National Weather Service United States Department of Commerce

Paul Iñiguez, Science & Operations Officer, NOAA/NWS Phoenix, AZ


There is a good chance (47%) that 2017 will be the warmest year on record in Phoenix, AZ.


Through November, the average temperature for 2017 in Phoenix, AZ is 78.9 °F. This ties 2014 for hottest year on record (so far). But will 2017 end up being the hottest year on record, beating out 2014? Let's use a few tools to make an educated guess where this year will rank among the hottest on record.


Daily average temperature 2014 vs 2017

Fig. 1. Daily average temperatures in Phoenix, AZ for 2014 and 2017 (through November 29).


How will 2017 end? We can utilize the one-month outlook for December 2017 (below) issued by the NOAA Climate Prediction Center (CPC). Based on the CPC outlook, there is approximately a 55% chance that the average temperature for December will be in the upper (warmest) third of the climate distribution, a 33% chance it is in the middle third, and an 11% chance it is in the lower (coolest) third.


NOAA CPC Outlook for Dec 2017

Fig. 2. Temperature outlook from NOAA Climate Prediction Center for December 2017 (issued 16 Nov 2017).


Using these numbers, we set up a computer simulation where it will randomly pick a mean temperature for December based on those probabilities. That would mean a 55% chance the temperature for December is at-or-above 56.5 °F, a 33% chance it is between 54.4 °F and 56.4 °F, and just an 11% chance it is 52.5 °F or cooler. To put limits on the extremes, we'll say December 2017 can't be warmer than 61.5 °F (the warmest December on record occurred in 1980 with an average temperature of 61.3 °F). The coolest December on record was much cooler (46.6 °F in 1911); the climate is trending so strongly that values that cool are exceptionally unlikely to occur. In the past 40 years, the coolest December was in 1978 with 51.7 °F, so we'll use 51.5 °F as a lower bound.


December Average Temperatures

Fig. 3. Historical values of the average temperature during December in Phoenix, AZ.

The long-term trend for December is an increase of +3.9 °F per century (purple line).


These parameters now allow us to simulate December 2017 based on Phoenix's climatology and the prediction from the NOAA CPC. We randomly select a value from the upper third (56.5-61.5 °F) fifty-five times, randomly select a value from the middle third (54.4-56.4 °F) thirty-three times, and randomly select a value from the lower third (51.5-54.3 °F) eleven times to build out a sampling of potential values. From these 99 values, we then randomly select one for the value of December 2017, combine it with the observed data from January-November 2017, and come up with a final value for the average annual temperature. We repeat this process one million times.


Example of how simulations are built

Fig. 4. Depiction of how values for December 2017 were simulated. Process was repeated one million times.


The result? Based on one million simulations, there is a 47% chance that 2017 will be the hottest year on record, a 96% chance it'll end up ranking in the Top 3, and >99% it will be in the Top 10. This is the continuation of a long-term trend of increasing temperatures in Phoenix and much of the United States (link). The average annual temperature in Phoenix is warming at a rate of over half a degree per decade (5.9 °F/century).


Annual trend of temperatures in Phoenix.

Fig. 5. Long-term trend of annual temperatures in Phoenix, AZ, along

with likely 2017 value plotted. The annual trend is +5.9 °F/century.