The strong storm system that affected our area last night is finally moving off the coast of the eastern U.S. Saturday.

This storm system had it all: severe thunderstorms, strong winds, heavy rain and winter weather. Many of you experienced those high winds Friday night. Exactly how strong was the wind?

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Peak wind gusts from different locations in the Valley from Friday night and Saturday morning.

The strong winds resulted in numerous power outages across the Valley Friday night and there are still residents without power Saturday morning. As of 9:45 a.m. there are around 800 residents without power in Cleveland, 600 in Pittsburgh, and a couple hundred around the Youngstown area.

Local power outage map as of 9:45 a.m.

The hardest hit areas locally were in the higher elevations of southwestern Pennsylvania where thousands of residents are without power Saturday morning.

The hardest hit areas from last night’s winds include Tennessee, Kentucky and Michigan. There are still over a million people in the United States without power this morning.

Power outage map for the United States as of 9:45 a.m.

What caused the strong winds last night?

If you were awake from the noisy wind, then you would know that there were not thunderstorms in the Valley last night. You might be wondering, then, what was the cause of the strong winds from last night’s storm system?

Typically, the intensity of low pressure systems are measured by how low the pressure at the center of the storm system falls.

The lower the pressure at the center, then the stronger the storm system. Friday, the pressure at the center of the storm fell to around 976 millibars (typical surface sea-level pressure is around 1013 millibars) which is quite uncommon for the southeastern United States.

In strong storms, the barometric pressure changes significantly over a small area which produces a force called the pressure gradient force.

The pressure gradient force is a fundamental concept in meteorology and describes the force that drives air from areas of higher pressure to areas of lower pressure. It results from differences in air pressure between two points in the atmosphere and is responsible for the movement of air and the formation of wind.

The pressure gradient force arises because air naturally moves from regions of high pressure to regions of low pressure, due to differences in air density caused by temperature and other factors. The greater the difference in pressure between two points, the stronger the pressure gradient force.

The strong pressure gradient force from yesterday’s storm system was the cause of the strong winds. This can be visualized on a map by using isobars which are lines on a weather map that connect points of equal pressure.

Below is a surface map from the storm system yesterday. The thin grey lines are isobars. Noticed how tightly packed the isobars are in Illinois and Indiana. This is indicative of a strong pressure gradient force which results in strong winds.

Surface map from yesterday courtesy of NOAA.

Thankfully, the storm that caused these strong winds is now moving off the coast. Good riddance!