The Weather Wire
September 2008 Volume 16 Number 09
Avg High - 85.1
Avg Low - 57.8
Snow - 0.0"
Season Snow - 46.3"
Precipitation - 4.03"
Avg High - 77.4
Avg Low - 47.3
Avg Snow - 2.1"
Avg Precip - 1.14"
High Elevation Tornadoes
With all of the high elevation tornado activity during the month of August I thought it would be interesting to look at some pictures of these tornadoes and try to answer a few questions that surround tornado activity here in Colorado. First let’s look at some pictures:
This was the Lemon Gulch tornado that formed north of Castle Rock and west of Parker taken from the Skyview Weather headquarters outside Castle Rock. This tornado occurred on August 24th just after 500pm. There were other tornadoes that formed that day along a convergence line that set up in central Douglas County and extended into eastern Arapahoe County. Multiple weak tornadoes occurred all along this line with the Lemon Gulch tornado receiving much of the coverage by local media. This tornado thankfully occurred over open terrain ripping up some scrub oak and snapping off a couple pine trees.
Just a day earlier on the 23rd there were multiple tornadoes that occurred near 11 Mile Reservoir in South Park including a “waterspout” that developed over the open water. According to the official NWS report:
TORNADO TOUCHED DOWN ON NORTHWEST CORNER OF RESERVOIR AND TRAVELED 5 TO 6 MILES SOUTHEAST ALONG THE WEST SHORE OF THE RESERVOIR DAMAGING CAMPGROUNDS AND OVERTURNING SEVERAL CARS AND RV TRAILERS. UNKNOWN NUMBER OF MINOR INJURIES TREATED
If you search YouTube, some incredible video of this waterspout can be found, but beware of some colorful language. There were numerous reports of tornadoes all along hwy 24 west of Colorado Springs that day. A particularly strong cell spawned a tornado near Woodland Park shearing off pine trees east of Westcreek. Here are some images of velocity and reflectivity from that particular storm.
This is an image of base reflectivity taken from the Pueblo radar site. Notice the fish hook that I have drawn over the radar to outline what is known as a “hook echo”. A hook echo is a great indicator of tornadoes as raindrops or hail in the cloud is picked up by the radar showing the spinning motion within the cloud itself. The best way to spot a tornado on radar is to look velocity images and spot “couplets” or areas where air is traveling at different directions at high speeds. Below is a velocity image taken from Pueblo radar:
The arrows in white drawn over the velocity image show wind direction relative to the radar site. The Green colors are traveling towards the radar site while the red colors are traveling away from the site. That is a great example of a velocity couplet. The boxes are tornado warnings and the dots are lightning. This storm weakened as it moved off the higher terrain and did not produce any additional tornadoes on the plains. Here is a link to see video of the tornado near Woodland Park, http://www.youtube.com/watch?v=gshu7vFiRi0.
High elevation tornadoes like the ones that occurred on the 23rd and 24th of August are quite rare, especially for that time of year when the spring is typically severe weather season. This led me to ask the question, What is the highest elevation tornado ever reported? Well according to Tom Skilling’s weather Blog,
“the highest observed tornado in the U.S. was photographed by a hiker at the 12,000ft level in Sequoia National Park, CA on July 7, 2004….the most violent high-altitude twister, an F4, struck in Wyoming between about 8,500 and 10,000 feet on July 21, 1987. Another high-level twister (F3) struck at 11,000 feet in the Uinta Mountains of Utah on August 11, 1993. It is quite likely that many high-altitude twisters go undetected and unreported in the sparsely populated mountains of the western United States.”
More typical of high plains tornadoes in Colorado are what is referred to as a “land spout”. A land spout is defined as, “A small, weak tornado, which is not formed by a storm-scale rotation. It is generally weaker than a supercell tornado and is not associated with a wall cloud or mesocyclone.”
Supercell - An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).
The tornadoes outside of Castle Rock were more land spout in nature as they formed along a convergence line and not in a single rotating supercell thunderstorm, also there was not a well defined wall cloud or single mesocyclone, instead there were numerous smaller circulations in which funnel clouds and weak tornadoes formed, the strongest being the Lemon Gulch tornado. The storm near Woodland Park was more supercell in structure as the entire storm was rotating and is more typical of tornado producing storms on the plains, except this storm was at 8,500ft above sea level.
Other sources used for definitions include:
Some areas of drought are beginning to appear once again across much of eastern Colorado despite above normal precipitation during the month of August.
The map below shows forecasted temperature deviances for September 2008. As can be seen, it is expected that virtually all of Colorado to have normal temperatures for the month of September 2008. The exceptions being the northeast corner of Colorado where slightly cooler temperatures than average are anticipated and southwest Colorado where slightly above normal temperatures can be expected.
The map below shows forecasted precipitation deviances for September 2008. Normal precipitation is anticipated for all of Colorado as September is one of the drier months in the year.
Drought conditions are expected to improve across eastern Colorado over the next 2 months, with no change expected western areas of Colorado.
What a wild month with record breaking heat, snow in the mountains to the west, high elevation tornadoes, and finally some good rainfall. The streak of 90 degree days ended on the 5th of August at 24 consecutive days shattering the old record of 18 days. There were two high temperature records broken as well on the 1st and 2nd of the month with 104 and 103 degrees breaking old standing records of 100. The month started out looking like it would end up being above normal in temperatures until mid August when an unseasonably cold upper level low pressure system slowly moved through the state. This storm system brought some significant snowfall to the mountains with snow visible on Mt. Evans for days after the storm. This storm system produced two record high minimums on the 15th and 16th. This cold spell helped to offset the heat early in the month resulting with a monthly mean of 71.4 degrees which is just below normal. The rains began to show up during the second week of August with a monthly total of 4.03" which is 2.21" above the normal of 1.82". August was the first month since October 2007 to have above normal precipitation. By the end of August the severe weather season usually slows down considerably, but not this year. There were a couple days late in the month that spawned tornadoes near 11 Mile Reservoir, Woodland Park, Castle Rock and parts of Arapahoe County. This type of tornadic activity this late in the thunderstorm season is very rare, especially in the higher elevations.
Summer loses its grip in September and fall begins on the 22nd as days become noticeably shorter. There has never been a 100 degree day in September with a record high of 97 for the month. 90 degree days become increasing more rare after the second week of September with average highs in the lower 80s at the start of the month and middle 70s towards the end. Thunderstorm season winds down with more isolated storms and severe storms with hail and tornadoes are very rare. The seasons first measurable snowfall can occur during September with about 2" on average, but many years snow holds off until October. The earliest freeze in Denver history occurred on the 8th of Sept. in 1962. The coldest temperature ever recorded in September was 17 degrees on the 29th in 1985. The high for that day was only 29 degrees and 9.0" of snow fell over the city. The taste of winter can certainly make an early visit to Colorado, but there have also been 79 years without any snowfall recorded at all out of 126 years of history, so there is about a 47% chance that we will see measurable snow in the city this September.
Sunrise/Sunset (Jul - December Denver area)
May 2008 to September 2008