Tuesday, July 15, 2014

Two Sides of the Same Coin

Temperatures at 1:00 p.m. CDT July 15.
It was pleasantly cool here in the Midwest today. Too cool for swimming, but great for just about anything else outdoors. It's a nice break from the humid weather of the past several days.

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
There is a strong upper level trough over the central U.S., and that is responsible for the cooler air dropping south through the U.S. However, it's not really the "polar vortex". When we have weather patterns such as we have this week it is important to look at the big picture, big as in continental, if not hemispheric.  Why?  While you may be enjoying cool weather this week, you can bet that somewhere to the west and east abnormally warm weather is occurring.  When there is a large, strong trough of low pressure over one part of the country, then it is likely to be sandwiched in between two corresponding large, strong ridges of high pressure to the west and east. Beneath those ridges of high pressure air sinks and warms. That is what's happening in the western U.S. this week as the central U.S. enjoys refreshingly cool weather. Five days ago temperatures 100°F and higher were confined to interior California and the southwestern deserts. Yesterday a broad swath of maximum temperatures100°F and higher occurred from Arizona north through Washington state.
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.
Over the winter there was a persistent trough of low pressure over the central and eastern U.S.  That directed the cold air into those regions. Since there was a strong trough, there had to be a corresponding strong ridge.  The axis of the western ridge was located off the west coast and accounted for warmer than normal temperatures throughout the southwestern quarter of the U.S. Alaska experienced their 8th warmest winter on record as the ridge generally deflected intrusions of Arctic air. The axis of a flatter eastern ridge was located in the eastern Atlantic. The jet stream located between the cold air in North America and the warmer, tropical air over the eastern Atlantic directed many large and intense storms into northern Europe. 

The mean 500 millibar pattern (left) and the temperature departure from normal
for the period December 1, 2013 through March 31, 2014.

Very often, the large intense upper air trough/ridge patterns that develop during the winter span much more than the North American continent and their effects extend far beyond the U.S. and Canada.

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