Anchoring Bias and Ensembles

Anchoring bias causes us to rely heavily on the first piece of information that we encounter.  I think of it a lot when I am interpreting model guidance, especially ensembles.   

Here's an example, which I draw from the new 82-member Utah Snow Ensemble.  One of the first products I often pull up is the plume for Alta-Collins.  I find it impossible not to immediately look at the snowiest ensemble member, which for last night's run is producing about 28.5" of snow from 3" of water.  

Like a drug, my heart rate doubles and I feel the adrenalin rush.

Then I pull up the four-panel total snowfall forecasts through 240 hours.  Again, my eye moves immediately to the lower right, which is the maximum snowfall.  Beautiful!  28.5"!

The problem is that such amounts are extreme outliers and highly improbable. What you don't see in a first glance at the plume is that more than 90% of the ensemble members aren't producing any snowfall.  That would probably be the best anchor for a forecast.  

Fight your primal urges when looking these forecasts, especially if your eye is drawn to the maximum.  

Crazy Heat in Phoenix

The wheels have come off in Phoenix or maybe better put burned off.  They have now set or tied daily records on what appears to be 8 consecutive days, set an all-time record of 117 for September and set an all-time record of 113 for October.  The murderer's row of daily records is:

Sep 24: 108
Sep 25: 113
Sep 26: 110
Sep 27: 113
Sep 28: 117
Sep 29: 113
Sep 30: 108
Oct 1: 113

The eight-day mean average high temperature of 111.8F obliterates the previous eight-day mean average high temperature record after September 1 of 108.6, which was set in the seven day period ending on 11 Sep 1979.  

The graph below shows the 8-day average mean temperature for 24 Sep - 10 Oct each year since records began in the Phoenix area, and illustrates just how anomalous the 8-day period has been.  

Min temperatures during this period have also set a record for the 8-day period.

Forecast highs for the airport are in the high 100s to 110 for the next several days.  What a stretch of fall misery.