Joe Lundberg

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Small-Scale Weather in the Midst of a Large Storm

February 27, 2013; 10:36 AM ET

Wednesday, 11:30 a.m.

The weather over the past 24 hours or so from the Plains and Midwest into the mid-Atlantic and parts of New England has been, in a word, stormy. Examine the surface pressure analysis from this morning:

The circulation around the parent storm is huge, extending from central and southern Ontario and Quebec all the way to the Gulf Coast states and from the eastern Plains to the East Coast. From that, it was easy to deduce that many places would have an extended period of clouds, many more would have a fair bit wind, and there would be rain and snow to contend with.

That's the big picture. When you start to narrow it down to smaller regions, it became more of a challenge, especially any place where you would have to deal with multiple precipitation forms. I liken it to watching my cat Gus. He's a playful sort, and you know he's going to do something goofy sooner or later. But figuring out WHEN he's going to do something stupid, or WHAT he's going to do, is next to impossible! Yes, this is the cat that LOVES to play fetch with its collar. Don't ask...

In trying to predict what would happen in the marginal areas, two main things became important: precipitation intensity, directly tied to upward motion, and elevation. In the case of the precipitation intensity, it's pretty simple, really. The stronger the upward motion there is in the atmosphere, the more likely you are to see heavier precipitation.

Think of it this way. In an unstable air mass, you'll see lots of puffy cumulus clouds form during the course of the day. If there is enough moisture and instability, then those cumulus clouds will get tall enough to produce some showers. In the summer, those rising columns of air tend to send clouds high into the atmosphere if conditions are right, allowing thunderstorms to develop. The deeper the instability, the taller the cumulonimbus clouds, and the heavier the rain that falls out of them.

In the case of this storm, the air was marginal for snow across central and northern Illinois in one instance, as well as across portions of Pennsylvania. When the upward motion was relatively weak, the precipitation intensity was light, and that allowed sufficient time to allow falling snowflakes to melt into raindrops. However, any time the vertical velocities were ratcheted up, then the precipitation rates increased. When that happens, it tends to cool the column of air locally. And that can be just enough to make the difference between rain and sleet or snow.

Then, when you throw in elevation, it really complicates matters! In a case like this, the tops of hillside and mountains tend to be cooler than the valleys below, so it's easier to maintain snowflakes there. Furthermore, the presence of mountains tends to enhance upward motion rates, especially if the mean flow is directed up and over that boundary.

Even though the difference in elevation from parts of Centre County to the Susquehanna River Valley in Pennsylvania is little more than 2,000 feet, that's plenty enough to see some wild variations in the weather at any given moment. And true to form, the precipitation over the past 24 hours went from a little rain to rain mixed with sleet to more sleet than anything else, then to snow as the precipitation rates increased and the air aloft cooled as a result ever so slightly! You can literally go a distance of a few miles and see all different forms of precipitation falling, along with vastly different results in terms of snow accumulation. Try to forecast that for each specific location in a given geographic area! It'll age you a few years!

These types of things are hardly exclusive to my neck of the woods. Any place where there's a substantial difference in elevation, a change from one type of environment to another (say, from land to ocean) or local terrain effects, there is apt to be some potentially dramatic differences in the way the weather works. The flow over the Great Lakes is but one prime example. The flow over the Chugach Range south and east of Anchorage is another. Or the local effects of the flow coming in through the Juan de Fuca Strait between Vancouver and northwest Washington, setting up a Puget Sound convergence zone.

Knowing these various local 'microclimate' effects can certainly increase your odds at hitting a forecast. I know, for instance, my cat Gus is going to just do something off the wall, especially if there's cat nip or one of his favorite toys involved! But sometimes he just doesn't do what you think he will do, or when you think he's likely to do it.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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About This Blog

Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.