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Bridging climate science, citizens, and policy


3rd Most 90°F+ days In Denver’s History

A second cool front moved over the Denver, CO area on the 12th of August, preventing temperatures from climbing over 90°F for only the second day this month.  Through the 12th, the NWS recorded 10 90°F+ days, including 3 days at 98°F – narrowly missing another 100°F reading.

Total 90°F+ Days

The total number of 90°F+ days for Denver so far in 2012 is now 56!  That value ties for 3rd most 90°F+ days in Denver’s history, which occurred in 2002.  Unlike 2002, Denver will very likely record additional 90°F+ days, which will move 2012 into at least sole possession of 3rd most 90°F+ days.  Still ahead of 2012 and 2002 in the record books: 60 days in 1994 and 61 days in 2000.

The high temperature in Denver could threaten 90°F, but might fall just short.  Tomorrow and Wednesday should see 90°F+.  The NWS forecasts includes another cool front to move through the area – giving us high temperatures of only 75°F or so!  To put that in context, nighttime low temperatures were just slightly cooler than that early last week (69°F).  Through Friday then, Denver should record 58 90°F+ days.  The weekend could provide one or two more 90°F+ days (likely just one).

That takes us just past mid-August.  There will be a decent number of chances through mid-September to record additional 90°F+ days.  So at this point, I feel confident that 2012 will challenge 2000 for the year with most 90°F+ days.  In contrast, I am more convinced than last week that Denver will record no additional 100°F+ days in 2012.

The above does not mean that the drought affecting Denver or Colorado is anywhere near over.  We need serious precipitation and that isn’t likely to occur for a number of weeks still.  Perhaps as we move into autumn and ENSO returns to neutral or weak El Nino conditions, some sub-tropical moisture will find its way over the US again.


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Denver’s Temperatures Continue March To Records

On the heels of the warmest July in Denver’s history, the first five days of August were also warmer than normal.  Due to a cool front that made its way through the metro area Friday night, Saturday’s high temperature was only 83°F.  Sunday was just as warm as Friday, however, with highs of 97°F and 98°F, respectively.

Through the first five days of the month, the average high has been 93.0°F.  The average temperature over those five days was 77.1°F – a clear reflection of how relatively cool Saturday’s temperatures were.  The departure from normal tracked above 4°F, but is only 2.7°F now.  You can bet that departure reading will edge back up toward 4°F given the lack of weather systems on the horizon.

I still think Denver’s 100°F+ days are likely over for 2012.  Despite my knowledge of future climate projections for the area, I sincerely hope 100°F+ days remain rare in my personal future.  As many other cities across the US can attest, 100°F+ days are simply miserable, in addition to being dangerous to people’s’ health.

The Denver area continues to experience Severe to Extreme drought conditions (see figure below).  I don’t think the last week’s rains will make a serious dent in those conditions.


Figure 1.  Drought conditions across Colorado as of July 31, 2012.  The orange contour indicates Severe drought conditions; the red contour indicates Extreme drought conditions; the brick-red contour indicates Exceptional drought conditions.

In the past couple of weeks, conditions have shifted spatially but haven’t worsened substantially.  Some areas have actually seen slight relief from Extreme to Severe conditions.  This is a shift from three months ago when, as the table in the figure shows, 0% of the state experienced Extreme conditions while 65-73% of the state experienced similar conditions in the past two weeks.  Weather conditions over the next few weeks will determine the level of drought the state experiences.

Consecutive 90°F-day streaks

Saturday’s high of only 83°F (which felt fantastic!) also stopped the streak of consecutive 90°F+ days from early July through early August at 24.  Once the NCDC confirms the temperatures, this streak will match the longest streak in Denver’s history, first set from July 13th through August 5th, 2008.  Denver’s earlier streak of 15 consecutive 90°F+ days should tie for 5th on the all-time list.

Total 90°F+ Days

The record for total 90°F+ days in one calendar year is also in serious trouble.  Through the 5th of August (yesterday), Denver had already recorded 50 such days in 2012 (2 in May, 17 in June, 27 in July, and 4 in August).  That is enough days to tie for 9th on the all-time list.  It seems incredible to someone who has lived in the area for a long time, but the all-time record of 61 90°F+ days seems easy to reach at this point in 2012.  Denver has already surpassed 90°F today, and the NWS predicts similar highs for the next four days.  That will mark 55 90°F+ days, good for a tie for the 4th most 90°F+ days and only 6 days from the all-time mark.  The GFS model provides a glimpse for days beyond Friday and the pattern might change over the upcoming weekend: 90°F is the forecasted high for both days.  Given recent history, I can easily envision highs of 91°F or 92°F, but I look forward to days that can no longer climb above 90°F.


Hottest July on Record for Denver, CO

As expected, July 2012 went down in the history books as the hottest July in recorded history (140+ years) for Denver, CO.  A large number of cities echoed this theme throughout the High Plains and Midwest.  I will cover some of those places in future posts.  This post is all about the capital city of Colorado.

The average temperature in Denver during July, 2012 was 78.9°F.  That value is 4.7°F above normal – so Denver regularly experiences rather warm Julys: 74.2°F.

July 2012 is also very likely to go down in the books as the hottest month ever in Denver’s history.  I posted this information recently and here it is again: the confirmed hottest month in Denver history prior to July 2012 was July 1934 – during the middle of the Dust Bowl years – with an average temperature of 77.8°F:


77.8 1934 #
77.7 2005
77.6 2008
77.3 1936
77.1 1939
76.9 1966, 2003
76.8 1954
76.7 2000, 2001
76.6 1901
76.4 1980

Beating a monthly average temperature record by 1.1°F is significant.  It is even more significant when you consider June 2012 was also the warmest June on record at 75.0°F, which was an astounding 7.6°F warmer than normal!  June 2012 beat June 2004 for the warmest June on record by an also significant 1.5°F.  To put it in a nutshell: 2012 has been extremely warm.

July 2012 missed another top-10 listing – this one for precipitation.  With the weak return of monsoon flow near the end of the month, 0.06″ of rain fell, which pushed July 2012’s precipitation total all the way up to … 0.48″.  That is 1.68″ below average.  But it was enough to rank July 2012 as the 11st driest July on record, missing the top-10 by 0.03″ (1920).

Consecutive 90°F-day streak

I have also recently written about the long strings of 90°F+ days Denver has experienced (suffered?).  Through the end of July, Denver has recorded 21 straight days of 90°F+ maximum temperatures.  That is the 2nd longest such streak in Denver’s recorded history (the longest so far is 24).  There was another long streak from late June through early July: 15 consecutive days.  2012 will be only the 4th year that 2 separate 10+ day streaks occurred during the same calendar year.  The longest streak could be seriously challenged this year: today and the next two days’ high temperatures are predicted to be 90°F+.  Saturday could end the streak as the upper-level ridge of high pressure shifts to the east slightly and a weak cool front moves over Denver from the north: Saturday’s high is expected to be 85°F, then return to 90°F+ Sunday.

The total number of 90°F+ days so far in 2012 is now 46: 2 in May, 17 in June, and 27 in July.  Those 27 90°F+ days in July also set another record.  The previous record was 26, set in 2000 and 2008.  In Denver’s recent climatological period, an average of 32 90°F+ days occur.  8 more days of 90°F+ temperature need to occur in 2012 to get into the top-9 list with 50 days.  The year with the most 90°F degree days was 2000 with 61.  Given the extensive heat that has occurred so far in 2012, I would not be surprised to see that record challenged.  Of course, it will take at least 2 more weeks to see how realistic such an event might be.

The seven days of 100°F+ in July 2012 ties the record set in 2005.  Of course, the six 100°F+ days in June 2012 set its own record.  The combination of 100°F+ days in June and July (13 days) also set a significant record by almost doubling the record set in 2005.

Lastly, this summer could be a really bad anomaly that is mostly natural or it could be caused in part by our changing climate.  Tying specific events such as the 2012 heat wave and drought to climate change is an extremely difficult proposition given the relative lack of robust observational data.  It is further challenged by the occurrence of multi-decadal droughts and heat waves that occurred in the geologically recent past (i.e., within the past 2000 years).  Clearly, those events were not anthropogenic in nature.  Discerning the influence of anthropogenic forcing on current and recent droughts is not straightforward.  As with many other phenomena, we might have to wait decades still before such signals are identified in the observational record.  Suffice to say, this year’s North American heat wave and drought are severe and anomalous.  It remains to be seen how much resilience and capacity is possessed by affected societies.  One bad year is easily survived given the state of technology we possess.  How many bad years might be necessary to challenge that technological state?  How robust are our agricultural and water resource policies, to name just a couple?  How much of a shock can our economic system handle given current weaknesses that remain unaddressed?

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41st Day of 90°F+ Heat In Denver, CO

While 90°F might sound like a low threshold for many across the US Midwest, the total number of 90°F days for Denver in 2012 is racing for the record books.  An average year yields 32 90°F days in Denver – or at least an average year in a cooler climate.

Through the 26th of July, 2012, Denver, CO has recorded a total of 41 days of 90°F+ maximum temperatures.  Wednesday was the last day that could have registered a sub-90°F temperature, but 90°F was the recorded high for the day.  As such, the latest streak of 90°F+ days continues: 16!  That streak edges out the 15-day streak Denver already recorded earlier this summer.  So not only have temperatures been warmer than normal more often than normal, they’ve been warmer than usual for extended periods of time.  That means that ecosystems haven’t had their normal chance to recuperate from such high temperatures – a point that I will spend more time on in an upcoming post.
That streak is likely to continue: the forecast for the next 7 days includes highs in the mid- to upper-90s.  Denver could witness a July with only 4 days below 90°F.  So far this month, the average temperature departure from average is +4.7°F.  That isn’t as high as June (+7.6°F), but the general trend is clear: 2012 is very warm for Denver, CO and other locations across the mid-section of the US.

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13th Day of 100°F+ Heat In Denver, CO

This is a short update to a recent series on this topic (last post here).

Through the 23rd of July, 2012, Denver, CO has officially recorded a total of 13 days of 100°F+ maximum temperatures.  The previous year with such heat was 2005, when the NWS recorded 7 such days.  We are on the cusp of doubling the previous number of 100°F+ days.  Due to Denver’s latitude (~40N) and altitude (~5,200 ft. above sea level), 100°F days are rare.  This year is developing a series of very anomalous heat and drought observations.

Denver has also now recorded 38 days of 90°F+ days, so only 12 more such days need occur the rest of this year to make the top-9 list.

Denver is also in the midst of another consecutive 90°F+ day streak: 13 through yesterday (11 Jul – 23 Jul).  That is in addition to the previous 15-day streak the city recorded from late June through early July.  The streak will continue through today before slightly cooler temperatures (only 89°F?) occur Thursday, then right back into the 90s starting Friday.

The thought of autumn and cooler temperatures is very appealing.


10th Day of 100F+ Heat In Denver, CO

Through the 19th of July, 2012, Denver, CO has officially recorded 10 days of 100F+ maximum temperatures.  The previous year with such heat was 2005, when 7 such days were recorded.  Due to Denver’s latitude (~40N) and altitude (~5,200 ft. above sea level), 100F days are rare.  This year is developing a series of very anomalous heat and drought observations.

Denver typically experiences 32 90F+ days per year.  With most of the summer still to go, Denver has already surpassed that mark.  It appears the city is targeting the top-10 90F days in a year.  That record is held by 2000 when there were 61 days of 90F maximum temperature or higher.  Note that six years since 2000 populate the top-9 list: 2000, 2002, 2005, 2006, 2007, and 2011.  There are only four years prior to 2000 in the top-9 (three are tied for 9th): 1994, 1978, 1964, 1960, and 1874.  The 9th place years recorded 50 90F days.

So far in 2012, there were 2 90F+ days in May, 17 in June, and 15 so far in July, for a year-to-date total of 34.  Denver likely recorded an additional 100F day today and will do the same thing tomorrow.  The extended weather forecast doesn’t call for a high less than 90F for the next 7 days.  Beyond that, above-average temperatures are forecasted for both the next month and the next 3 months.  90F daily maxima are not unheard of in the first couple weeks of September and I don’t at this point expect conditions to be cooler than that this year.

15 consecutive days were 90F+ from the 22nd of June (102F!) through the 6th of July.  If all those records hold up to quality control, that should tie for 7th on the list of most consecutive days with 90F or higher:

2008 JUL 13TH — Aug 5th……24
2011 JUL 15th — AUG 1st……18
1901 JUL  6TH — JUL 23RD…..18
1874 JUL  1ST — JUL 18TH…..18
2000 JUN 29TH — JUL 15TH…..17
1987 JUL 18TH — AUG  2ND…..16
1934 JUL  7TH — JUL 21ST…..15

Notice how the majority of these dates begin in July, not June.  Note further that there were three other times when such a streak began within one day of June 22nd (see full list).  The reason the streak stopped on the 7th of July was a temporary appearance of North American monsoon moisture that made its way from the Gulf of Mexico around the western periphery of the massive high pressure system that is at the heart of the record setting heat wave and drought afflicting the US this year.

So: will 2012 challenge 2000 for the most 90F days in one year?  There are only 16 more days before Denver makes the top-9 list, then 11 more to tie for 1st place.  This isn’t a record most of us want to see happen, of course.  I would much rather see the monsoon return with a vengeance and alleviate the precipitation deficit under which the area is suffering this year.

Will another noteworthy consecutive 90F streak occur?  Denver has 9 in a row through yesterday.  With today’s likely 100F, another streak of 10 joins the list.  As mentioned earlier, conditions don’t look like they’ll change any time soon.  2012 could see two extended streaks of 90F+.  Stay tuned.

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8th Day of 100F+ Heat In Denver, CO

This is just a short update to my post yesterday about the early-summer heat wave affecting Denver, CO.

With the 101F maximum temperature yesterday (a new daily record), Denver has hit at least 100F 8 days already in 2012.  That might not sound like much to folks in Arizona or Texas, but 100F days are rare in Denver due to both our latitude and altitude.

As the 2nd link is kind enough to describe, the 8 days now exceeds 2005, which saw 7 such days.  One big difference between 2005 and 2012 is that in 2005, the 7 days of 100F+ heat occurred near the end of July, not prior to July 4th.  Put simply, not only is this heat wave anomalous in and of itself, but the timing of the heat wave is especially anomalous.  As I wrote in the post yesterday, June 2012 was the warmest June on record for Denver by a significant margin: +7.6F.  June 2012 beat out the previous record average June temperature by 1.5F (1994).  That is a significant margin of heat over an entire month.  It also follows an anomalously warm May and April.

There is good news: the string of consecutive 90F+ days might end this weekend.  Any temperature relief may not be accompanied by precipitation relief, which is also desperately needed.