First blast of Canadian air

From the glimpses I've taken at the developing pattern I'm becoming more and more interested in how fall 2012 is shaping up for North America.

This week will feature a significant pattern change beginning in the Pacific and extending across North America by next weekend.

Pacific jet energy is poised to dump into western NA by the middle of next week. The low in the deep south, responsible for all the heavy rain from Texas through Mississippi will provide a path for this energy to dig into one of the first large full-latitude troughs of the season. Upstream, the previously suppressed flat ridge over the eastern Pacific explodes poleward into Alaska.

Meanwhile, we can't forget the train of monster tropical cyclones recurving in the Western Pacifiic. These recurving typhoons can trigger a wave response downstream across the Pacific into North America. Upper level ridging arises over the northwest Pacific in response to the huge latent heat release and upper level divergence associated with these storms. This ridging translates to a trough around the Aleutians and to another ridge over the eastern Pacific into Alaska.

Combine forces and all of a sudden we have this massive blocking ridge forming along the west coast of North America.

What does this mean for you?

Expect a strong cold front to plow through the Plains during the middle of this week. A line of thunderstorms, some strong to severe will accompany the front across the Midwest. Meanwhile parts of the northern Plains could be looking at their first snowfall of the season behind the front as the storm intensifies over the Great Lakes. The cold front will continue eastward, through New England and the Mid Atlantic region by next weekend. In the wake of this storm system, temperatures will stabilize around 10 degrees below the normal for this time of the year.

Here's the GFS ensemble mean 6-10day 850mb temperature anomaly forecast (degrees C):