The Weather Wire
November 2007 Volume 15 Number 11
Avg High 68.2
Avg Low 38.8
Snow - 3.0"
Season Snow - 3.0"
Precipitation - 3.03"
Avg High 51.5
Avg Low 23.5
Avg Snow - 10.7"
Avg Precip - 0.98"
What is the Jet Stream?
Now that we have moved out of summer into fall the jet stream plays a major role in our everyday weather. One can imagine the jet stream as a river of air flowing high in the atmosphere steering our winter weather systems through the mid latitudes. The jet stream is caused by the uneven heating of the earth. Cold air is denser than warm air and along edges of the cold to warm air the atmosphere has a slope to it. Try to imagine it as a bowl of cold air and a mound of warm air. The sides of the bowl represent the temperature difference between the air masses. The larger the temperature difference the steeper the sides of the bowl. Now this may be difficult to imagine, but the temperature difference with respect to distance is the driving force of wind. Where there is the largest change in temperature in distance there is a steeper side to the "bowl" and the strongest winds reside where the slope of the "bowl" is greatest.
The jet stream winds separate the large scale warm air form the cold air. Some areas have larger temperature difference than others and the winds are stronger in these areas. These areas of strongest winds are called jet streaks. These jet streaks are responsible for movement and location of the comma clouds or storm systems we see on Satellite imagery. The jet stream is a large feature and so are the storm systems associated with it, some storms can be thousands of miles across and affect more than half of the U.S.
In the summer months the jet stream weakens as the North Pole warms and the temperatures difference from north to south decreases. Jet stream winds decrease and is pushed north into Canada rarely dropping into Colorado. Daytime heating is the driving force in summer as warm moist air becomes unstable in the afternoon resulting in afternoon and evening thunderstorms. As the sun sets daytime heat is lost and thunderstorms usually subside overnight only to redevelop the next day.
This winter the jet stream has remained north of Colorado making for a rather tranquil start to the fall season. If the jet stream drops further south our weather pattern will become more active here in Colorado during the months of November and December. November is usually one of the snowiest months in the mountains along and west of the Continental Divide due to the jet stream migrating south steering winter storms into our area.
Most of Colorado continued to see dry conditions, with little improvement in drought conditions along all borders of the state.
The map below shows forecasted temperature deviances for November 2007. As can be seen, extreme Western Colorado should see above normal temperatures with normal temperatures anticipated elsewhere.
The map below shows forecasted precipitation deviances for November 2007. Below normal precipitation is expected for most of Colorado for November 2007.
As can be seen in the below map, little overall change is anticipated for much of Colorado through January. Areas to our north such as Idaho, Montana and western Wyoming will see improving drought conditions as they have been experiencing drought now for up to 10 years in some locations.
October of 2007 was a warmer than average month with an average high of 69.2 degrees which was 2.2 degrees above normal. Low temperatures were 2.9 degrees warmer than average. There was a 24-hour precipitation record broken as 2.65" of rainfall fell between the 13th and 14th at DIA. This was due to an isolated thunderstorm that tracked through the area. This was the only thunderstorm for the month and small hail was reported as well. Lesser amounts of rainfall fell throughout the city generally in the 0.5 to 1.5" range. There was only one snowfall event for the month where 3.0" of snow was officially reported, but southern areas of the city received much higher amounts with 6-9" common along the C-470 area extending into parts of Aurora. There was an unusually high amount of sunshine for the month as 86% possible sunshine was observed compared to the 72% which is average. Despite all the sun and above normal temperatures there was only one day that tied a record high. 80 degrees was observed on the 25th which tied an old record set back in 1959. For the year at DIA we are exactly 1.0" below average precipitation and 3.2" below average snowfall. The storm track needs to become much more active over the next two months to achieve normal numbers for the year.
November can be a roller coaster type of month with warm spring like days as well as powerful winter snowstorms just days apart. November is the 2nd snowiest month for Front range locations just behind March. In the mountains this is the time of year where snow begins to pile up creating the "base" and all the ski resorts open for the season. Overnight lows really begin to drop during this time of year with average lows well into the 20s. There can be periods very cold temperatures as the first arctic air can intrude from the north. The record low for November is a bone chilling -18 degrees. On the flip side the record high is 80 degrees which was set just last year. Between the two there is a total temperature spread of 98 degrees! There is less sunshine on average observed during the month of November with only 64% possible sunshine, this is less than any other month during the year. With a warm start to this November we will likely be above normal in temperature and below normal in snowfall and overall precipitation. The storm track will most likely become more active the last couple weeks of the month with bringing some much needed moisture to the state.
Sunrise/Sunset (Denver area)
October 2007 to March 2008