The jet stream has been and will be for the next couple days, a little wacky. A deep trough has formed over the central continent, right over Manitoba as seen in the image directly below:
As a result of cold arctic air moving much further southwards than normal, snowfall has been falling in parts of the US this week, to some this has been the first May snowfall in decades. According to Jeff Master's blog, about 4 cm (1.5 inches) of snow fell in Sioux Falls, South Dakota yesterday morning. This is the city's first snowfall in May since 1976 and first snowfall of more than 1 inch (2.5 cm) in May since 1944! Some snowfall is expected to fall in Kansas today. In Topeka, Kansas, if they manage to get some snowfall accumulation today, it would only be the second time in its history to have measurable snow in May. The only other time was in 1907, more than 100 years ago. Normally at this time of year they should be seeing temperatures near 20°C!
This deep trough sets the stage up for the formation of a ''cut-off low'' this weekend as the low pressure system becomes cut-off from the polar jet stream. This will spell cold weather for that part of the US for the next several days. The low will ever-so-slowly make its trek eastward over the next week.