Why the Relentless Western US ridge last summer?

Many people have been asking why has it been so incredibly hot and in addition, dry? Myself included, we all know that typically summers are warm to hot here in Northwest Montana with our fair share of upper 80 degree days and even 90 degree days with the odd 100 degree day here and there. The strange thing is typically we see these strings of hot days break after a week or so with either a cold front passage or a thunderstorm outbreak bringing some relief. Both of those features have been largely absent this summer and the result has been too many scenes like the one below.

 The pattern so far this year has featured a much stronger than normal Western North American upper level ridge with a corresponding much stronger than normal Hudson Bay trough. In weather language we refer to this phenomenon as the positive phase of the PNA (stands for Pacific North American) pattern. The graphic below illustrates how that affects us. 

While the PNA pattern is often talked about a lot more during the winter months, it can also occur during the summer. Obviously it would not be as cold during the summer months and the upper high in the positive phase of the PNA on the left would be centered more over the Western US rather than offshore. While we have been having heat and drought in the Northwest, our neighbors in the Great Lakes and Northeastern US have had just the opposite, a cool and very wet summer. The Southwest US from Southern California to Southern Nevada, Arizona, Southern Utah, Colorado and New Mexico have all had a wet summer season. It is all tied together as the ridge and trough, the upper high and upper low work in tandem and have remained in position since the middle of May thus prolonging our hot and dry spell.

     What is truly remarkable though is the lack of precipitation anywhere from British Colombia and Alberta to Washington, Idaho and Montana. The upper ridge has been positioned in such a way as to block any shortwave disturbances from migrating up from the south generating our thunderstorms and also blocking any incoming Pacific troughs with cooler air and showers from coming in from the west. The ridge has been centered on an axis generally from East Central British Colombia south to the Four Corners region when typically our ridges are centered just east of the Continental Divide during the summer months. With the ridge axis centered to our east the door is open for our common nighttime thunderstorm outbreaks that often bring refreshing rains and spectacular lightning displays or the occasional pacific storm that brings some rainfall and cooler breezes. In summary, it has simply been a very dry and stable pattern that has provided little shower and thunderstorm development. What little thunderstorm development we have seen has produced some locally heavy rainfall in small areas and the usual abundance of lightning that has started most of the fires around the area. Until the ridge axis collapses or shifts further east, the dry conditions will continue unless we get a miracle from God.  From a climate standpoint though, now that we are almost into mid September as of this writing, the upper high over the West will begin to break down and shift offshore as tropical activity picks up and interacts with the jet stream delivering more energy and dynamics to spin up storms that can produce beneficial rains for us. September and October are typically the drier months of the year for Northwest Montana but as we know from last years record rains in both months that is not always the case.