The upper level (18,000 foot) trough is a bit stronger this year but in the same location as 2009.
The above normal Gulf of Alaska water temperatures have enhanced the western high pressure ridge by pushing the jet stream further north. The movement of the jet stream is indicated with the red arrow. The strongest portion of the jet stream (highest winds) is shown with the warmer colors over Alaska. The jet on the east side of the ridge dives southeast in the center of the country and bottoms out over the Ohio Valley where the trough is located.
The PNA index is used to measure these pressure patterns across the Northern Hemisphere. When the PNA is positive, the ridge is very strong out west and the trough/low pressure is strong east as it has been since last winter. You can read about it more HERE.
The southeasterly jet becomes the conveyor belt for frequent rain and thunder events leading to above normal rainfall across much of the Great Lakes, Midwest and Ohio Valley. This also leads to more frequent periods of below normal temperatures.
For perspective, July in Cleveland, Ohio finished the 6th coolest since 1900.
The average high temperature in Cleveland from July 15th to July 31st was the the second coolest on record!
So the warmer Gulf of Alaska water and the drought out west have significantly boosted the high pressure ridge out west which is reinforcing the trough/cool pattern further east across the midwest and Ohio Valley. This is not a complete list of the drivers of this summer pattern. The weak El Nino, which garnered a lot of attention during the spring, has had an influence too but not to a major degree. The weak El Nino will factor into our winter outlook which I'll talk about in the fall. Let me know of any other major contributors that I've missed.
Bottom line, it doesn't look like this pattern is going to change anytime soon!