Terry’s Take: The PNA and our fall weather
The downward turn in temperatures will be the result of energy entering Alaska and buckling the jet stream. This releases a lobe of cool Canadian air that should arrive here around the beginning of October. This is coincident with the PNA flipping strongly back to a positive phase at that time. You can see how this is forecast by the red line in the graphic.
Now the disturbing thing about the +PNA to me is that is overall a darn dry pattern. Again in the graphic you can see how since late July we have generally been in a positive or neutral phase of the PNA. We all know how dry the ensuing weather has been over the Midwest. Here in the Quad Cities we are down 7.50″ on rainfall since July 1st. That means we have lost most of the rain surplus we gained earlier this year that off-set the dryness of 2012. That’s a bummer!
Anyway, I have a graphic posted which shows what +PNA’s do for fall precip. (October) across the country. You can see much of the cornbelt is on the dry side, especially the eastern Midwest and Great Lakes. For that matter, much of the country’s rainfall is no better than normal. No green anyway in the U.S. signifying above normal precipitation. Northwest flows are notorious for their general lack of moisture.
They do bring in clipper type systems which produce significant fluctuations in temperatures. Warm ahead of the clipper, much cooler behind it. It looks to me like this push pull pattern will be ongoing through much of October and perhaps November. Temperatures may end up close to normal but it won’t feel like it with all the ups and downs. I hope I’m wrong but my gut (and the signals) tell me that October will be another dry month over much of the Midwest.
By the way, if the dry cycle is not broken by late November I think that will have big implications on the upcoming winter, cutting snowfall significantly. That is not a forecast but a “trend” I will be watching closely the next 4-6 weeks.