(Image source: GOES/NOAA)
Today, a very muscular blocking high pressure system located over the Western US flexed. After having set off record fires and heat waves it reached deep into the Arctic and drew moisture and more unstable air down from over Hudson Bay across the Rockies and over top of Arizona and New Mexico, igniting powerful thunderstorms which blanketed large areas in hail, heavy rainfall and lightning. A second moisture stream drawn into the high’s circulation from the Pacific also fed these storms.
You can see the bright, high, cold cloud tops now firing over Arizona and New Mexico.
To the east, an upper level low pressure system is just now starting to draw this concentrated moisture into two other feeding, damp air flows. The largest draws straight up from the Caribbean over Florida and then rushes up the US East Coast. The second, pulls moisture from tropical storm Dalila in the Pacific, draws it over Mexico, then pulls it over Mississippi and up the back side of the Appalachians.
The action of this powerful blocking high over the US West and associated upper level low over the East is likely to result in very moist, rainy conditions for a large section of the country east of the Mississippi River. On the Gulf Coast, as much as 5-8 inches of rain is expected. But a wide swath shows potentials for 1-5 inches over the coming week.
This persistent wet and extremely moist flow raises the risk of flash flood conditions where major storms light off. Record rainfall over many areas has already left the ground saturated and atmospheric conditions are very unstable, setting off the potential for powerful storms.
The broad sweep of these convergent moisture flows also sets up the possibility that even more violent conditions may emerge. Large blocking highs were associated in all the major flood events that have occurred around the world so far this year. In one example, noted by commenter Colorado Bob, Pakistan suffered 120 degree heat under a blocking high during May and June. The high then swept a massive flood of moisture up over India and into the Himalayas. The result there was devastating floods that left hundreds dead in a virtual tsunami of mud and water.
The strength of the current upper level pattern, drawing moisture from the Arctic, the Pacific, a tropical weather system, and from the Caribbean sets in place the components for major instability to meet with four rivers of moisture over the Eastern US. It’s a dangerous set of circumstances that may result in current rainfall forecasts under-shooting long-term totals. This week has already seen a number of torrential downpours over broad sections of the US East Coast. But this flow taps even more moisture than what was previously in place, drawing from multiple sources across an area spanning more than 4,000 miles to link rivers of moisture with unstable air. Let’s hope these convergent flows don’t set off flood events similar to those seen in Europe and India this year.
Fair warning: the mangled Jet Stream now has the Eastern US under the gun.
(Hat tip to X-Ray Mike for his comments on strange storms in Arizona today).