Saturday, May 10, 2014

Preliminary 2014-2015 Winter Analogs

After some consideration, I've decided to create a set of analogs for the upcoming winter, using the same parameters that gave me such high success this past winter.

This post will be divided into two sections of analogs. We will begin with the analog years that matched any two of my five parameters I set forth for this winter.

The image above shows 500mb height anomalies across the northern hemisphere, where cool colors signify stormy and cool weather. Warm colors define the presence of warm and quiet weather. In this image, which shows 500mb height anomalies averaged out across these 9 analog winters, we see significant negative height anomalies across the western coast of North America, defining a textbook negative-Pacific North American index (PNA) pattern. In negative PNA patterns, low pressure dominates the West US, leading to warm weather in the Central and East US. We see ridging in southern Canada, spilling over into the northern United States. There is no clear blocking pattern over the Arctic Circle, meaning cold weather may be even more difficult to come by.

The temperature composite for these same analog years confirms this assumption. We see warm weather dominating the northern United States, maximized in the northern Plains. This sort of temperature pattern is typically associated with an El Nino, where we see warmer than normal weather in the Northern US, with cooler than normal weather in the South or West US.

The precipitation anomaly for these same years confirms the textbook El Nino set-up, as we see below normal precipitation affixed over the southern Ohio Valley down towards the Gulf Coast, and the Southwest US along the Gulf Coast experiencing above normal precipitation. This belt of above normal precipitation looks to be due to the abnormally active subtropical jet stream, another characteristic of an El Nino. We also see a stripe of wetter than normal conditions along the East Coast, which can be attributed to that El Nino-induced subtropical jet stream.

Let's now go over my preferred analogs, which match three of the five chosen parameters that fit the upcoming winter.

Looking at the 500mb height anomaly composite for the four years that match three of my five parameters, we see a much different story than we saw with the less-preferred analog set we just went over. We now see a well-defined blocking situation over the Arctic Circle, with strong positive height anomalies in Greenland and into Canada. This exemplifies the negative phases of the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO). The negative phases of these indexes both allow cold air to push into the United States, and the negative NAO permits the storm track to push north and threaten the East Coast with more coastal snowstorms.

The temperature composite for these four choice analog years is also a change from the other analog set. Rather than most of the United States being warmer than normal, the negative AO and NAO seem to influence temperatures to be well below normal across much of the nation, maximized in the southern Plains. We still see warmer than normal conditions in the upper Midwest, but these warm anomalies are more restricted than they were in the first analog set. Unfortunately, this could mean the second straight colder than normal winter, as I place more trust in this analog set than the one we first analyzed.

The precipitation composite for these four analog years shows a very stormy East Coast, with anomalies anywhere from 4 to 6 inches above normal, most enhanced in the Southeast. This precipitation pattern is still somewhat aligned with a typical El Nino pattern, though the dryness in the Southeast argues against this somewhat. I'm not as ready to accept this precipitation composite, considering how dominating the El Nino may be this winter, but as always, the analog set will be refined in coming months.