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Severe thunderstorms cause damage across Philadelphia region
Severe thunderstorms and winds swept through the Delaware Valley on Saturday, flooding streets and businesses, and knocking down trees and power lines.
PHILADELPHIA - Severe storms tore through parts of Philadelphia on Saturday, leaving behind downed trees, power outages, flooding and building damage.
City officials said the damage was caused by straight-line winds from several severe microbursts, not a tornado.
What we know:
According to the National Weather Service, a microburst is a localized column of sinking air, also known as a downdraft, that forms inside a thunderstorm. Microbursts are usually 2.5 miles wide or smaller, but they can create damaging winds when that air crashes to the ground and spreads outward.
In Philadelphia, officials said at least four severe microbursts produced straight-line wind gusts of 60 to 70 mph across parts of West and South Philadelphia on Saturday afternoon.
The storms brought down trees and wires, damaged buildings, flooded roads and knocked out power to thousands of PECO customers.
How a microburst forms
A thunderstorm can hold large amounts of rain and hail high in the cloud through rising air, known as an updraft.
When that updraft weakens, the suspended rain and hail can drop quickly toward the ground. That sinking air accelerates downward, then spreads outward in multiple directions once it hits the surface.
That outward rush of air is what can snap trees, damage roofs and make storm damage look widespread.
Warning for Microbursts (Radar image from weather.gov)
Why it is not a tornado
A tornado is rotating air that extends from a thunderstorm to the ground.
A microburst is different. It is a rush of air that plunges downward and then spreads outward in straight lines.
That is why officials described Saturday’s Philadelphia storm damage as straight-line wind damage, not tornado damage.
How strong can a microburst be?
Microbursts can be dangerous. The National Weather Service says microburst winds can reach up to 100 mph or higher, which can be comparable to an EF-1 tornado.
That means even when there is no tornado, a severe thunderstorm can still cause major damage to homes, trees, power lines and vehicles.
Wet vs. dry microbursts
There are two main types of microbursts: wet and dry.
Wet microbursts happen with heavy rain. Dry microbursts can happen when much of the rain evaporates before reaching the ground, but the sinking air still creates damaging winds.
The type most people notice in storms like Philadelphia’s is usually the wet kind, because it comes with heavy rain, flooding and sudden wind damage.
Why warnings can be difficult
Microbursts can form quickly and do not always last long.
The National Weather Service says forecasters may have only short lead times because microbursts can be difficult to detect and may even happen between radar scans.
That is why severe thunderstorm warnings should be taken seriously, even if there is no tornado warning.
What to do during a microburst
If a severe thunderstorm warning is issued, move indoors and stay away from windows.
Avoid driving through flooded roads, stay clear of downed wires and do not walk or drive around fallen trees or damaged structures.
If you see a downed wire, a blocked roadway, structural damage or an immediate threat to life or property, call 911.
The Source: This article was written using information from the National Weather Service and the City of Philadelphia.