Friday, October 20, 2023

About Those Seasonal Forecasts

 NOAA released an updated seasonal forecast yesterday, which immediately got everyone in a lather with much of Utah being "in the green" for January to March precipitation.  

But let's all take a deep breath here.  First, these outlooks have never been very reliable (or all that useful for skiers).  Remember the outlooks from last season?  My "favorite" was the one issued by Accuweather last September.

That didn't age well in the Sierra and Utah that's for sure.  

Second, let's talk a bit about what the NOAA seasonal outlook actually means.  It is a probabilistic outlook, meaning that they are not forecasting those areas in green to definitely receive above normal precipitation.  Instead in the first two shades of green, they are saying that the odds are leaning to above normal.  For example, southern and central Utah are in the first shade of green, which means they are giving a 33–40% chance of above normal precipitation.  In any given season, if you have no idea what will happen, there's 33% chance of above normal, a 33% chance of near normal, and a 33% chance of below normal, so this is a very minor shift of the odds. 

But let's look a bit closer at the rationale behind the NOAA forecast.  Such forecasts are based in part on precipitation during prior El Nino events (this winter is expected to be an El Nino winter) and ensemble forecasts produced by statistical and numerical models.  Here's a snippet from the discussion issued with the seasonal outlook:

"Despite a strong El Nino event highly likely this upcoming winter and spring, the NMME and C3S dynamical model guidance depicts high forecast spread and seasonal precipitation outcomes - both in sign (i.e., wet/dry) and location. This is quite different from similar forecast guidance preceding the 2015-2016 El Nino event. Given this, the forecast probabilities for either above-normal or below-normal seasonal precipitation are quite modest for the western CONUS. Elevated odds for above-normal precipitation is depicted for parts of California and Nevada during DJF 2023-2024 and JFM 2024, with slightly elevated probabilities extended across the central Rockies. The best convergence of ancillary information, forecast tools and model guidance is the primary basis for the selection of this area."

I have underlined a key phrase, which essentially says there is a lack of correspondence in the guidance.  And this guidance isn't very good in the first place!  Walk away from the craps table now! 

Note also that the northern edge of the green shading is draped across Salt Lake County.  If they are anticipating any loading of the dice for the central Wasatch, it is very slight.  

I continue to stand by my usual outlook for the central Wasatch.  We simply cannot provide a reliable outlook of what is going to happen with snowfall this season near and above about 7000 feet.  If you guess below, near, or above average, you have about a 1/3 chance of that guess being right.  Below 7000 ft, there is probably some loading of the dice for below average snowfall and snowpack simply due to the fact that we are now living in a warmer climate system, but as we saw last year, Mother Nature can still bring the goods with the right pattern, so I'm not going all in on that.  

Finally, stop worrying about this winter.  A bad year in the Cottonwoods is better than a good year in Colorado, so just plan on skiing it if it's white. 


  1. Depends on where in CO haha

  2. Doubtless you know 12Z 20-Oct-2023 GFS has 13 inches between Weds 6pm and Thurs 6pm. Mean of NAEFS plume looks to be the same, though Canadian members look to be 5 inches. Max 40 inches. Not that I'm obsessed.


    PS, Notwithstanding the omens, 2 laps Mambo Friday the 13th, and another 2 Saturday the 14th. Not a bad start to the season, clipped a few rocks in predictable spots. A group dropped Main Chute during the eclipse, another group followed shortly after. They had fun by their hoots and hollers.

    Alas it appears another year w basal facets high north.

    1. And now 06Z 21-Oct-2023 GFS has 18 inches between Weds 1pm and Thurs 3pm.

      HOWEVER, Canadian members of the NAEFS have DESERTED the storm having a mean of ZERO.

      Looks like there may be a processing error on the Canadian output, God forbid they have better snow insight than GFS.

      We DESERVE at least 1 two foot snow storm before Halloween

    2. Well, the GFS is one of many possible outcomes. Best not to rely on it unless you know when it is going to be right ;-).

      It's a tricky forecast. I didn't discuss why in my latest post, but this is a case of downstream development and those can have a lot of issues with regard to the track and intensity of the developing trough.