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Weather Q & A - Round Two
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I asked for anyone interested to submit their weather-related questions to me and we had quite a few takers. The second round covers everything from thundersnow, to varying weather from the mountains to the plains and lastly why the heck is Boulder so windy.
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How thundersnow develops.
Kitty wants to know, “Thank you for helping me understand weather- a task that my 9th grade since teacher could not master :) But what the heck causes thundersnow? And how come we haven’t heard about it until this year? :)”
This is a very good question about one of the lesser known weather conditions we can experience in Colorado. Simply put, thundersnow is nothing more than a thunderstorm in which snow reaches the ground instead of rain.
Normally, in order to produce thunder and lightning you need low-level humidity, low-level instability and strong updrafts. In the winter, these conditions don’t normally exist – there is little humidity and updrafts are negligent. However, on the Great Plains it can occur when cool air is at the surface, cool air above and a warm front containing a large amount of moisture moves in between. This creates a region of strong convection currents that starts well above the ground. You then have conditions similar to a thunderstorm and you get thundersnow.
Little is really known about thundersnow as it is estimated that less than 1% of snowstorms have thunder associated with them – about three events a year are reported in the United States. Thundersnow may in fact occur more often but we simply don’t hear it because snow does act like an acoustic suppressor limiting the transmission of sound. While a normal thunderstorm’s thunder may be heard from miles away, thundersnow is usually only heard within a mile or two of it occurring. Current studies indicate it is most common in Colorado as well as Missouri, Kansas, Nebraska, Iowa, Minnesota, Wisconsin, Oklahoma and northern Texas.
Tim asks, “Can you help me understand the difference between weather in the mountains and weather in the city? What should I expect the difference to be between here and say, Vail or even Winter Park?”
Oh, boy, this one is a tough one! To be honest, there really is little correlation between the weather in the mountains and the weather we receive along the Front Range. Yes, the storms that move across the mountains do make their way to Denver however by the time they get here they have changed considerably and our topography completely changes their complexion.
This past winter virtually every snow basin in the state was well above normal and near record levels while the Denver area remained well below normal. Just last Thursday we saw a little sampling of this as snow was falling across the mountains at elevations greater than 9,000 feet. In Denver we had some pretty good rain. Further east on the Plains there were severe thunderstorms and even reports of a funnel cloud near Punkin Center 50 miles east of Colorado Springs.
Probably the biggest reason we have such stark contrasts in weather conditions within the state is geography. If you ever look at a topographical map of Colorado you would see that Denver actually sits in a bit of a bowl with higher elevations surrounding it on three sides. Click here to view a shaded relief map of Colorado - if you zoom in on Denver you will see what I am talking about. That elevation change alone can affect what one side of the city sees in comparison to another. On a state level, the mountains and their associated ridges all serve to steer weather one way or another and actually change the weather as it passes over. When you watch the evening weather, you will hear terms like the Palmer Divide, the Grand Mesa, Raton Ridge and others – these smaller areas all affect our weather and help to ensure that no weather forecast is perfect.
Jim must live in the western part of the metro area as he wonders, “Why are south Boulder County / north Broomfield / Lafayette so windy?”
This photo shows damage to property in
south Boulder that took place on 8 January
1969, the day after one of Boulder's worst
windstorms. Photo by Edward Zipser.
The answer is at least partially tied to the previous question – geography. Boulder and the surrounding area find itself a mere 20 miles from the Continental Divide and some of the highest elevations in the state. On the other side however are the plains. As the air comes over the mountains and reaches the plains, that steep slope allows those winds to literally pick up speed and momentum. Throw in a fast moving jet stream overhead like we sometimes have in the winter and you are in for a pretty good blow. By the time these same winds reach Denver though, they have decreased in intensity by a good 50%.
That completes round two of the Weather Q & A. If you have a weather question of your own, feel free to add it to the comments section and I will get to it soon! Have a great weather day!
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