This is an important factor in the Winter ahead, especially given the understanding that La Nina events tend to feature Southeast US ridging overall. Recently, an easterly QBO has been observed. A negative (easterly) QBO often supports higher latitude blocking and ridging, while a more positive (westerly) QBO supports some resistance to high latitude blocking in those areas. The QBO is a very important atmospheric index which monitors the quasi periodic oscillation between the equatorial zonal wind – from easterlies to westerlies. Having confidence in the orientation of SST anomalies in the Tropical Pacific is a key part of the winter forecast and serves as a foundational piece of the forecast as a whole. It helps us to understand where we are heading during the winter ahead and how global circulation patterns may behave. This analog set matches quite closely with ongoing and expected conditions. Using a weighted analog set featuring a lean towards eastern-based La Nina events of at least moderate strength, we are able to compile an analog set of the SST anomalies in the Tropical Pacific. The most frequently observed types of La Ninas are “central based”, when the coldest anomalies are in the central Tropical Pacific and “east based”, when the coldest anomalies lean eastward, closer to Central and South America in the eastern tropical Pacific. This year, colder than normal sea surface temperatures extend across both the Central and Eastern ENSO regions, with a slight lean towards the eastern regions closer to South America. La Nina events can be broken down into subcategories separate from their strength, based on location. La Nina conditions are observed when colder than normal temperature anomalies exist in the aforementioned tropical regions of the Pacific Ocean. In a general sense, the more anomalous the positive sea surface temperatures, the more convection that can then exert a stronger forcing mechanism on the adjacent regions of the atmosphere, reverberating throughout the globe.Īlmost all forecast model guidance now agrees that we will experience weak La Nina conditions during the upcoming winter. The equatorial waters of the Pacific ocean that comprise the ENSO regions breed a great deal of thunderstorm activity, which then accordingly results in atmospheric ridging and subsequently moves downstream. This convection, most frequently observed in warm and moist climates, releases latent heat that then rises up into the atmosphere, forming ridges of higher atmospheric air pressure. Tropical forcing more specifically refers to concentrated areas of showers and thunderstorms, otherwise referred to as convection, in meteorological regions of the tropics. The El Niñdo-Southern Oscillation (ENSO) is “an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean.” ENSO conditions are one of they key drivers to the Winter pattern, and can often be broadly referenced as “tropical forcing”. The role of ENSOĪnticipated ENSO Conditions: Moderate or strong La Nina This will lead us to conclusions which we believe will be the guiding forces for us during the seasons ahead. Instead of focusing on individual numerical indexes and values, we are going to try to paint a picture of the atmosphere and what it’ll be doing over the next few months based on several global and hemispheric oscillations, conditions, and phenomena. We have been piecing together ideas for this upcoming winter since then, and we are excited to finally have a finished product. It is the product of several months of intensive research, which often begins in the prior Spring. We say it every year, but it is worth repeating: Seasonal weather forecasting is one of the most challenging aspects of meteorology. NYC’s 30 year average annual snowfall is 28.9″, and we expect slightly above normal snowfall this season. The second half of the winter, however, is expected to be warmer and less snowy than normal. The Winter of 2021-2022 is likely to begin with a colder and more wintry pattern than normal in NYC.
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