|500 millibar map at 7:00 a.m.CDT |
Saturday, April 26.
The system currently crawling across the Midwest is called a closed low. Closed lows typically are strong systems with a distinct center of cyclonic circulation. On upper air charts the lows will be completely encircled by one or more height contour lines. Here is the 500 millibar map from May 4, 2013 showing a strong closed low centered over northern Arkansas. Note that there are three contour lines encircling the center. The more lines encircling the center, the more intense the low.
|Strong closed low on May 4, 2013|
Compare this to an open trough of low pressure. On the map below the trough along the west coast is open.
|500 millibar map for May 17, 2013 with open trough along west coast.|
Open troughs tend to be progressive. Small troughs tend to move faster than the
large troughs that extend north to south across the U.S.
Closed lows, on the other hand, move slowly. If the closed low becomes disassociated from the westerly flow of air, it is referred to as a cutoff lows. In both cases, the presence of a closed or cutoff low means an extended period of wet, cool, and stormy weather. Such is the case this week. Complicating the picture this week is the existence of what is called an "omega block", a pattern that resembles the Greek letter omega. This pattern tends to occur during the late winter and early spring in the Northern Hemisphere. It's main feature is the presence of the upper level high poleward of the jet stream. These warm highs tend to be stable and persistent, retarding the eastward movement of weather systems.The pattern is very evident on this morning's 500 millibar chart.
|The 500 millibar map for 7:00 a.m. CDT April 28.with omega block pattern.|
|500 millibar forecast for 7:00 a.m.Thursday, May 1st.|
|Quantitative precipitation forecast for the five days ending 7:00 p.m. CDT Saturday, May 3rd|