Derecho risk to continue in southern US before storm threat shifts farther north
Long-tracking and intense thunderstorms have bombarded areas from the southern Plains to the Southeast with damaging winds, hail, tornadoes and torrential downpours in recent days. AccuWeather meteorologists warn that these same areas will remain at risk of strong thunderstorm complexes into the start of this week before a change in the weather pattern occurs.
Through much of last week, complexes of thunderstorms were developing and moving along the northern edge of a dome of heat centered on Texas then plunging southward toward parts of the Gulf and southern Atlantic coasts. As the pattern continued to ramp up, some long-tracking thunderstorms approached the official threshold that meteorologists use to label such storm systems as derechos.
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The official scientific criteria of a derecho, as described by the National Weather Service's Storm Prediction Center, pertains to a swath of wind damage that must extend either continuously or intermittently for more than 400 miles with a width of at least 60 miles. But even where the storm complexes fail to officially become a derecho, they can still bring dangerous conditions and significant damage over a broad area.
At least two such complexes last week are candidates to be labeled a derecho. One began on Thursday afternoon in southwestern Kansas and the Oklahoma Panhandle and rolled toward northeastern Texas. A new complex of storms erupted in northeastern Louisiana Friday morning and continued to track southeastward across the lower Mississippi Valley during the daylight hours Friday. The combination of the two complexes produced hundreds of severe weather reports ranging from high winds to large hail.
More thunderstorms erupted over portions of the southern and central Plains and Southeast states into Friday night.
Another large complex formed and rolled swiftly through the east and southeast across parts of the Ozark Mountains and on through the lower portion of the Mississippi Valley and Delta region during Saturday night, bringing the threats of flash flooding, large hail, damaging wind gusts and even isolated tornadoes throughout the corridor.
The same complex and spinoff thunderstorm complexes impacted the south-central and southeastern states through Sunday night. Numerous wind and hail reports came in across northern Louisiana, southern Arkansas and Mississippi throughout the morning on Sunday and continued into Sunday evening as storms barreled through the region.
Even through the overnight hours on Sunday, a few tornadoes were spawned along with multiple incidents of damaging winds and large hail.
Where the storms targeted the same areas within hours or a couple of days, the risk of flash flooding increased substantially, with Flash Flood Warnings issued Sunday night in places such as Meridian, Mississippi.
Areas in the Southeast will likely pick up 6-12 inches of rain this week even as the derecho risk subsides. This is because there will be an ongoing potential for more thunderstorm activity - just not as organized or as fast-moving as the storms that developed through the weekend.
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As the zone of low pressure and associated frontal boundaries shift southeastward on Monday, locations from southeast Louisiana to central Florida and parts of South Carolina will face some risk of thunderstorms turning severe and bringing threats such as flooding, hail and damaging wind gusts.
"The threat for derechos or at least long-lasting severe thunderstorm complexes will decrease after Monday over the southern Plains," AccuWeather Long-Range Meteorologist Brandon Buckingham said.
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"The jet stream pattern will change its shape as to block much of the thunderstorms from slicing southeastward over a great distance from the southern Plains to the Southeast," Buckingham said. "Instead, there will be more locally grown heavy to severe thunderstorms in the Southeast states, but conditions are likely to turn volatile farther north."
AccuWeather's long-range team of meteorologists anticipates an uptick in discrete severe thunderstorms - known as supercells - over the central and northern Plains from Tuesday to Friday of this week.
"These are the sort of powerful thunderstorms that can produce large tornadoes," Buckingham said.
The same pattern will cause warmth to build substantially over the North Central states and into part of the Northeast.
Because large wildfires continue in both western and eastern Canada, there is the potential for smoky conditions to expand, causing air quality to decline in the Midwest and the Northeast.
As a large storm forms over the Southeast states in the part of the atmosphere where jets fly this week, steering breezes will funnel moisture into the region from both the Gulf of Mexico and the Atlantic. This increase in moisture will not only continue to promote drenching showers and gusty thunderstorms in the region, it may also lead to more widespread flooding problems.
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As rain expands northward into more of Georgia, eastern Tennessee and the Carolinas, tropical moisture may join in from the Caribbean as well. This extra moisture may help to alleviate drought in the Florida Peninsula, but it may also worsen the flooding in that area. There is some data suggesting that a weak tropical system may move along with that moisture from the Caribbean and across Florida.
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