Saturday, September 20, 2014

Let the Hype Begin

The latest (and some of the previous) GFS forecast is calling for a bonafide fall trough to move into Utah for next weekend.  The coveted 0ºC 700-mb temperature contour is draped across the Great Salt Lake at noon next Saturday.

Before waxing the skis, some recognition of the chaotic nature of these long-term forecasts is needed.  We start with the North American Ensemble Forecast System, which consists of 21 forecasts from the US Global Ensemble Forecast System and 21 forecasts from the Canadian Ensemble Forecast system.  The plot below is known as a spaghetti diagram and includes contours of 500-mb geopotential height from all the members, along with a thick red contour for the ensemble mean.  This chart shows that most ensemble members put a trough over the western U.S. but the coverage of spaghetti suggests quite a bit of variability in terms of placement and strength.  
Source: NWS
Moving to the gold standard ECMWF, I didn't have time to hunt down a spaghetti diagram this morning, but the mean of their ensemble also puts a trough in the west (note that there are slight variations in the time all these forecasts are valid).  

Source: ECMWF
My take on this is that it looks like we may have a deep trough entering the western U.S. late next week or next weekend, but we'll have to see how it shakes out in terms of strength and timing.  I suspect, however, that these forecasts are enough to jumpstart the weather hype machine.


  1. The GSL is good and toasty, any tropical tap to be had? I love this time of year.

    1. Impossible to predict any details with reliability at this stage. There's still uncertainty in the timing and strength of the trough.

  2. Jim, I have a hike and bike trip planned to the Capitol Reef area this Friday/Saturday/Sunday. How bad do you think things will be down there? Any expert guidance appreciated!