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.

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″.

Denver’s Warm December Start Didn’t Last

Through the 20th of December, Denver Colorado’s average temperature measured 7.1°F warmer than average.  I told a friend at the time that a monumental shift in weather would have to take place in order for the final month’s tally to approach normal (31.2°F).  Well, we got that shift.  7 of the next 11 nights would record lows below 10°F, with the month’s lowest low recorded on the 26th of December (-2°F).  7 of the next 11 days would record highs below 32°F.  We started the month on a very different note (Slide 8): four of the first five days saw highs at or above 60°F!

This 2-week period of below-average temperatures was the first such event since February 2012.  Every other month this year was characterized by above-average temperatures.

The precipitation deficit continued through December.  Denver finished 2012 with 10.11″ of liquid equivalent precipitation, which is 4.19″ below normal.

2012

Denver’s 2012 climate summary is as follows:

Highest temperature: 105°F (June 25 & 26), which tied daily, monthly, and all-time temperature values.

Lowest temperature: -6°F (January 11)

Average maximum temperature: 68.4°F (+3.7°F above normal)

Average minimum temperature: 39.3°F  (+3.0°F above normal)

Mean temperature: 53.9°F (+3.4°F above normal)

73 days above 90°F (33 more than normal; almost double the normal number of 40 days)

19 days with max below 32°F (1 fewer than normal)

132 days with min below 32°F (25 fewer than normal)

Total precipitation: 10.11″ (4.19″ below normal)

Snowfall: 38.5″ (15.3″ below normal)

In summary then, Denver was much warmer and drier than normal in 2012.

2013

Looking ahead, low-frequency climate patterns (e.g. ENSO and IPO) are currently neutral and weakly negative.  For the next few months, Denver should see near-average temperatures and near- or below-average precipitation.  Since the Denver area is currently experiencing `Moderate` drought conditions, additional below- or near-average precipitation conditions will likely further worsen drought conditions.  Any recovery from this drought is likely to be long-term.

This has direct implications on peoples’ lives.  Water supply will be strained in early 2013.  Agriculture was hit hard by the drought last year and will likely need to plan for continued drought this year.  That translates to higher consumer prices for staple goods.  Pine forests continue to face stressed environmental conditions (e.g. pine beetle epidemic and drought), setting the stage for another season of terrible forest fires.  Stakeholders worked to mitigate some of the climate effects in 2012.  They will have to remain vigilant and informed in 2013.

Denver Weather & Climate: July 2008, Take 2

I wrote about Denver’s temperature and precipitation status one week ago. There is more data now that continues the story set forth in that post. The record streak of 90 degree plus days has continued since then, including today’s ( 8/4/08 ) high of 95 degrees. The record: 23 consecutive days. Tomorrow’s high temperature is forecasted to be about 85 degrees, due to an expected cooler air mass to Denver’s north moving into northern Colorado.

In addition to the streak, which was just fun to keep track of and not really indicative of anything terribly substantial, Denver’s precipitation continues to threaten to set its own record. Through the end of July, we have the following information, provided by the Denver/Boulder NWS Forecast Office:

Jan 1 – Jul 31 2002; Jan 1 – Jul 31 2008

5.34 inches 3.28 inches

Difference: 2.06 inches

Normal (Jan 1 – Jul 31) 10.25 inches

What’s significant about 2002? It currently ranks as Denver’s all-time driest year on record, dating back to 1878. So through seven months of 2008, Denver’s official precipitation measurements add up to a mere 3.28″. As you can see, that’s ~3/10ths of the normal value. That’s more significant than the >90F temperature streak. The most recent month where precipitation was more than average was October 2007, which is quite a ways back now. This summer was forecasted to be drier than average in a general sense, which has unfortunately come true. And really, Denver hasn’t had the worst of it.

According to the U.S. Drought Monitor, the eastern half or so of Colorado is experiencing some level of drought. But look at the southeastern portion of the state. Seven counties (Cheyenne, Kiowa, Otero, Bent, Prowers, Las Animas and Baca) are experiencing Severe Drought, parts of four counties (Bent, Prowers, Las Animas and Baca) are experiencing Extreme Drought and the bottom portion of one county (Baca) is experiencing Exceptional Drought.

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I forgot to mention that Denver hasn’t officially received any rain yet in August either. A trace was recorded on the 3rd, but what little precip did fall didn’t measure up to 0.01″ or more. The NWS only discussed data through the end of July, so I’m not sure how things compare through yesterday. I’ll bring in more information as the NWS records it.