(Image source: NOAA)
I’ve honestly never seen anything like this before. An upper level low and associated frontal systems had stalled near Tennessee this weekend. Then, on Sunday, the low abruptly turned west, riding against the now-feeble tide of the Northern Hemisphere Jet, its center transitioning all the way to eastern Arkansas last night even as associated frontal systems extended all the way to Utah. Today, the low continued to churn westward, against prevailing winds and centuries of weather precedent, and is now located near the Texas panhandle. You can watch the most recent flash animation of this extraordinarily strange weather sequence here.
Models show the low continuing to shift into the Desert Southwest, embedding itself into a monsoonal flow from the Pacific even as it pulls moisture in from the Gulf of Mexico. Such a situation will likely increase the ferocity of typical monsoonal rainfall events for this region with the potential for sudden downpours, locally extreme rainfall accumulations, and a heightened chance of severe thunderstorms.
A very moist Atlantic flow is now also flooding in from the east behind this retrograde low. For the US East Coast, the new and strange pattern will cause weather systems to now come in off the Atlantic rather than from the continent. In the hot conditions expected over the next week, added moisture may result in the firing of locally powerful storms or even the influx of tropical wave-like weather to the East Coast region. Greatest impacts will be seen for Florida, the Central Atlantic Appalachian chain, and the Great Lakes region.
This odd combination of moisture flows and a retrograde low pressure system will combine to create conditions for 1 inch or greater rainfall over broad swaths of the US over the next week. Areas most affected include the Eastern US, the US Gulf Coast, much of Texas, and Arizona and New Mexico. Areas hardest hit by rainfall are predicted to be West Texas and the border between New Mexico and Arizona, where rainfall totals could exceed 4 inches this coming week.
(Image source: NOAA)
A substantial erosion of the Northern Hemisphere Jet Stream has permitted this strange pattern of weather to emerge for the US. Usually, higher levels of summer sea ice and greater areas of summer snow cover in the far north would tend to increase the speed of the Jet, allowing it to push weather systems more rapidly along. But a massive erosion of this sea ice and related colder air has slowed down the Jet, setting up the conditions for the current retrograde to move backward against the prevailing weather flow for many days.
A chain of not one, but two retrograde lows appear to be moving against the prevailing weather pattern in the Northern Hemisphere. Doug, in a previous comment’s post, provided this chilling GOES sequence of the two lows — the first over the Central US and the second over the Pacific Ocean — moving together in retrograde.