So...about that Saturday snow, or lack thereof - what happened? As Twitter follower @manp put it, "@MattNoyesNECN what happened to our 2 to 4" in Boston?" Then, there is the overarcing question of, what happened to our winter? The weather fanatic, like Twitter follower @WeatherFred phrases it, "@MattNoyesNECN Matt why have the computer models been so bad this year?"
The answers could be found in analysis of global patterns and teleconnections, including Pacific and Atlantic pressure patterns and resultant wind patterns, but the most basic and pertinent answer is: split flow. A "split flow" is the term meteorologists use to refer to two separate and distinct jet stream wind patterns - a northern jet stream, called the "polar jet," that typically carries cold and energetic disturbances, and a southern jet stream, the "subtropical jet," carrying moisture and warmer disturbances. In order to get a sizeable storm, we need to see "phasing" - that is, the northern cold energy and southern warm, moist energy must combine. This contrast of air and combination of energy is precisely the recipe needed for hearty winter storms to blossom, and in a typical winter we see this phasing taking place somewhere over the United States. The precise location of the phasing between these two jet streams determines the storm track, and is rarely stationary throughout a winter, meaning that snowstorms are distributed across the Lower 48 based upon where and when the phasing of the polar and subtropical jet streams allow for storms to blossom.
This year, that phasing has been happening in the same place with remarkable consistency and regularity - over the Western Atlantic. We've seen storm after storm intensify rapidly just off the coast, passing south of New England. Occasionally, the pattern will break down and a storm will form over the Central Plains, running into the Great Lakes - this has allowed for snow from Oklahoma and Texas into the Midwest from time to time, but those storms have been less frequent than their Atlantic counterparts, and mainly the result of a polar jet stream disturbance that is exceptionally strong, and is able to dig far enough south to briefly access the subtropical jet stream moisture before recoiling north again, into the Great Lakes, sweeping warmth ahead of it and into New England. So, essentially, the split flow has resulted in two possible outcomes for New England: a miss to the south, or rain.
The image included here is a representation of the 7-day mean 500 millibar heights and anomalies. I'm going to focus on the mean heights, which is the left of the two image panel. For those not familiar with reading a map like this, the basic tenet is that the temperature of the atmosphere determines the height of a column of air. Warmer air is less dense, so the air column is taller - colder air is more dense, so an air column is shorter. Looking at a map like this, lower heights, generally found closer to the North Pole, represent cooler average air temperature through the atmosphere, while higher heights represent a warmer atmosphere. To approximate the jet stream, look for tighter packed contours - that is, tight gradients in height/temperature. In the last seven days, averaged on this map, we see two zones of tight clustering of the height lines - one dipping from Canada across the Great Lakes (this is the polar jet) and the other running from Texas to the Carolinas (this is the subtropical jet). Where do they finally merge into one stream? Off the Eastern Seaboard, over the Western Atlantic, and - not coincidentally - this is where the storms have been forming.
The best representation of where the jet stream is located, however, is actually found higher up. This image depicts the average wind speed at 200 mb (typically 40,000 feet in altitude) and anomaly - again, I'll focus on the average, which is the top of this two panel image. Note the strong wind over the Pacific Ocean, indicating a fast, "phased" jet stream where the polar and subtropical jets are both contributing. Then, notice the absence of strong wind over the Continental US - indicative of our split flow. Where does the wind increase substantially? Over the Western Atlantic, where the northern and southern streams phase again. This is, again, where our storm development has been taking place.
So, the questions about where and why winter has been absent in the Lower 48 hinge upon this split flow. Similarly, the answer to why the few storms we've seen have posed challenges are also found in this split flow. Consider this: the key to a good forecast is determining precisely where the phasing of northern and southern stream will occur. A difference of a few hundred miles seems like a lot to us in terms of driving or travel, but when put in the context of the atmosphere, that is a relatively short distance. So, leading into this past weekend, which turned out to produce hardly a flake of snow in many communities, the key question from days out was: will the phasing of northern and southern streams occur earlier than it has been so far this winter, or out over the ocean again? For all the reasons I explained in weathercasts in days leading up to the weekend snow chance, my intuition on the weather pattern was that this storm would, yet again, develop just a little too late. Having said that, to forecast a near or total miss would mean disregarding millions of dollars of forecasting technology - check out the precipitation forecast from one computer guidance product (the NAM) two days prior to the storm. The deep blues indicate precipitation amounts of near and over an inch of liquid, essentially meaning 10"+ of snow for a huge swath of New England! Then, contrast that with the final precipitation forecast from the same computer guidance just hours before onset of snow (for some) Saturday. What an amazing difference!! All of this hinges upon precise location of phasing - an earlier, farther west phase means the storm blossoms sooner. A later phase, means a miss. Like so many times this winter, the later phasing was the correct solution.
There are a number of other lessons to learn from this winter, but in the interests on keeping this from being an encyclopedic post, I'll save those for later posts. I hope this helps to answer some lingering questions on the absent winter of 2011/12.