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Denver Climatological Preview - July 2009 |
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A Preview of Thornton's July Weather - A Stormy Month
In fact, the worst hail storm on record in Denver history hit western sections of the city on July 11th, 1990. Baseball and softball sized hail from this ferocious storm caused millions of dollars worth of property damage. July is the most stormy month of the year in fact with thunderstorms occurring on average 11 days per month - or once every three days. These slow moving storms are one of the reasons July is Denver's second rainiest month with an average of 2.16 inches of rain (second only to May). However, these storms are also often very localized and cause widely varying precipitation amounts across the metro area. It is not unusual for one area of down to be deluged while others remain entirely dry. With these severe storms, flash flooding remains a possibility. Our hottest month, July daytime temps average 88 degrees and is when we typically see the most consecutive days above 90 degrees. 14 times 10 or more consecutive days above 90 degrees have been recorded entirely in July or at least partially starting in June or ending in August. The longest 90 degree streak last 18 days and actually occurred twice - once in 1901 from July 6th through July 23rd and again in 1874 from July 1st to July 18th. Just eight years ago in 2000, 17 consecutive 90 degree days occurred from June 29th through July 15th. Many of us will hearken back to July 2005 which was a record heat setting month. Six record high temperatures were set, all in the 100's. In fact on July 20th the mercury soared to 105 degrees tying the all time Denver high temperature set on August 8th 1878 and breaking the 102 record for July 20th. Other records were 102 degrees on the 16th, 101 on the 19th, 105 on the 20th, 104 on the 21st, 102 on the 22nd, 102 on the 23rd, and 101 degrees on the 30th. July 2005 became the 2nd hottest July and also the 2nd hottest Denver month ever. In all there were 25 days registering 90 degrees or higher which ties the 3rd most ever with 1963. Looking back at 2008 - Record setting heat and very dry Last year saw Denver break a record that had held for 107 years. 19 consecutive 90 degree days from July 13th to July 31st broke the old record of 18 set in 1901 and in 1874. The streak in fact continued to a total of 24 days until August 5th. Here in Thornton though, we were a bit cooler than at DIA and recorded a streak of "only" 18 straight 90 degree days. Despite that and the fact there were 26 ninety degree days in July 2008, the month only ranked as the third on top 10 warmest. Temperatures ranged from a high of 100 degrees on the 20th down to a low of 51 degrees on the 9th. Overall the month averaged a temperature of 77.6 degrees. 1934 holds the warmest spot with an average of 77.8. Here too, Thornton averaged a bit cooler than the Denver measurements with an average temperature of 75.3. Our highest temperature was 98.0 degrees recorded on the 30th and the coolest was 51.2 degrees on the 13th. The 26 ninety degree days was not a record either tying with 2000 as the second most. 1964 holds the number one spot with 27 days. Further, despite the heat, not one day set an individual day record. Cooling (and heating) degree day statistics are used to measure energy demand. A cooling degree day is tallied for each degree the average temperature on a given day moves above 75 degrees. So if the average temperature on a day was 80 degrees, it would count as five cooling degree days. These statistics have been tallied since 1969 and 2008 finished 395 cooling degree days ranking the month in 2nd place. The top six years for cooling degree days are all in this decade: 2005 (405), 2008 (395), 2003 (376), 2001 (373), 2000 (368) and 2006 (363). Not only was the month hot, it was also dry – the third driest in fact. A mere 0.24 inch of precipitation was collected which was well below the 1.92 inches normal. The driest July on record was 1901 when a measly 0.01 inch of precipitation was recorded, coincidentally also the previous holder of the 90 degree day consecutive streak. Thornton fared a bit better recording 0.45 inch of precipitation during the month last year. Eight thunderstorms were recorded at Denver International Airport which was three below normal. Two of those dropped small hial at the airport. The peak wind gust for the month was 47 mph from the west. Click here to view Thornton's climate record for July 2008 Outlook for July 2009 The National Weather Service 30-day outlook models suggest that precipitation during July 2009 will be slightly above normal and the outlook for temperatures indicate normal readings. For planning purposes Denver's climatological normals are listed below. For More Information Temperature Normals and Extremes for July Monthly Temperature, Rainfall and Snowfall Extremes for July
* Historical weather statistics gathered from the National Weather Service's Denver / Boulder forecast office data archives. |
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Change is of course the one constant in Denver's weather but come July, things actually get pretty consistent. The standard formula for a day in July is a sunning morning, clouds developing in the late morning and early afternoon. Come mid-afternoon, thunderstorms are rolling off of the foothills and into the metro area and the eastern plains. These storms do occasionally reach severe status containing hail, gusty winds and heavy downpours of rain. 
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