During the month of April 2013, Denver, CO (link updated monthly) recorded a 74°F difference between maximum and minimum temperatures. This fact tells us nothing about how temperatures compare to climatological norms however. For the entire month, Denver was 5.7°F below normal (41.7°F vs. 46.4°F). The maximum temperature of 80°F was recorded on the 29th while the minimum temperature of 6°F was recorded on the 10th. Here is the time series of Denver temperatures in April 2013:
Figure 1. Time series of temperature at Denver, CO during April 2013. Daily high temperatures are in red, daily low temperatures are in blue, daily average temperatures are in green, climatological normal (1981-2010) high temperatures are in light gray, and normal low temperatures are in dark gray. [Source: NWS]
There is a big disparity between 2013 temperatures and normal temperatures, especially daily maxima. Three outbreaks of Arctic air impacted Denver during the month, which set record low temperatures on four different days. This graph also shows something else that is eye-opening: five daily maximum temperatures were equal to or lower than the climatological daily minimum temperature! As someone who was ready for spring to spring, April was a disappointing weather month.
But it also got me to thinking about the difference between spring 2013 and spring 2012. As many of us remember, temperatures in the US in 2012 were very warm compared to climatological norms. So how different were temperatures in Denver in February-March-April 2013 versus 2012? I decided to take a look. Let’s start with extending the dates in Figure 1 back to the beginning of February 2013:
Figure 2. Time series of temperature at Denver, CO during February-April 2013. Daily high temperatures are in red, daily low temperatures are in blue, climatological normal (1981-2010) high temperatures are the top dark gray line, and normal low temperatures are the bottom dark gray line. [Source: NWS]
This graphic simply demonstrates the same story that I wrote above as well as in my March and February Denver Climate Summary posts. February was obviously colder than normal due to extended cold air masses over the area. March and April were also colder than normal, but this was due to vigorous mid-latitude cyclones that brought Arctic air masses south over the area. This is evident by the significant dips in both maximum and minimum daily temperatures: there was one in the beginning of March, another in the end of March, and three in April.
With this chart in mind, let’s look at the difference between 2012 and 2013. First, daily maximum temperatures:
Figure 3. Time series of maximum temperature at Denver, CO during February-April 2012 and 2013. 2013 temperatures are in brick-red, 2012 temperatures are in red, and climatological normal (1981-2010) high temperatures are the dark gray line with green crosses. [Source: NWS]
My memory of 2012’s maximum temperatures was close to reality. February 2012 was colder than I remember, but this was likely affected by the warmth of April 2012 and the record-setting daily highs in the summer of 2012. Figure 3 shows a very large difference between daily maximum temperatures in 2012 and 2013, especially after the 22nd of March. I didn’t remember the cold snap on April 3, 2012. This graphic shows, by proxy, the lack of spring synoptic storms in 2012. Daily maximum temperatures rarely fell below the normal for the date. Instead, April temperatures were as much as 20°F warmer than normal on some dates, but regularly 10°F warmer than normal. In contrast, 2013 temperatures were often 25-30°F colder than normal. The difference between two years’ temperatures is a measure of interannual weather variability. I have more on that below.
Figure 4. Time series of minimum temperature at Denver, CO during February-April 2012 and 2013. 2013 temperatures are in blue, 2012 temperatures are in green, and climatological normal (1981-2010) high temperatures are the dark gray line with brown pluses. [Source: NWS]
Again, February 2012’s temperatures were similar to February 2013’s. The specific dates of temperature swings obviously varies between the two years. March 2012 and March 2013 also look similar, up until the 22nd of March (see maximum temperatures above also). Thereafter, the time series diverge with much colder air in place over Denver four different times through the end of April. 2012 had warmer than normal minimum temperatures through most of April. The combination of warmer than normal nights and days, combined with a relative lack of precipitation in 2012 set the stage for the record-setting warmth in the summer as well as the rapid decline in drought conditions, which are still largely present now.
I have written hundreds of posts on the effects of global warming and the evidence within the temperature signal of climate change effects. This series of posts takes a very different look at conditions. Instead of multi-decadal trends, this series looks at highly variable weather effects on a very local scale. The interannual variability I’ve shown above is a part of natural change. Climate change influences this natural change – on long time frames. The climate signal is not apparent in these figures because they are of too short duration. The climate signal is instead apparent in the “normals” calculation, which NOAA updates every ten years. The most recent “normal” values cover 1981-2010. The temperature values of 1981-2000 are warmer than the 1971-2000 values, which are warmer than the 1961-1990 values. The interannual variability shown in the figures above will become a part of the 1991-2020 through 2011-2040 normals.
Precipitation was above normal again during April 2013, extending this new trend to three months. During the month, 1.87″ of liquid water equivalent precipitation fell, compared to 1.71″ normally. The wettest April on record was in 1983 when 4.56″ of precipitation fell. There were three notable weather events during April: a 6″+ snowstorm on the 9th, a 7″+ snowstorm on the 15th, and a 5″+ snowstorm on the 22nd. In total, the NWS recorded 20.4″ of snow.
The recent precipitation surplus reduced northeast CO drought severity in the last three m months, but did not break it yet. Above-average precipitation will have to fall for longer than three months for that to happen. The NWS expects continued drought conditions across most of Colorado through the next three months. Additional improvement in eastern Colorado might occur, but NOAA and the CPC expects western Colorado drought to remain the same or worsen.