Radar image 091219

The red and yellow echoes with lightning bolts indicate thunderstorms, but there was no rain falling where the green bands were, crossing the New River and Roanoke valleys and the Blue Ridge from northwest to southeast. These radar echoes appear to be related to convective rolls in the atmosphere, where moisture collects in horizontal slowly rotating tubes, enhanced by swarms of dragonflies and migrating birds flying in from the Ohio Valley.

Searing heat for mid-September and swarms of dragonflies plagued the Roanoke and New River valleys and nearby areas on Thursday. But Friday, we get a break from the heat.

Roanoke hit 97 degrees, a record for Sept. 12, beating out 96 from 1933, but perhaps more impressively, it was the hottest it has been this late on the calendar in any year since 1991, when it hit the same temperature on Sept. 16. Only nine previous days Sept. 12 or later on the calendar since 1912 have hit at least 97 officially in the Star City, topped by 99 on October 6, 1941.  Blacksburg, which set a record high Wednesday at 94, was a bit cooler at 91 on Thursday, not a record , but still 14 degrees above the normal high for Sept. 12.

Meanwhile, in addition to pop-up storms with bursts of cloud-to-ground lightning and heavy downpours on small areas, National Weather Service radar on Friday also picked up squiggly green bands moving northwest to southeast over the New River Valley and the Blue Ridge generally from Roanoke southward. The banded nature, combined with cloud streaks of visible satellite, suggested "convective rolls," broad slowly rotating tubes in the atmosphere where dense moisture concentrates.

However, it is likely these echoes were enhanced by swarms of dragonflies traveling southeast out of the Ohio Valley and West Virginia. There were numerous reports of swarming dragonflies along the Blue Ridge south of Roanoke during the early evening, and similar reports occurred with radar echoes in previous days in states to our northwest. The National Weather Service office in Cleveland reported that its echoes on a dry day were likely related to these dragonflies plus other insects and migrating birds. It is not unusual for Doppler radar to pick up "biological" echoes such as this, but such large concentrations of dragonflies are unusual in our region, with many who experienced it saying they had never seen anything like it before.

Not sure what it will do for the dragonflies, but a "backdoor" cold front sliding in from the north and northeast will return our region to the "wedge" pattern, with easterly winds banking cooler, damp air against the mountains. Temperatures on Friday will likely stall in the 70s, 20 or more degrees cooler than Friday's highs. That isn't really "fall-like" weather but it will feel cool after recent record heat. There may be some drizzle or light rain showers, but amounts are likely to be light. Much more of our region is now considered "abnormally dry" in the latest U.S. Drought Monitor map, with parts of Floyd, Franklin and Montgomery counties having moved into "moderate drought." It will take something more than what falls Friday to make a dent in that.

The wedge will slowly erode over the weekend, with some scattered showers and gradually warming temperatures, possibly pushing 90 again by Monday.

A tropical disturbance in the Bahamas may soon become a tropical depression or possibly even Tropical Storm Humberto, bringing more squally rain to islands ravaged or scraped by Hurricane Dorian last week. There is still much conjecture about its eventual intensity and track, but at this point, it appears likely it will be more to the east than we'd like to see for a real drought-easing rain, possibly even retracing Dorian's path. But there is time for this to change.

Down the road, there may be a fairly strong cold frontal passage late next week that could kick up some storms and bring cooler weather behind it. We'll have to monitor that in days to come to see if it has some fall-triggering potential or is just another pretender with a quick bounce-back of summer heat for late September. As the second paragraph points out above, it has been nearly 100 degrees before in the first week of October, so summer can sometimes hang on long past its welcome to those dreaming of fall colors or even snowflakes.

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Contact Kevin Myatt at kevin.myatt@roanoke.com. Follow him on Twitter @kevinmyattwx.

 

Since 2003, Kevin Myatt has penned the weekly Weather Journal column, and since 2006, the Weather Journal blog, which becomes particularly busy with snow. Kevin has edited a book on hurricanes and has helped lead Virginia Tech students on storm chases.

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