Evolution of upper level disturbances for today’s storm (top) and the January 26, 2011 “Commutageddon” storm (bottom). (WeatherBell.com (top) and NWS (bottom))

The last time we had at least two inches of snow in Washington, D.C. was January 26, 2011. And it was a blockbuster - dumping 5-10 inches of thundersnow at the height of rush hour. The storm stranded thousands of cars for hours. We named the storm Commutageddon. Others called it Carmageddon.

Today’s storm effects in D.C. will bear little resemblance to that paralyzing event and there are good meteorological reasons for it.

Examining the graphic at the top of this post is in­cred­ibly instructive. In a lot of ways, the Commutageddon storm and today’s event are remarkably similar, but the differences between the two events have a profound impact on the kind of weather D.C. experiences.

The graphic above displays the evolution of upper level disturbance (about three miles high in the atmosphere) driving each storm. Pay attention to black ‘x’s (surrounded by red and pink areas) in each of the graphic panels representing the center of the two disturbances (at two different time steps).

Both disturbances are fairly impressive, taking a similar track - from the lower Mississippi river valley to the Mid-Atlantic. These are historically good snow tracks for D.C. since they are to our south, keeping us on the cold side of the storms. But what’s important here is that the two disturbances are in opposite phases of development.

Do you notice how today’s disturbance unwinds and becomes less well-defined over time (top panels above)? At 7 a.m. today, notice the series of closed circular contours around it in western Mississippi. By early tomorrow morning, when the disturbance reaches the North Carolina coast, the contours open up. That signifies a weakening or decaying disturbance (rising pressures) - one which is unwinding or spinning down.

A water vapor view of Commutageddon in 2011. It was a very well-defined, mature storm when it hit D.C. with a classic “comma-head”.

In contrast, notice how the Commutageddon disturbance winds-up (bottom panels above). The curved counters surrounding the disturbance in the lower Mississippi river valley on the evening of January 25 become closed near the Virginia Capes the evening of January 26. This signals a deepening or intensifying storm (falling pressures).

Related: What made Commutageddon special?

So, in 2011, we were dealing with a strengthening storm, today we’re tracking one which is weakening. Thus, it should not be surprising forecast precipitation amounts are lower.

Also, the 2011 disturbance tracked probably about 100 miles north compared to today’s disturbance. As we know from our accumulation map for today’s storm, 100 miles makes a big difference.

Finally, during the 2011 storm, while cold air was still somewhat marginal, there was more of it in place leading up to the storm than this year.

So when you consider together this year’s weakening disturbance, farther south track, and lesser quantities of cold air relative to Commutageddon, it’s easy to understand why snowfall amounts will be much less.