The challenges of forecasting a derecho

Digital Originals

ROANOKE, Va. (WFXR) — On Monday, Aug. 10, a derecho made its way across the Midwestern part of the country, leaving behind a swath of damage. One week later, many are still without power in portions of Iowa and cleanup is ongoing.

The definition of a derecho, according to the Storm Prediction Center, is a widespread, long-lived wind storm. Technically, a derecho produces measured wind gusts 58 mph or greater over an area 240 miles or longer. They’re usually associated with bands of quick-moving showers and storms. While derechos produce wind damage, it’s typically in the form of “straight-line winds”.

The derecho that impacted Iowa produced peak wind gusts of more than 100 mph. Emergency management officials reported a gust of 112 mph in Linn County which is home to nearly a quarter-of-a-million people and is also the county that Cedar Rapids is part of. Crops were flattened, trees toppled, and various objects were thrown around.

Going back to June 2012, the derecho that moved through the Commonwealth caused about one million power outages and a peak wind gust of 81 mph was recorded in Roanoke. In total, five people died in Virginia as a direct result of the derecho, including a firefighter from Boones Mill. More than five people died in the Commonwealth as a result of health-related illnesses as well. A state of emergency was declared for Virginia after the storm.

While meteorologists can forecast potential severe weather outbreaks a few days in advance, predicting a derecho can be difficult.

Phil Hysell, the Warning Coordination Meteorologist at the National Weather Service Office in Blacksburg, says it’s the small-scale meteorological aspects that can make a difference in whether a severe storm becomes a derecho.

“We have a really good understanding of the conditions required to develop lines of thunderstorms, but once those lines of thunderstorms develop when do they take on those characteristics of becoming a derecho and are long-lived,” Hysell said. “That really boils down to interactions with each cell within the line, the processes with rain droplets inside those clouds: we’re talking about really small scale processes that are really difficult to measure and to predict with much-advanced notice.”

Usually, meteorologists can look at a radar to better identify a derecho-type event. Radar signatures corresponding with a derecho-type event can show a line of storms “bowing out” and holding their strength over a period of time. Yet, by studying radar signatures, this may only provide a few hours notice.

In the past, derechos are often associated with hot weather and a passing cold front. Nonetheless, lines of storms have developed with these conditions in place, but they weren’t classified as derechos.

“I think that’s where a lot of research still needs to be done…is what conditions are favorable for derechos versus what conditions are not.”

While Hysell knows there is a challenge with forecasting derechos well in advance, he says meteorologists generally know ahead of time when a severe weather threat is possible.

“We know ahead of time that there’s going to be the threat for severe weather,” Hysell explained. “We may not know that it will meet the definition of a derecho —and we really don’t know if it will be a derecho until after the fact — so I think the important message is if there’s the threat for severe weather for damaging winds, I don’t think it really matters if it’s a derecho or not.”

When lines of thunderstorms approach from the west or northwest, the unique challenge with Southwest Virginia is whether these storms will make it over the mountains or deteriorate. Hysell says the National Weather Service in Blacksburg is doing better in anticipating these storms due to new meteorological research available.

Whenever there is a threat for severe weather in the area, it’s best to stay alert and prepared —whether it’s a derecho or a severe thunderstorm capable of producing damaging wind gusts.

“We need to take the same precautions,” Hysell exclaimed. “We need to make sure we have multiple ways to receive weather warnings. We need to make sure we have our places of shelter identified, so when that warning is issued — whether it’s for a derecho or not — we know where to take shelter.”

Many people rely on receiving weather alerts through their phone. Wireless Emergency Alerts (WEA) will automatically send Tornado Warnings to cellphone users when a warning has been issued by the NWS in their area. However, that’s not the case with severe thunderstorms.

“Right now, Severe Thunderstorm Warnings are not activated through Wireless Emergency Alerts. So if we issued a Severe Thunderstorm Warning for a line of storms that may turn into a derecho, you’re not going to get those alerts through your phone.”

Everyone is encouraged to have multiple ways to receive weather alerts and warnings. The only way to receive alerts directly from the NWS is with a NOAA Weather Radio All Hazards. Hysell suggests having an AM or FM radio on hand to get weather alerts as well. There are also numerous private weather and news apps, including the WFXR News app, that will send you weather alerts for your area.

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