This unseasonably cool pattern across the Great Lakes and upper mid-west hasn't been completely unexpected. Back in April and early May, we talked about how this summer would feature periods where temperatures would drop below normal. Take a look at the 12 day stretch from July 15th to July 26th. The area of the US that experienced below normal temperatures this year is exactly the same as 2009.
2009 was one of our years we used as a guide in formulating our summer outlook. Remember, this does not include this week's cool-down. I'll redraw this anomaly map at the end of this week.
The upper level (18,000 foot) trough is a bit stronger this year but in the same location as 2009.
Notice where the ridge (high pressure) is located: The western US where the present drought is the most extreme. Drought drives temperatures higher due to the dry ground. The dry ground and heat creates a high pressure ridge. This ridge keeps the dry conditions going further which drives the temperatures higher....and on and on.
Water temperatures in the Gulf of Alaska are off-the-charts above average this summer (as it was last winter).
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!
