Warm Sectored and Rain Shadowed
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Posted December 7, 2007

Category: Weather

The weather pattern for the first week of December has not been conducive for snow in Denver. One reason is the lack of upslope. To get the best upslope a low pressure system needs to pass to the south of Denver. This is because the counterclockwise wind around the low will produce an upslope flow for Denver. An upslope enhances uplift, cooling and therefore snow. A second reason is the temperature being too warm. When a low passes to the north of Denver, not only does it promote a downsloping flow but it also pulls up warmer air from the south. Both precipitation events for Denver this December has been light rain events. When a low pressure passes to the north of Denver in helps promote a "rain shadow" effect. The Rockies block moisture from getting to eastern Colorado and the downslope flow weakens the uplift needed to generate precipitation.

Below is a "rain shadow" for Denver. Notice the significant precipitation in western Colorado but very little in eastern Colorado. The Rockies are blocking moisture penetration coming from the west and the downsloping weakens the dynamic uplift that does make it over Denver.



The example below is getting "warm sectored" in Denver. When a low is passing to the north of Denver then Denver will be in the warm sector. These southerly winds out ahead of the low pressure have twice caused the precipitation that did fall in Denver to be rain instead of snow. The warm sector is also enhanced by downsloping air. Since the wind flow to the south of a low pressure system will be from the west and southwest in produces a downslope for Denver. Downsloping air will warm adiabatically.