Wednesday, November 29, 2017

The Deception of a Single GFS Forecast

Having cut my teeth in an era before ensembles, I confess some tendency to pull up the medium range forecasts produced by the Global Forecast System (GFS) when I arrive at the office during times of drought to scan for the next hope for flakes.

This is a colossal mistake, especially in the current pattern.  

The predictability at 4-7 days seems to have been remarkably low the past few weeks, leading to individual GFS forecasts that might be described as "all over the place," although one might argue that in general they have trended to drier as the forecast lead-time decreases.

Here's an example of the types of changes that one sees.  The 162 hour forecast from the GFS initialized at 0600 UTC 27 January November shows a major storm for all of the mountains of Utah on Sunday afternoon.  Great hope right?

Two days later, the 114 hour forecast from the run initialized at 0600 UTC last night says NO SNOW FOR YOU!

Let's consult an ensemble forecast.  We had a problem with our 0000 UTC NAEFS processing, so I'll use the plume forecast for Alta-Collins from yesterday.  Look at that spread.  There are some members generating over 3" of water and 30" of snow and others absolutely nothing.

I personally like plume diagrams, but the members producing the heaviest precipitation are the ones that attract the eye's attention and cause the heart to race.  Close inspection of that diagram shows that most members (about 40%) are producing less than 0.5" of water.  Thus, if one never bothered looking at the GFS, but instead the ensembles, the possibility that we're not going to see much is there.  On the other hand, the possibility of a larger storm is small, but not zero.

Humans like single model runs.  They are easy to look at and interpret, and they produce plausible forecasts.

However, for medium-range forecasts, they can be deceptive.  They do not provide estimates of the range of possibilities, and in a pattern like this, that's a problem.

I go to bed at night in a pattern like this expecting the worst (dust on dirt) and hoping for the best (major dumpage.


  1. Please start spreading the word, as there are new chem trails everyday in Utah, just look up:

  2. ECMWF also showing a quicker, less moist shortwave for Sun-Mon. For those relying on single models, a look at the single model run of the GFS, ECMWF, UKMET, Canadian Model, and eyeballing the mean pattern, might offer the same, if not better result.

  3. Are 10 outcomes enough to illustrate the uncertainty? Why not do 100 or more? Is it limited by computer time, cost?

    1. 10 really isn't enough, but it is better than nothing. The NAEFS has more members than that, but even still, it probably isn't enough.

      Computer time is always limited. There is a tradeoff that must be made between grid spacing (smaller means more time - halving grid spacing requires at least 8 times the computer power), number of members, and sophistication of the model dynamics and physics. We don't really know the best way to do that tradeoff, and the best mix probably depends on the problem you are working on.

      If you combine the Canadian, US, and European Center ensembles, you have close to 100 members. The European Center ensemble currently had 51 members and is you could have just one, that would be it, but it costs money, which is why we don't have it in our processing suite at the University of Utah at present.