior bad judgements.'

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  11. I find the snow WAY more interesting than the rain.

    Reminds me of a bumper sticker I saw many moons ago:

    Mother Nature Bats Last.

    Enjoy

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  12. Hi Cliff, this morning we found the oddest formation in a bucket of ice in the backyard. It looks roughly like a chicken-foot-shaped protuberance of solid ice poking up to 2" above the surface of ice in the bucket. What could this be and how did it happen? There is nothing inside the ice that it could be forming around...
    Thanks!

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  13. Yesterday, awoke to clear skies and sunshine, once it rose (I was up at 5, sun rose around 7:40) and remained bright and sunny all day, up to at least SeaTac as my sister and a family friend and I drove to a niece's place for cookie baking. By the time I arrived home to my place in Tacoma around 4:30, my grass was mostly bare but a few spots still had some, though not much snow, outdoor lights on, including the 4 string of Christmas lights on a tree out front when I arrived home.

    Today, it's cloudy, snowing and/or raining, or a mixture of both and at the moment, snow, mostly or entirely, beginning to dust my car and possibly the pavement. According to the Weather Channel, it's 37, with the high around 39, but may not be as "warm" as they show it.

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  14. So much variation with different outcomes ... seems like something is missing. What about nesting a land use model like LSM Noah or whatever the latest and greatest is -- Just for the lowland / urban bits?

    How much fine grained urban modelling of the lowest 1000' is in your model already? This doesn't seem like a *meteorology* failure to me, but a problem with urban ecological modelling not being present.

    Land use data is available on a 3m grid now -- do we need to model the urban canyons (rooftop heights very over the urban area) directly? Could we use the personal weather station data to get a finer grid of temps with elevation *in the cities*?

    During this last week I followed your models carefully and also looked at the 'radar' plots on wunderground/nexrad (how high up do those sample?), and it was clear the 'radar' was showing snow that reached the ground or not depending on very local conditions way down here on actual earth ;)

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    Replies
    1. We already use the Noah LSM in our regional model.

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    2. Dang. And there I thought I had a good idea for you. :) Well, it would really be interesting to here your ideas on 'future directions' after you've mulled this one for a bit.

      At least you must have the data to look for systematic biases (it sounds like model prediction skill by elevation is one of them)

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  15. Just a quick update. Tacoma has gone from snow/rain mix to pure snow since around 11-11:30 and everything is completely white now and the temps have dropped to 33 from the upper 30's.

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  16. " I am starting to get tired of these marginal snow situations......"

    As opposed to Uber perfect, spot on, absolute forecasting?

    It's called life and one gains confidence by adapting and dealing with the outcomes as opposed to trying to outsmart dozens of micro climates in a given city

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  17. Can't win em all. If you wanted a perfect record than San Diego is about the only place to pull that off.

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  18. It's been snowing very hard in Puyallup for several hours, already 2 inches of accumulation with no signs of it letting up. This looks like another forecast failure.

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  19. And the trend continues, low did not slide down as forecast and is shifting north from Southwest WA. throwing cold moisture from the east. We have heavy and continuing snow at the Eastside highlands

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  20. Snowing in Gig Harbor since 11 am. Monitored WSDOT Tacoma twitter for the endless collisions. Thinking I could make a snowman! Still coming down and sticking. Will measure the inches in another hour. So much for changing to rain. 32 -33 F steady state. If the temp dips the roads will be ice.

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  21. Kinda gets a skeptic wondering if these local weather forecasts can miss the mark in just a span of 2 or 3 days, why is any rational person (the key qualifier being "rational") believing the climate doom prophets (or should that word be spelled "profits") projecting the end of human life as we know it in 15, 20, 50 or _______ (fill in the blank) years from now?

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    Replies
    1. Actually there are many cases where it is easier to predict something long term compared to short term. For example, it is near impossible to correctly predict if anyone who reads this sentence will die in the next 2 or 3 days, but it is very easy to predict that anyone reading this now will be dead in a100 years

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  22. Almost 3 new inches in NE Puyallup. Temp fluctuation between 33 and 31. Dropped to 31 during the heaviest of snow fall. Did have some freezing rain when things started around 1100. A little of everything today to keep it interesting. Thanks for keeping us informed Cliff!

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  23. Well they got our part of the forecast right. We got a lot of snow in Western Kitsap County. Our neighborhood was without power for 36 hours (after 40 hours without power the previous storm). Ah, the problems of living in the woods.

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    Replies
    1. Completely missed a few miles away: 0 inches near lowland Suquamish.

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  24. Yes, I am getting tired of it too... Let's go back to normal, with rain, with highs in the upper 40's, low around 40.

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  25. On the waterfront in downtown Bremerton, less than half an inch. Relatives on Kitsap Lake had 8 inches much of which is still in place. Manette must have had a couple inches, still a little left yesterday. Top of the ridge west of Silverdale only a little. I am suspecting some narrow snow bands hit some places, narrow part leading.

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