Upper Low Instability Brings Storms, But Why?

The Northeast has been "under the upper low" (as we say in meteorology) since Friday. This hurricane-looking system has spawned thunderstorms across the region, making for some good pics to be uploaded to our Photo Gallery, including great rainbow shots and more lightning. Read more to find out why these particular storms are conducive to both.

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But what does "upper low" really mean, and why do these storms form?

First of all, the storm does look somewhat like a hurricane on our Color Water Vapor satellite, from which a capture (yesterday afternoon) is shown below. And it is a large Low Pressure system with inward spiraling bands, like a hurricane. But it is mainly at upper levels of the atmosphere (called an "upper low"), and the surface reflection of the system is weak, with unremarkable pressure and winds -- unlike a hurricane.

With the lack of a significant surface weather system (containing moisture or energy), this storm can't produce its own storms. What the upper low does, however, is it presses colder air down closer to the Earth's surface than it would normally be this time of year (an inverted dome, if you will). This causes lower temperatures here at the surface to be lower than usual (feels like fall today here in Central PA)... but it also causes the atmosphere to become more unstable, and this is key.

Instability is defined in meteorology as the amount of cold air above warm air. Well, I just said colder-than-usual air was already in place; now when the sun heats the earth, causing it to warm, this increases the difference in temperature, hence the instability. This is why you can barely pick out the upper low system on radar overnight, but by late morning towering cumulus clouds are everywhere and showers and thunderstorms blossom (check out the radar from yesterday afternoon below, courtesy our new "Enhanced Radar" product on our Pro site).

Because of the cold air aloft, small hail can also be a feature of these storms, even though the storms don't usually contain wind or tornadoes, like classic severe thunderstorms (check the storm reports from yesterday below, notice how they are mostly hail). Because these storms are generally cellular in nature, with no surrounding cloud cover, they are conducive to rainbows. With extra cold air poking down into the lower atmosphere, they are conducive to lightning (remember lightning requires hail).

Below is a look at "the upper low" itself on the GFS Model 00-Hour Initialization yesterday morning. The concentric rings represent isobars (lines of equal pressure - if they are "closed" it qualifies as a "low pressure") at the "500 mb" level in the upper atmosphere. The colors show atmospheric lift, a measure of how intense the pressure system is.

I hope this helps you understand the science behind your weather. If you have any additional questions or comments, leave a Comment below.