A Jam in the Atmospheric Stream

500 millibar map at 7:00 a.m.CDT
Saturday, April 26.

A sluggish upper level weather pattern is setting the stage for a wet week across the eastern half of the country. The stage was set on Friday when and upper level low pressure system came ashore in the Pacific Northwest. By Saturday the low had dropped to southern California, and then kicked east on Sunday. This low is what helped produce the unstable atmosphere which spawned the tornadoes across Arkansas on Sunday and in Mississippi and Alabama on Monday.

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.

Since Friday there has been significant precipitation in the central U.S. as a result of the closed low on the west side of the omega block. Rain gauges will get a workout this week in the eastern half of the U.S. as the upper level low intensifies and moves slowly to the northeast over the next 48 to 72 hours. By Wednesday the center of the low will spin over the upper Great Lakes and this system will dominate the weather from the Continental Divide to the east coast. 

500 millibar forecast for 7:00 a.m.Thursday, May 1st.

Most precipitation will occur Tuesday and Wednesday, although wingspread cloudiness, scattered showers and thunderstorms, and cooler weather will be possible through the end of the work week. Present indications are that the closed low will open up and accelerate eastward on Friday. That should leave much of the eastern half of the U.S., except perhaps the northern Great Lakes, in good shape for the weekend.

Quantitative precipitation forecast for the five days ending 7:00 p.m. CDT Saturday, May 3rd