The Weather Wire
July 2010 Volume 17 Number 07
Avg High - 84.1
Avg Low - 53.6
Snow - 0.0"
Season Snow - 60.6"
Precipitation - 1.60"
Avg High - 88.0
Avg Low - 58.7
Avg Snow - 0.0"
Avg Precip - 2.16"
Dry Thunderstorms/Outflow Boundaries
Not all thunderstorms in Colorado produce measurable rainfall as all it takes to be considered a thunderstorm is the presence of lightning and thunder. There are many days along the Front Range of Colorado where there is limited moisture and yet enough instability to produce high based thunderstorms. What is meant by “high based” is the bottom of the cloud has formed well above the surface, usually 8,000 feet or more above ground level. The greater the distance from cloud bottom to the ground the greater the evaporation of the rainfall before it reaches the surface and in many cases there is not any rainfall left at all. This time of year from late June into mid or late July is one of the most common times of year for “dry thunderstorms”. This is mainly due to the changing of seasons as the cool of spring has left and the heat of summer is upon us yet the monsoonal moisture has not arrived from the desert southwest quite yet. Once deeper moisture arrives later this month the threat of dry thunderstorms will lower through August until we transition into fall. Then as monsoon moisture departs the state dry thunderstorms will be likely once again by early to mid September.
Some of you have heard us as at Skyview Weather talk about temperature and dew point spreads. What we are referring to is the air temperature and dew point at the surface which is a good indicator how much evaporation will occur as well as how strong the potential wind gusts will be. At the base of the cloud temperature and dew point are the same, hence condensation occurs and clouds form. While at the surface there may be upwards of a 50-60 degree temperature and dew point spread. For example if it is 90 degrees outside and a current dew point of 30 degrees there is a spread of 60 degrees. This magnitude of 60 can be used to predict potential wind speeds or gusts from thunderstorms; in this case a 60 mph wind gust is highly likely. These wind gusts are caused by the evaporation of rainfall aloft cooling the air around it. This cooler air rushes to the surface and spreads out from a central point. These winds can travel quite a long distance slowly weakening the further away from the central point. These winds as they spread out can be known as outflow boundaries. Outflow boundaries can then push against the foothills or slam against another outflow boundary from another storm and can be a triggering mechanism for new thunderstorm development. Many times these outflow boundary driven storms are “popcorn” type thunderstorms as they often strengthen and weaken rapidly. Many times outflow boundaries and their associated winds can be surprising to some. They can be that gust of wind out of nowhere while having that outdoor bbq, playing golf, fly-fishing, setting up that big outdoor tent, etc.
With the widespread precipitation of the last several months few areas of drought remain over Colorado, though some dry areas have developed in western Colorado.
The map below shows forecasted temperature deviances for July 2010. As can be seen, normal temperatures are expected the eastern 2/3s of Colorado with above average temperatures the western 1/3 of the state.
The map below shows forecasted precipitation deviances for July 2010. Normal precipitation is expected for the eastern 2/3s of Colorado with below normal precipitation the western 1/3 of the state.
Little in the way of drought remains in Colorado.
July of 2010 was very wet for most areas of the Front Range as DIA reported 3.70” of precipitation for the month and 13 days with measureable rainfall. Normal precipitation for DIA is 2.16” and with a surplus of 1.54” for July we are now 0.25” above normal for the year. This July missed out on being in the top ten wettest July’s ever recorded by only 0.01” out of 10th place. What a difference a month can make! July of 2010 was also a little warmer than average with a mean temperature of 74.4 degrees compared to 73.4 on average. There were 18 days with high temperatures of 90 degrees or more with 15 days during an average July.
August is typically the last month of true “summer” along the Front Range as temperatures towards the end of the month begin to fall off and thunderstorms become less likely. Upper level winds are typically quite light this month reducing the necessary wind shear for severe storms with slow moving moderate and heavy rainfall producing thunderstorms being most likely which are fed by monsoonal moisture being drawn northward from the Desert Southwest. The average high for August is 86.0 and the average low is 57.4 producing a monthly mean temperature of 71.7 which is cooler than July. Average precipitation for the month is 1.82” with 9 days of measureable precipitation. The record high for the month is 105 degrees which is tied for the hottest in Denver history and the record low is 40. August of 2010 will likely be around normal in both precipitation and temperature.
Sunrise/Sunset (July - Dec Denver area)
May 2010 to Sept 2010