Tuesday, December 27, 2016

Tracking the Effects of La Nina

Last year the Stateline experienced the impacts of an El Nino winter, which means on average the Midwest experiences a warmer and drier winter compared to normal. Typically, but not always, the reverse a La Nina will then set up for the following year. This year a La Nina Advisory has been issued for the winter, but it is a fairly weak set up before transitioning to ENSO (El Nino Southern Oscillation) neutral conditions in January. Every El Nino and La Nina are different, but a generalized outlook would show a couple things. High pressure builds in the east Pacific as the air in that region sinks. The active pacific jet weakens as it gets closer to the west coast with a more active pattern in the west Pacific. However, the storm systems still need to find a way around the blocking high pressure. In general this creates a more active pattern for not only the Pacific Northwest but for portions of the Midwest and Great Lakes region, as the jet stream brings the storm tracks overhead. While in the south they stay drier with this typically more active area now much more quiet. This means the Stateline area has a slightly higher chance to see more precipitation. Not only that but with the polar jet forced north because of the high pressure system, it dislodges colder arctic air, and areas in the northern Plains will typically have a colder than average winter, and with colder air trapped in the northern half of the country, the south will typically be warmer.

But again this is just an idealized depiction of La Nina, for more information and detail on ENSO and both La Nina and El Nino, you can click here or here.

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