Why are we so dry? While we are still several weeks away from setting any records for a lack of rainfall in the Pueblo area, it has clearly been a dry September so far, and all indications are that the dry and generally warm weather may continue well into this month. September typically marks the end of a wet monsoon season in Colorado, but other factors are also contributing to this stretch of dry weather. The main culprit appears to be a large weather systtem over the northern Pacific Ocean known as a "Rex Block." This consists of a strong high pressure system alongside a strong low pressure system. As seen in the satellite imagery above, the high pressure system is located over southern Alaska. This weather pattern is highly stable and "blocks" any disturbances in the jet stream from moving into the Western U.S. and the Rocky Mountain region.
The current satellite imagery of water vapor (above) clearly shows a strong "deformation band", a juxtaposition of moist and dry air in the upper atmosphere, which is indicative of the Rex Block. It also reveals the extremely dry air to the south of the Rex Block, which continues to move into the southern and central Rockies.
The high pressure system over the North Pacific is a common pattern associated with the moderate La Nina event that we are currently experiencing. La Nina is characterized by above average sea surface temperatures in the equatorial Pacific. These blocking events typically last a week or two, and sometimes longer. It appears that this pattern will last at least another week, so expect the relatively dry weather to continue through at least the last week of Summer. The graphic below shows the strengthening blocking pattern forecast for Wednesday, September 15, 2010. Note how the jet stream is deflected from the central rocky mountains. The very cold air sitting over northern Canada is part of a second strong low pressure system, which is also contributing to the quiet and dry weather over our area by maintaining an overall stable pattern of temperatures and winds.