Late on Friday, June 29, a straight-line windstorm known as a “derecho” and associated severe thunderstorms hit the Chesapeake Bay watershed. Downed trees and power lines cut power to up to 5 million customers from the Ohio Valley through the mid-Atlantic; as of the morning of July 1, there were still more than 3 million without power. Sadly, lives were lost as well, mostly due to falling trees. While the high temperatures and humidity that helped fuel the derecho have abated slightly, conditions are still above normal, and could lead to dangerous heat exhaustion for the recovery efforts and those without power.
The derecho formed at the Illinois/Indiana border Friday afternoon and traveled east rapidly. The NOAA Storm Prediction Center noted that the storm traveled roughly 600 miles in 10 hours—an average forward speed of 60 miles per hour. The storm fed on instability caused by extreme heat and humidity in the Midwest into the Ohio Valley until it reached the Chesapeake Bay watershed, where record heat topping 100 degrees in many locations combined with high humidity led to heat indices around 110 during the day.
CBIBS buoys captured data as the storm hit the Chesapeake Bay. Each of the ten CBIBS buoys recorded wind gusts above tropical storm force (35 knots), and the Stingray Point CBIBS buoy saw winds above hurricane strength (65 knots). The buoys also recorded dramatic pressure drops—as much as 2 millibars in 20 minutes—during the storm. To see how conditions changed at CBIBS buoys near you, you can create graphs using the CBIBS Data Graphing Tool.
Peak wind gusts at CBIBS stations from Friday night’s event were (1 knot=roughly 1.15 miles/hour):
- Susquehanna: 37 knots
- Patapsco: 44 knots
- Annapolis: 49 knots
- Upper Potomac: 46 knots
- Gooses Reef: 57 knots
- Potomac: 49 knots
- Stingray Point: 71 knots
- Jamestown: 45 knots
- Norfolk: 39 knots
- First Landing: 43 knots
Because of the extreme damage caused by the storm in neighborhoods around the watershed, many people may think that a tornado touched down near them. But because of the nature of this line of thunderstorms, this is not the case—the damage left by this storm was caused by strong straight-line winds. Tornadoes were unlikely to form, and of the 1,200 storm reports received by the NOAA Storm Prediction Center from this weather event, only two (both in Ohio) are for unconfirmed tornadoes.
While storms like this are rare, always check the forecast, radar, and conditions at CBIBS buoys before heading out on the Bay. NOAA Weather Radio broadcasts official National Weather Service information—like the severe thunderstorm watches and warnings issued in advance of the June 29 derecho—24 hours a day, 7 days a week.