Anatomy of a blizzard

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My last newsletter was about a historic storm, hurricane Sandy, and this newsletter, will be about another historic storm, the blizzard of 2013 (Winter Storm Nemo). In fact, the newsletter is going out a little later than usual because of the storm!

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One of the more frequent questions I have received about this storm (except for “how much snow did you get?” — 28 inches at my house on the coast of Maine!) has to do with how such a storm forms, so I thought I would use this newsletter to look at the factors that came together to lead to such a tremendous amount of snow over a good portion of the northeastern U.S. along with hurricane force winds over the adjacent coastal waters.

As is frequently the case with mid-latitude storms, a look at the upper level (500 millibar) pattern provides significant insight into the evolution of a strong surface system. So we will look at the 500-millibar charts along with the surface weather charts for a 24-hour period from Friday morning Feb. 8, 2013, through Saturday morning Feb. 9, 2013.

In general, at the 500-millibar level, meteorologists look at waves in the flow pattern to determine how much energy is present, and also how the transport of warm and cold air will occur. When the flow pattern dips more toward the equator, we say that a trough is present, and typically these troughs lead to the formation of surface low-pressure centers, which, as most folks are aware, are systems that can produce significant precipitation and strong winds. Usually at the 500-millibar level there are many troughs of varying different wavelengths and amplitudes, which will move at different speeds. Sometimes these troughs interact very little with one another, but occasionally they will interact in a way that leads to two troughs combining and resulting in one much stronger trough. The effect is not at all unlike waves on the surface of the ocean. Sometimes at the beach, one can observe two waves coming together and resulting in a much bigger wave, but sometimes the two waves remain separate and hit the beach as rather wimpy features.

In the atmosphere, when two waves combine, we speak of the “phasing” of 500-millibar troughs, and that is what happened in the case of Winter Storm Nemo.

Let’s look at the charts and see. Figure 1 is the 500-millibar chart for 1200 GMT Friday, Feb 8. Looking in the eastern U.S, two troughs (shown by dashed lines) can be seen: one extending from Michigan southwest to Texas, and another from Virginia south to the Carolina coastal waters. We would expect to find surface low-pressure centers downstream of these troughs, and if we look at Figure 2, which is the surface chart valid for the same time, we see that this is indeed the case with lows located over Lake Erie and near Cape Hatteras. These systems were separate and distinct at this time, and each was producing precipitation — mainly rain with the coastal system, and snow and mixed precipitation to the north and northwest of the Great Lakes system.

Moving ahead 12 hours, Figure 3 is the 500-millibar chart for 0000 GMT Saturday, Feb 9. Note that both troughs have progressed east, and also that they are a little closer together. Essentially, the more western trough is moving more quickly and is catching up to the eastern trough. Looking at the corresponding surface chart (Figure 4) we see that the coastal low has strengthened rapidly as energy from the more western trough has begun to affect it, and while the western low is still present, it is starting to get swallowed up into the expanding circulation of the Atlantic low. The rapid strengthening of the ocean low has led to many more isobars surrounding it, and the system is producing hurricane force winds at this time. The phasing process of the two upper level waves is well underway.

After another 12 hours (1200 GMT Saturday, Feb. 9), looking at the 500-millibar chart (Figure 5) reveals that the two troughs have completely merged resulting in a closed, strong upper level low just southeast of Nantucket, Mass. The surface chart for this time (Figure 6) shows a hurricane force low over the outer Gulf of Maine, and the western low is no longer indicated. The phasing is complete. At this time very heavy snow and strong winds were occurring over eastern New England.

Another factor, which plays a key role in the blizzard conditions, is the strong high pressure anchored over eastern Canada. Notice that this high remains generally in place through all three surface charts. The high provides the required cold air, which allows the heavy precipitation to fall as snow. The presence of the high also enhances the pressure gradient which, in turn, leads to the strong winds. Without the strong high, a strong storm may still have occurred, but winds would not have been as strong, and the areal extent of the snowfall would not have been as great with warmer temperatures leading to more rain.

By watching the 500-millibar charts for the possibility of troughs phasing and leading to a stronger feature, it is possible to anticipate that a rapidly developing surface low will evolve, and for mariners, this means strong winds and high seas at any time of year.

Two quick follow-ups from previous newsletters:

First, after the last newsletter about hurricane Sandy, there were a couple of comments that deserve my response, and I apologize for not providing these earlier. One comment asked about the system apparently stalling for a time before making landfall. This occurred during the time when the system was transitioning to an extratropical low and was being “picked up” by the mid-latitude 500-millibar flow. In a sense, the system was beginning to reorganize a bit, and this led to the somewhat erratic movement for this brief period of time. Another comment asked about the “sucker punch” when winds shifted suddenly from E to S in the New York City area. This was a direct result of the fact that at that time, Sandy was no longer a hurricane, and was a powerful extratropical low. Hurricanes have nearly circular isobars, and so the wind shifts around them are very gradual. Mid-latitude lows, on the other hand, have fronts which feature abrupt wind shifts, and Sandy at that time was likely developing these features.

Second, almost a year ago, I wrote about the changes in the arrows showing the motion of highs and lows on surface charts produced by NOAA’s Ocean Prediction Center. The OPC has reversed their decision to eliminate some of these arrows, so to any of you who followed up on that newsletter by making your voices heard, it worked!

Before I finish, here’s a quick plug for the Ocean Navigator School of Seamanship weather seminar which will be offered in the Portland, Maine, area on the weekend of April 27-28: It’s a great opportunity to learn more about marine weather, and to get questions like these answered. More info is available on the Ocean Navigator website. Hope to see many of you there!

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