|Temperatures at 1:00 p.m. CDT July 15.|
Now that the hype about the "polar vortex" has died down, I thought I would try and describe what actually is happening across the country this week. While it is a bit unusual for mid-July, it isn't unprecedented by any means. The circumstances by which this pattern set up may be a little out of the ordinary, but unseasonably cool weather in the summer has occurred before and will occur again.
|500 millibar chart for 7:00 a.m. CDT July 15|
|Maximum temperatures for July 14. Temperatures 100°F and higher are shaded in red.|
Meanwhile, the cooler air pushed south through the central U.S. The cold front has helped trigger flooding rains in the eastern U.S. The leading edge of the cool air is expected to push to just off the east coast by Thursday morning, but will likely stall because of the large high in the western Atlantic.
The development of the large trough over North American this week is attributed, in part, to Typhoon Neoguri which slammed into Japan late last week. This was an immense storm, and it induced a "ripple" in the jet stream over the northern Pacific. The atmosphere is a fluid, and what happens upstream will have an effect downstream. This caused a trough to form over Alaska, with a ridge downstream over the western U.S. and the developing trough over the central U.S.
|500 millibar map showing a trough over Alaska (red line), the ridge in the western U.S. (blue dashed arrows), and the trough over the central U.S. The chart is for 7:00 a.m. July 15.|
|The mean 500 millibar pattern (left) and the temperature departure from normal|
for the period December 1, 2013 through March 31, 2014.