Weatherdem's Weblog

Bridging climate science, citizens, and policy

Leave a comment

Current & Future U.S. Heat Waves

A substantial portion of the U.S. population experienced a heat wave during the past week.  Due to the number of people affected, the media spent some time on the topic.  As opposed to places like Las Vegas or Phoenix, where the “heat is supposed to happen”, folks normally accustomed to rather pleasant summer conditions experienced real heat again.  Heat waves of various intensity happen every year.  This  heat wave is rather intense – it is breaking some heat records.  Some interesting factoids:

Temperatures at Newark Liberty Airport in New Jersey were recorded at 98 degrees at 1 p.m. local time on Friday, as the mercury hit 93 in Central Park. John F. Kennedy Airport in Queens, New York, recorded temperatures of 100 degrees on Thursday, beating out the previous record set for that date a year ago, and on Friday the heat index there reached 108.

Electricity usage soared to an all-time high in New York City as the work week closed out, provider Con Edison announced, as service hit a peak of 13,214 megawatts around 2 p.m. local time. The previous record was 13,189 megawatts on July 22, 2011, according to the company.

So, some serious heat and serious energy consumption.  The latter proves interesting to look at in more detail: if warming trends continue, power plants will be unable to operate like we expect them to due to water and infrastructure cooling requirements.  That spells trouble for people: the worst heat waves of the future might be accompanied by temporary brownouts and blackouts.  How manageable will heat waves be with no A/C?

What about the warming trend?  If we stay on our current greenhouse gas emissions pathway (the highest considered by climate models), look at the potential number of weeks with 100°F+ temperatures in 2090-2099:

 photo A1FI-warming.gif

Figure 1. Projection of A1FI emissions pathway-derived number of weeks (2090-2099) per with daily maximum temperatures exceeding 100°F.

With this heat wave fresh in mind, imagine what it will be like later this century when there is more than one excessive heat wave per year in the Midwest and along the east coast.  Instead of five days of misery, what will 25 days be like?  How about 50 days of 100°F heat in Virginia, North Carolina, Kentucky, Illinois, Iowa, and Nebraska?  When 100°F daytime heat dominates one, two, or even three months every year and high nighttime temperatures accompany it, this week’s heat wave will seem refreshing by contrast.

That’s how we feel in Denver, CO this year.  Instead of 73 total 90°F+ days – 13 of those days at 100°F+ – in 2012 (with June 2012 7.6°F warmer than normal), summer 2013 has been closer to average.  Yes, it’s been warm and only one 100°F day occurred so far this year, but it feels almost pleasant in comparison to last summer when the heat was relentless for months on end.

Three days of excessive heat is difficult to experience.  Three months is currently unimaginable.  How much worse future heat waves get is mostly within our control.  The sooner we significantly cut greenhouse gas emissions, the better things will end up for all of us.  But as the above graph demonstrates, the future could be quite hot if we continue along our current emissions pathway too much longer.


1 Comment

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.


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.


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.


73 90°F+ days (so far) in Denver CO in 2012

I fully expect this to be the last update of this statistic in 2012.  Hopefully, Mother Nature doesn’t prove me wrong.

As of September 11, 2012, with a high temperature of 90°F, there have been 73 90°F+ days – far and away the record number for one calendar year.  That is 12 more than the previous record of 61 set in 2000!!

This NWS link has the  list of the years with most 90°F+ days in a year.  50 90°F+ days used to signal a very warm summer.  60 and 61 days in one year were previous anomalous records – amazing counts all considered.  2012 blows all of those away.  I cannot emphasize strongly enough how unreal the heat wave was this year for Denver and hundreds of other cities and towns across the US in 2012.  Additionally, the chance that so many 90°F+ days have occurred in six years since 2000 is not and cannot be random.

Here is the breakdown of 90°F+ days per month in 2012 (to date):

2 in May

17 in June (6 of those were 100°F+ days; 2 of those tying Denver’s all-time maximum temperature of 105°F)

27 in July (7 of those were 100°F+ days)

20 in August (thankfully none of them 100°F+ days, although 4 of them were 98°F and another 3 were 97°F)

7 so far in September

There have been two nice breaks to the 90s already this month.  A few days ago, the high temperature was only 70°F.  One month ago, that was slightly warmer than our nightly low temperature.  Moreover, the low temperatures dipped all the way down to 47°F for two nights in a row – what a relief!  There was a brief and light amount of rain associated with a cold front that passed through the region in my neck of the woods. Another cold front passed through the area in the early evening today.  Temperatures are already down in the low 60s and it has rained a little more tonight again.

We’re not totally out of the woods for 90°F+ days.  It’s still the first third of the month of September.  If another ridge builds over the area, the temperature could climb back above 90°F, but each day it gets a little less likely.  This area saw no relief from Hurricane Isaac in terms of precipitation either.  Areas in the Mississippi and Ohio River Valleys saw significant drought relief after Isaac’s landfall near New Orleans, but the West and High Plains remain quite dry.

I can say unequivocally I would rather not challenge the top-10 90°F+ day list again any time soon.  Given the state of our climate system, I may not get what I want however.  The probability that we will witness more summers like 2012 are increasing with time.


67 90°F+ days (so far) in Denver CO in 2012

As of the September 1, 2012, there have been 67 90°F+ days – far and away the record number for one calendar year.  That is 6 more than the previous record of 61 set in 2000!

The list of the number of 90°F+ days in a year can be found at this NWS link.

Here is the breakdown of 90°F+ days per month in 2012 (to date):

2 in May

17 in June (6 of those were 100°F+ days; 2 of those tying Denver’s all-time maximum temperature of 105°F)

27 in July (7 of those were 100°F+ days)

20 in August (thankfully none of them 100°F+ days, although 4 of them were 98°F and another 3 were 97°F)

1 so far in September

August 2012 went down in Denver history as the 5th warmest and 4th driest on record.  The average temperature for the entire month was 75.0°F, 2°F cooler than August 2011.  The total rainfall for the month occurred during one storm – 0.11″.  That monthly total is 1.58″ below the monthly average and 0.09″ more than the driest August on record in 1924.

Today and the next two days are forecasted to be 90°F+, followed by two days of mid-80s.  It would be incredibly significant if 2012 recorded 1/6 more 90°F+ days as the previous record.  This week might not see all of the 90°F+ days (although I certainly hope they end on Tuesday!) as short heat waves can set up in mid-September.  Many folks along the Front Range are looking forward to autumn.

Leave a comment

Denver Ties Record For Most 90°F+ Days

On the 26th of August, 2012, Denver, CO tied the record for the most number of 90°F+ days at 61!

This ties the year 2000 for the most number of 90°F+ days in one year.

After setting the record for the hottest July in Denver history and a very warm start to August, 8 out the past 10 days were below 90°F.  The past week and a half has felt very, very comfortable when compared to the record number of 100°F+ days recorded in June and July (13).

What is the difference between that record-setting July and August 2012?  July’s average temperature was 78.9°F!  The average temperature through 25 days in August 2012 is only 74.1°F, or 4.8°F cooler.  The biggest difference hasn’t been the maximum temperatures, it has been the minimum temperatures: instead of high 60s, nightly low temperatures have been much more comfortable in the high 50s.

The record number of 90°F+ days will fall – that is certain now.  The next three days are all forecasted to be 90°F+, and Thursday’s forecasted high of 89°F will also obviously challenge 90°F.  90°F+ days can also occur in September, so the total number may not be recorded on Wednesday.

While August 2012 will not be in the top-10 warmest all-time, it just might challenge the 4th-driest August on record.  Current month-to-date precipitation is only 0.11″.  1924 was the driest at 0.02″, 1900 and 1917 were 2nd driest at 0.05″, and 1960 was 3rd driest at 0.06″.  Just behind 2012 is 1974 at 0.16″.  It won’t take much of a rainstorm to boot 2012 out of the top-10, however.  The 10th-driest year one record is 1985 at 0.28″.  It does not look likely at this time that any additional precipitation will be recorded at Denver.


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.

Leave a comment

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.