Sunday November 16, 2008It has been a dry autumn in Colorado, to say the least. Denver officially remains nearly five inches below normal on precipitation as of today. Since January 1, we've received 9.81 inches of liquid precipitation. Measuring precipitation, especially snowfall, can be very tricky. We will discuss that in more detail later in this blog. Keep in mind that official readings for Denver are taken by the National Weather Service at Denver International Airport. The varying terrain and elevation have a profound effect on precipitation totals in our area. Here is an example of how variable precipitation readings can be across the Front Range.On Friday morning, Denver officially received 1/10 of an inch of snow, which amounted to 1/100 of an inch of liquid precipitation. According to the CoCoRaHS volunteer observation network, some areas just south of Denver received up to 3.6 inches of snow, with liquid totals of 0.17 inches. So, while Denver's official precipitation deficit can seem a bit alarming, some unofficial stations reported healthier snow totals. With Friday's storm in particular, the higher terrain south of Denver caused an area of enhanced snowfall, giving those places higher totals.Gathering accurate snowfall measurements can be difficult, mainly due to the wind that sometimes accompanies snow events in Colorado. Volunteer observers in the CoCoRaHS Network are asked to walk around their yards with standardized measuring sticks and take readings in a number of places. Then, the totals are averaged. The tricky part comes in when attempting to get an accurate reading on the liquid equivalent of the snow that has fallen. Snow falls into the observer's rain gauge. However, the wind can affect the amount of snow that is caught by the gauge. That is why we take something called a "snow core measurement". That involves turning the rain gauge upside down, depressing it into the snow all the way to the ground, sliding a clip board or piece of cardboard under the gauge to capture the snow, then melting it down. This gives a more accurate reading of the liquid equivalent of the snow that is on the ground. It takes a bit longer, but snow cores can be very revealing.All things considered, it really has been a dry fall so far across much of our state. But as we explained, the readings can be highly variable. Persistent high pressure ridges have been in place this year, preventing storms from dropping large amounts of snow on us. The snows we have received have been due to passing cold fronts, which traditionally move through very quickly and therefore have light snow totals associated with them. When Denver receives a "big" snow, it is usually due to a low pressure system passing to our south that causes "upslope" winds. When the air moves toward the mountains, it rises with the terrain. When air rises, it cools, causing the water vapor it carries to condense into precipitation. If it is cold enough, the precipitation falls as snow. Upslope events can last several days and drop copious quantities of snow. If you lived in Denver in March 2003, you know what we are talking about!This is a time of year when weather patterns change across the United States. The Polar Front moves farther south with the approach of winter. Lower sun angles and shorter days mean less incoming solar energy, and thus, cooler temperatures. Precipitation patterns largely depend on the position of high and low pressure. For most of this fall, Colorado has been dominated by high pressure. We can expect to remain below average on precipitation until we see more low pressure systems coming our way.We salute the volunteers of CoCoRaHS who trudge out every morning to record the latest snowfall. Just about anyone can become a CoCoRaHS volunteer. Please visit http://www.cocorahs.org for further information.
November 11, 2008Bands of heavy snow dumped 4-8 inches of snow on the northeast corner of Colorado on Monday, with the most falling over northern Yuma county. The storm that brought that snow missed Denver for the most part, with only a few flurries in the metro area. The storm system will swirl into Kansas today and head away from the region. IN the mountains, a strong northwest flow aloft will keep snowshowers in the forecast through the day, with 3-6 inches of snow possible on northwest facing slopes.Officially, we've only received a 'trace' of snow in Denver, but some accumulation has been measured in the foothills and along the Palmer Divide. A 'trace' of snow is anything from one flake up to 1/10 of an inch.Our first snow is late this year. The average date for the first snow in Denver is October 19th.The earliest measureable snowfall was September 3, 1961 and the latest November 21, 1934. Over the last few years our first snows have been as follows - 10/22/2007... 10/18/2006 ... 10/10/2005 ... 11/01/2004 ... 11/5/2003 ... 10/25/2002 ... 10/05/2001 ... 09/23/2000. While it will not be a big snow, it may indeed be our first this season. Previous Entries: