Cooling temperatures and beginning of autumn? From Mountain Weather Master, Joe Ramey

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Hello MWM,
Meteorological 2017 summer ends tomorrow, Thursday. That is a bit of a bold statement. But these well-above normal temperatures of late summer  look like they will be replaced with below normal temperatures for the foreseeable future. Saturday morning, we may see our first seasonal snow over the Park and Gore Ranges of NW Colorado.
Back in the dark ages when I went to school, I remember a professor teaching about the concept of singularities. A singularity is a meteorological event that marks a significant shift in the weather patterns. At the time, in the 1980s, we knew that the persistence tool, that is forecasting today what had happened yesterday, would statistically yield the highest accuracy. The problem with persistence is that you miss every singularity that will impact your users.
The next few days look like the singularity that introduces the fall season weather pattern and brings an end this warm summer.
In current satellite imagery, you can see a closed low spinning along the central California coast and progressive, open troughs working across the NE Pacific towards shore. The closed Low will open tonight and work across western Colorado bringing increased chances of showers and cooler temperatures for Thursday into Friday. Then early Saturday, a strong cold front, associated with the next progressive trough, passes. This will take 700mb (about 10,000ft MSL) temperatures below 0C. Steamboat Mountain and Winter Park could get their first snowfall of the season. SW Colorado will remain moisture starved. Temperatures across northern and central Colorado will drop around 15 degrees below today’s high temperatures. You can see this in the latest 6-10 and 8-14 day outlook.
The western CONUS will quickly shift from above normal to below normal temperatures. The storm track will tend to favor the northern Rockies. The monsoon as we know it (subtropical high near or east of the Four Corners with subtropical moisture bubbling up from Mexico) will be over. There is still a good chance that cold-core Pacific storms will work across the Intermountain West and pull good moisture up from Mexico. It is that pattern that makes September and October some of our wettest months of the year on the West Slope.
You can read the latest thoughts of the local NWS forecasters here.
So enjoy your shorts and sandals, and ready your wool.
Joe

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