June Heat in Colorado - Why Is It so Hot?
Disclaimer: This draft is an evolving research assessment and not a final report. The analyses presented have not yet been peer reviewed and do not represent official positions of ESRL, NOAA, or DOC. Comments are welcome. For more information, contact Dr. Martin Hoerling (martin.hoerling@noaa.gov)

A string of record breaking maximum temperatures and numerous fires and almost no precipitation in Colorado during the month of June 2012 had people wondering - What's up with the weather? For example Boulder, Colorado had the hottest average maximum temperature (91.2° F) for the month of June. The period from March to June was the driest in the city's weather records dating back to 1894. Is this natural variation? Is it climate change? Has this happened before? The pages below give some background information to help answer these questions.

  • Hoerling, M.P., M. Dettinger, K. Wolter, J. Lukas, J. Eischeid, R. Nemani, B. Liebmann, and K. E. Kunkel, 2012. Evolving Weather and Climate Conditions of the Southwest United States. Chapter 5 in Garfin, G., Jardine, A., Merideth, R., Black, M. and Overpeck, J., (eds.) Assessment of Climate Change in the Southwest United States: a Technical Report Prepared for the U.S. National Climate Assessment. Tucson, AZ: Southwest Climate Alliance. (in press)

  • McQueen, H. R. and H. J. Shellum, 1956: The Heat Wave From the Intermountain Area to the Northern Great Lakes, June 9-13, 1956. Mon. Wea. Rev., 84, 242–251.

Fires in Colorado

On June 26, 2012, lightning sparked the Flagstaff Fire above the NOAA building in Boulder, CO. The timelapse video below was taken by Dustin Henderlon shows the view from Boulder from 3 pm, June 26 to 10 am June 28th.

The fires in Colorado and elsewhere in the US have people asking what is causing this and is this more extreme than usual?

A good discussion on some of the factors leading to the large loss of property and 3 lives is here:

Red Zone: Colorado’s Growing Wildfire Danger

How does this compare to the historical record

According to the National Interagency Fire Center, through July 3, the acreage burned and number of fires are

  • Acreage - 2,199,484
  • Number - 28,420

This compares to the average over the period available 2003-2012

  • Acreage - 2,461,795
  • Number - 39,502

So far this year, the U.S is slightly below average for acreage and considerably below average for number of fires. To compare with a relatively bad year over this short period, 2006 had

  • Acreage - 3,779,450
  • Number - 57,609

In this 10-year record, on a most-to-least scale, so far 2012 ranks fourth out of ten for acres burned and ninth out of ten for number of fires. So while CO has experience severe fire loss, the US fire season as a whole has not been exceptional to date.

A recent web chat was held between weather and climate experts and communicators (including Marty Hoerling from NOAA/ESRL/PSD) on the fires and other extreme events of June. The video below provides more information on the possible causes of these events:

Other links of interest

Politics of Fire and Ice by Chris Mooney

Andy Revkin's Dot Earth blog
Given the regional context, what has been happening in Colorado?


Over the past year, Colorado has seen drier and warmer than normal conditions. Click on a station in the map (or the arrow next to the station name) to see the time series for July 2011 to July 2012 for that station.

Images generated by the High Plains Regional Climate Center

Climatology for Boulder, CO

The plots below show the record maximum, minimum and average temperatures (left) and the chance of precipitation (right) for Boulder, Colorado from 1950 to 2010. Some things to note about the graphs below in relation to 2012. Historically, the maximum high temperatures of the year (> 100 °F) occur in late June and early July. Also, note that there is a climatologically normal period of relatively dry weather at the end of June and into the beginning of July during the period between the usual wet spring season and the onset of the July-August monsoon season.

Temperature Trends

Since 1901, both the maximum and minimum June temperatures have risen. Maximum temperatures have generally increased by 1-1.5 degree Celsius (2-3 degrees Fahrenheit). Minimum temperatures have risen more in western Colorado than the east.

Precipitation Trends

Trends in precipitation are most important on shorter timescales. In the image on the left below, one can see that over the past 2 years, the southern states have had generally dry conditions while the northwest and northeast have been wetter than average. This is consistent with the precipitation pattern associated with La Niña - a cooling of the Eastern Pacific Ocean. As described in the Southwest Climate Assessment: "It is likely that most of recent dryness over the Southwest is associated with a natural, decadal coolness in tropical Pacific sea surface temperatures." (Hoerling et al, 2012)

Is there an historical analog?

In 1956, Colorado experienced a dry spring and a hot June. This was also a La Niña year as was the case in 2012 until late spring. The maps below compare the patterns of 1956 to those in 2012. These maps show a similar story in terms of circulation patterns and their effects.



500 millibar heights (May-June)

Given the trends of warmer temperatures and the 2012 drought conditions in Colorado, one might ask:

  • How have changing conditions over the Southwest been symptomatic of human-induced climate change?

  • Is recent Southwest dryness a symptom of human-induced climate change?

The charts below compare the observed trends of March-April-May (MAM) temperature and precipitation to two different model systems. These box and whisker plots show the observed values (green dot) compared to the range of values from the model ensembles. The range is denoted by the line (whisker) between the maximum (red asterisk) and the minimum (blue asterisk). The bottom and top of the box are the 25th and 75th percentile (the lower and upper quartiles, respectively), and the band near the middle of the box is the 50th percentile (the median). In each of the plots, the top half shows the temperature comparison and the bottom half shows the precipitation comparison.

Observed vs. CMIP5

The CMIP5 (Coupled Model Intercomparison Project Phase 5) dataset is a set of coordinated model experiments from 20 climate modelling groups around the world. This ensemble of different solutions can be used to examine the observed change in temperature with model simulations that include forcings due to increasing CO2. In the plot below, the observed temperature trend aligns with the rise in temperature due to human-induced forcings. However, the observed change in precipitation (drying) is not well simulated by the CMIP5 model runs.

Southwest U.S. CMIP5 vs. Observed

Observed vs. AMIP

The AMIP (Atmospheric Model Intercomparison Project) is a standard experimental protocol for global atmospheric general circulation models (AGCMs). The AGCMs are constrained by realistic sea surface temperature and sea ice so the output is forced by ocean surface processes instead of greenhouse gasses. In the plot below, the precipitation much more closely matches the AMIP solutions, indicating that the primary driving force of the reduced precipitation is a natural variation of the ocean related to El Niño/La Niña cycles.

Southwest U.S. AMIP vs. Observed

How do the heat and dryness fit into historical trends for the region?

A soon to be published report titled the "Southwest Climate Assessment" (Hoerling et al, 2012) can be used as a guide to answer these questions. Here are some findings:

Temperature and Precipitation

  • The decade 2001–2010 was the warmest and the third driest in the Southwest of all decades from 1901 to 2010

  • Average annual temperature increased +0.9°C +/- 0.3°C over the Southwest during 1901–2010, while annual precipitation experienced little change.


  • Streamflow totals in the four major drainage basins of the Southwest were 5% to 37% lower during 2001–2010 than their average flows in the twentieth century.

  • Streamflow and snowmelt in many snowmelt-fed streams of the Southwest trended towards earlier arrivals from 1950-1999, and climate science has attributed up to 60% of these trends to the influence of increasing greenhouse-gas concentrations in the atmosphere.

Paleoclimate Context

  • The period since 1950 has been warmer in the Southwest than any comparable period in at least 600 years, based on paleoclimatic reconstructions of past temperatures.

  • The most severe sustained droughts during 1901–2010 were exceeded in severity and duration by several drought events in the preceding 2000 years, based on paleoclimatic reconstructions of past droughts.

A comparison of the average 2001-2010 March/April/May (MAM) daily maximum temperature (top panel) and daily minimum temperature (middle panel). Units are °C with warmer (colder) trends shown in red (blue). Lower panel is the comparison of averaged precipitation. Units are the total change expressed as % of annual climatology, and positive (negative) trends are shown in green (orange).