A Look at Winter’s Pattern

Meteorological winter spans December 1st to the end of February, which falls on February 29th this year. The last few days of February likely won’t have a large effect on the overall winter pattern so leaving them off won’t drastically affect the overall results. So let’s jump in.

The area where El Nino is tabulated from.
The area where El Nino is tabulated from.
Above normal sea surface temperatures in the Pacific Ocean around the equator indicating the El Nino.
Above normal sea surface temperatures in the Pacific Ocean around the equator indicating the El Nino.
The NAO, AO, PNA for the winter season where all mainly above normal, aside from a brief negative of the NAO and AO in January.
The NAO, AO, PNA for the winter season were all mainly above normal, aside from a brief negative of the NAO and AO in January.

 

The major headlining variable controlling a large part of winter is the Strong El Nino in the Pacific Ocean. This El Nino was one of the warmest on record in terms of the normal place where readings are taken in the middle of the ocean. This is very important because El Nino has a huge effect on winter’s sensible weather and there are obvious signs that it was occurring during winter. The easiest sign that El Nino is occurring is that the subtropical jet stream which usually sets up near or just south of the Southern United States. This did occur this year and likely allowed for stronger low pressure systems during this year’s winter. This winter’s precipitation departure from normal plot shows the East Coast with above normal precipitation. Also, the warmer than normal sea surface temperatures along the East Coast likely contributed to the strength of the low pressure system in that area.

Winter temperature anomalies. Notice the lack of below normal temperatures in the United States.
Winter temperature anomalies. Notice the lack of below normal temperatures in the United States.
Winter precipitation anomalies. Notice the above normal anomalies in the East Coast and the Pacific Northwest.
Winter precipitation anomalies. Notice the above normal anomalies in the East Coast and the Pacific Northwest.
Winter averaged jet stream place. Notice the subtropical jet from the Gulf of Mexico to the Norheast.
Winter averaged jet stream place. Notice the subtropical jet from the Gulf of Mexico to the Norheast.
Mid Level geopotential heights. Generally, the below normal heights indicate low pressure and above normal heights indicate high pressure.
Mid Level geopotential heights. Generally, the below normal heights indicate low pressure and above normal heights indicate high pressure.

In order for there to be plenty of wintry precipitation, we need lots of precipitation, which we know we had, but we also need plenty of cold air. Last winter, it seemed we could never get out of the cold air. Cold air was locked in Canada and whenever the jet stream came in from Canada, it seemed another snow storm was forming during January and February. This year, warmer air than normal was locked in Canada. So, while we had winds in from Canada, cold air was harder to tap into for winter storms. The only storm with all snow this winter was the blizzard that buried the Washington DC area, which occurred on January 22nd to 24th. The Northeast Storm Impact Scale (NESIS) compiled by renowned snow storm researchers Paul Kocin and Louis Uccellini identified this storm as a category 4, or “crippling”, winter storm and is rated as the 4th most powerful snow storm since 1950. Most of the other storms that impacted the East Coast were snow mixed with rain and ice precipitation types. One reason this may be is because we had warm phases of the North American Oscillation (NAO) and Arctic Oscillation (AO) as a winter average meaning there was no blocking in the jet stream flow to allow for storm development and the teleconnection phases likely contributed to the lack of cold air. Furthermore, the 15 day period where the NAO and AO went to the cold phase and then trended back towards the warm phase that allowed for the January Blizzard, so that likely had an effect on the winter pattern. Even still, the Northeast ended up with a winter of around normal snowfall accumulation.

Finally, El Nino’s are also known for having plenty of moisture flowing into California, but this El Nino had the moisture flowing into the Pacific Northwest instead as a result of the average placement of the low pressure system and jet stream off the Eastern Pacific Ocean. The general low pressure system‘s placement was in the Gulf of Alaska and concurrently, the general place of the jet stream was north of San Francisco. When this placement occurs, areas north of the jet stream get precipitation, but those south of the jet stream receive minimal precipitation. This along with an average of above normal heights, which indicates drier air, in the Southwestern US brought the moisture into the Pacific Northwest instead of Southern California.

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