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.