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Tuesday, December 2, 2014

El Nino's dark side is revealing itself but Vermont weather still could get interesting

The critical ENSO "3-4" region has jumped to a warm 1.0 as of this past week and although the weather pattern doesn't directly respond to those types of changes, El Nino is making itself known and not in the way I would have wanted. The weather pattern in November more or less picked up where last year left off. A large ridge in the jet stream across Alaska and western North America, a mostly loose jet in the Pacific and extreme cold for November standards over large swaths of both the U.S. and Canada. As the month turned to December, the weather pattern flipped on its head and is now dominated by an entirely different set of weather features. The most critical of these features is a broad expanse of jet energy that has settled into the northern and middle latitudes of the Pacific Ocean. This has tightened the jet stream dramatically and caused much of the arctic cold in North America to make a hasty retreat. There are similarities here to our coined "evil empire" feature (large mid latitude Pacific Ocean ridge) but the two are different for a few reasons. The biggest of which will be the precipitation situation in California. The major impacts will be similar however with much of the eastern United States struggling to have enough cold air to support snow over the next 2-3 weeks. "Much" however is not all and I am hoping Vermont is one of the few exceptions.

We did experience a brief but damaging thaw over the weekend, but the "leftovers", from November's polar vortex have brought winter back to Vermont and a few inches of snow as well. The late part of the current week and the weekend are expected to be high and mostly dry. The one exception would be Saturday where some light snow from a disturbance in the retreating polar jet could grace the region with a very light accumulation. After the weekend and through most of next week, the polar jet will be of little use to us and our eyes will turn to the southern branch of the jet stream and a storm that will ultimately track toward the Carolina coast late this weekend or early Monday. This system will not be inhibited by any polar energy, and moisture from this system could work its way into the region early next week. Do we have enough cold air to support a big snow ? Maybe, but it will be of the stale variety, and this makes the prospects of a 15-30 inch powder-fest a daunting one.

Winter will remain loosely entrenched across the region through the middle part of next week, a few days after our potential early-week storm.  The threat for warm days and even some rain will persist however in this pattern through around the Winter Solstice as much above-normal temperatures dominate much of Canada. If there is a silver-lining, it is that the mean axis of the warmth-producing ridge should set up well to the region's west. I certainly don't expect sustained cold weather in such a set-up, but interior New England is positioned to have the best chance of retaining some sense of winter even as the blowtorch impacts large portions of the rest of the continent.

Some other "housekeeping" items that I want to mention in the update today. The "favorability index" this year will be scrapped. I still like the idea of an index but it proves confusing and most importantly has not been particularly accurate over the last few years. One of the big issues with the index, as it relates to snowfall in Vermont, is the varying degrees of what is optimal. For example, a maxed out negative AO and NAO in January usually means Vermont is in the "shaft" zone for snow but such a scenario can be fruitful very early or very late in the year. What has worked well is the tweets however. Hey, we live in the attention-deficit tech savvy world and the blog should embrace it. We will use twitter more and use complex and unreliable "favorability index" less (I should say not at all). Please feel free to follow and respond on Twitter.

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