(Front and associated trough sweep from east to west over Central US. Image source: NOAA)
An extreme US summer that has featured floods and heavy rains in the east and drought and devastating fires in the west boasts yet one more bizarre weather pattern: a backward moving storm system.
As of last week, a strong frontal boundary had swept into the southeastern US bringing with it another dose of heavy rains and storms. Then the system stalled. Over Friday, Saturday and Sunday, the front and associated low pressure systems have backed up, moving from east to west, passing over the Appalachians, then the Tennessee River Valley, then the Mississippi, until today it reached a central region stretching from Texas all the way north to the Dakotas.
This retrograde weather is a very uncommon, but not unheard of, event. In the context of an already strange summer, it adds yet one more anomalous weather pattern to the list. In the AQUA/NASA image below, we can see the position of this retrograde frontal boundary and low pressure system as of yesterday. Note how the eastern front essentially collapsed as it pushed westward forming a bow from Texas to the Dakotas in the image at the top of this post.
(Frontal System begins to move in reverse on Saturday, July 13. Image source: NASA)
The cause for this retrograde storm motion is an extraordinarily weak Jet Stream. Over the past two decades, the Northern Hemisphere Jet has continued to weaken as both sea ice and summer time snow cover rapidly eroded. This helped reduce the strong north-south temperature differentials that drove the Jet to move weather systems rapidly from west to east. Now, temperature differences between high and low lattitudes are greatly reduced and, as a result, the Jet proceeds more slowly.
One result is that weather tends to persist much, much longer over a given region. Droughts, heat, and dry weather persist in areas where the Jet is pushed into a south-north configuration. Rain, storms, and cooler weather persist in regions where the Jet is pushed into a north-south configuration.
In the case of this weekend, the Jet became so weak that prevailing local influences overcame a hemisphere wide system to drive weather patterns against its ebbing flow. In this case, though, the result may be a somewhat positive influx of rainfall to drought and fire stricken western regions. Let’s hope this doesn’t turn into too much of a good thing.
The weather system I reported on earlier today is still moving in its retrograde fashion, east to west, across the United States. It now stretches almost to Utah and appears to be dumping rain in a bow shaped arc from Texas into New Mexico and then re-curving northward back through the Dakotas and all the way through to Quebec. A moist upper level flow issuing from the Atlantic Ocean is still pushing the arching front westward.
You can see the progress of this retrograde storm in the most recent satellite image here:
(Image source: NOAA)
This retrograde action has now spanned 2/3 of the Continental US and is still proceeding westward. May well be something for the record books…