Denver’s April 2013 Climate Summary With A Bonus

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.

Interannual Variability

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

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.

4th Daily April Record Low in Denver & Record Snow in Boulder

I spent a lot of time on record temperatures in Colorado in 2012 – they were all record highs.  Due to annual weather variability, there are a couple of different records in April 2013: record lows.  There have been four record lows set or tied in Denver, CO this April:

9F on April 9th

6F on April 10th

22F on April 16th (tie)

21F on April 22nd

Needless to say, with record low temperatures due to vigorous synoptic cyclones that brought Arctic air masses down into the middle of the country, April’s average temperature is among the lowest on record.  I will have more to say about that next week after the month ends.  Denver may not record a bottom-10 moth because much more seasonable weather is on tap for the next week.  In contrast, two record highs were set in April 2012: 84F on the 1st and 88F on the 24th.

In other news, Boulder, CO set a monthly record for snowfall: 47.4″ through the 23rd!  The old record of 44″ was set in 1957.  The official snowfall measurement site for Denver (Denver Int’l Airport) recorded “only” 20.4″ of snow for the month-to-date.  With 60F+ temperatures forecasted from today through next Tuesday, DIA won’t challenge the top-10 snowiest Aprils (#10 recorded 21.0″ of snow).

Remember that one month’s, season’s or year’s temperatures, precipitation, or even drought are not indicative by themselves of climate change.  They are too heavily influenced by individual weather systems.  When I discuss climate change, I write about long-term trends (decadal to multi-decadal).  Natural variability influences individual weather events that overlie the long-term climate signal.  I’ve written before that climate change means we are more likely to see record high temperatures than record low temperatures.  The weather will continue to set both, but will set the former at a higher rate moving forward than the latter.  Of course, I for one am very glad there was more precipitation than normal for April.  Last year’s drought and record hot summer was not enjoyable to live through.  Denver-Boulder and the surrounding region will unfortunately need months in a row of above average precipitation to break the long-term drought.  This spring’s precipitation pattern slightly reduced the intensity and areal coverage of drought.  I will update my last drought post in the next couple of days.

Denver’s March 2013 Climate Summary

During the month of March 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 2.7°F below normal (37.7°F vs. 40.4°F).  The maximum temperature of 76°F was recorded on the 15th while the minimum temperature of 2°F was recorded on the 25th.

Figure 1. Time series of temperature at Denver, CO during March 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]

Precipitation was above normal again during March 2013, making a two-month trend.  During the month, 1.47″ of liquid water equivalent precipitation fell, compared to 0.92″ normally.  The wettest March on record was in 1983 when 4.56″ of precipitation fell.  There were two notable weather events during March: a 6″+ snowstorm on the 9th and the 23rd.  In total, the NWS recorded 23.5″ of snow, 13.5″ more than the normal of 10.0″ for the month.

While more precipitation fell than normal during the month, the drought impacting the region was still not broken.  Above-average precipitation will have to fall for longer than one month for that to happen.  The NWS expects continued drought conditions across most of Colorado through the next three months.  Some improvement in northeast Colorado might occur.  In contrast to February and March, the NWS projects warmer and drier than normal conditions over Colorado during the next three months.

Denver’s February 2013 Climate Summary

During the month of February 2013, Denver, CO recorded a 58°F difference between maximum and minimum temperatures (20°F less than January!).  This fact tells us nothing about how temperatures compare to climatological norms however.  For the entire month, Denver was 2.4°F below normal (30.1°F vs. 32.5°F).  The maximum temperature of 63°F was recorded on the 17th while the minimum temperature of 5°F was recorded on the 22nd.

Figure 1. Time series of temperature at Denver, CO during February 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]

Precipitation was finally above normal again during February 2013.  During the month, 0.77″ of liquid water equivalent precipitation fell, compared to 0.37″ normally.  For the first time in my life, rain fell across the Denver metro area in February!  On the 6th, it rained very lightly, just enough to make the streets and plants wet.  To add to the oddity and rarity of the situation, the ground was still wet with liquid on the morning of the 7th – it wasn’t cold enough to freeze the rain overnight. A similar event occurred in late January.  Conditions returned to normal in the second half of the month.  Measurable snow finally fell on the 20th and 21st of the month.  Then a significant winter storm hit the area on the 24th, dropping ~9″ of snow across the metro area.  In total, the NWS recorded 14.1″ of snow, 8.2″ more than the normal of 5.9″ for the month.

While more precipitation fell than normal during the month, the drought impacting the region was not broken.  Above-average precipitation will have to fall for longer than one month for that to happen.

Denver’s January 2013 Climate Summary

During the month of January 2013, Denver, CO recorded a 78°F difference between maximum and minimum temperatures.  Does that tell you anything about whether it was warmer or colder than normal?  No, it does not.  For the entire month, Denver was 0.4°F below normal (30.3°F vs. 30.7°F).  But the maximum temperature of 66°F was recorded on the 24th while the minimum temperature of -12°F was recorded on the 12th.

Figure 1. Time series of temperature at Denver, CO during January 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]

Precipitation was below normal again during January 2013.  0.31″ of liquid water equivalent precipitation fell during the month, compared to 0.41″ normally.  For the first time in my life, rain fell across the Denver metro area in January!  Two days after hitting the high for the month, the National Weather Service recorded 0.01″ of rain on the 26th.  I haven’t read anything regarding historical rain in Denver in January, but I think such an event is very rare indeed.  4.6″ of snow fell, which was 2.4″ below the normal of 7.0″.

Drought Continues To Affect Middle U.S.

The extensive and high magnitude drought that has afflicted various parts of the U.S. continues late into the 2012 calendar year.  These conditions will only be alleviated if significant changes in precipitation patterns occur in 2013.  I wanted to do a quick post on this topic as a significant winter storm is making its way across the country so I can do a before/after look next week.  This storm will largely impact the inter-mountain west as some places receive 1 to 3 feet of snow.  Some slight relief will also be felt across the High and northern Great Plains as a blizzard impacts the region today and tomorrow.  Here are drought conditions in the West as of last week:

The percent area values haven’t changed too much since early July, but the specific areas impacted have moved around somewhat in response to weather systems in the intervening time period.  The mountains in this region are experiencing their own drought, just as the inter-mountain plateaus are.

Look at the High Plains, which saw drought conditions expand and worsen quickly this summer and fall:

Only a tiny stretch of North Dakota is not currently experiencing any level of drought.  A majority of the High Plains is actually in Extreme drought (red or dark brown).  These conditions developed rapidly north of 40 degrees latitude (northern border of Kansas) this year.  These locations will benefit somewhat from winter snow, but need spring and summer rain to really get rid of this drought.

Denver Area

So far in the month of December, the average temperature for Denver, CO is 37.3°F, which is 7.1°F above normal.  The next week or so will include near and below average temperatures, which will reduce this anomaly.  With only 12 days left in the month however, December will likely be another month in 2012 that higher than normal temperatures were the rule.  Denver’s average November temperature was 43.5°F, 5.2°F above normal.  October bucked the 2012 trend with a -1.9°F anomaly (49.0°F average).  September was 2.9°F above normal (66.3°F average).

In November, there was a -0.34″ precipitation deficit from normal.  October was slightly more wet than normal: +0.20″ (1.22″ total), as was September: +1.99″ (2.95″ total).  This is largely why drought severity in the Denver area is only moderate and not severe or extreme.  Note that most of the positive precipitation anomaly in September was the result of one storm which rained overnight on the 25th into the 26th (1.95″).  Without this storm, local drought conditions would likely be worse.

Precipitation for 2012 remain anomalously low.  The NWS recorded 9.89″ of precipitation so far this year, compared to a normal of 14.17″ (-4.28″ or 30% deficit).  In contrast, 2011 was a wet year with 17.31″ through Dec. 31st.  We all hope 2013 returns to normal or above average conditions so that this drought ends.

Data from Denver-Boulder NWS Office.