Waiting for the Western Trough

Jan 1 2018 8:30 PM CST| 0 Comments | Wisconsin | Long Range Forecast

Man, it's cold.

High pressure over middle of the lower 48. Below average temperatures for pretty much everyone.Great lakes slightly warmer.

Through the first full month of meteorological winter, we've definitely had the cold you would expect this time of year. Temperatures have been far below average since Christmas. The snow on the other hand had been underwhelming especially for southern Wisconsin where they are still waiting for their first major storm, first 4"+ accumulation even. Northern Wisconsin has experienced a steady stream of snow from clippers. This pattern has been great for lake effect snow in the upper peninsula, lower Michigan, and great lakes overall. The pattern has been fairly consistent.

Riding west, trough east forces low pressure systems across the northern rockies southeast towards the great lakes and northern plains. Cold attacks the east.

If you are like me, by this point you are starting to wonder if Wisconsin will get the major storm we hold our breath for? That is a great question.

Our pattern has been dominated by northwest flow since November. We've certainly had our moments of warmth. Recently we've seen some very cold air spill into the western great lakes. We were near-record cold for New Years which is hard to do. It feels more like late January or February at this point, with cold northwest flow driving clipper systems through the region. This is much different southwest flow which will tend to drive low pressure systems in from the southwest or zonal flow.

PNA(Pacific North America) oscillation. Since Dec 1 has been on the boring end of the spectrum. We haven't had a strong negative PNA since November.

In November we saw an abnormally strong negative phase of the PNA oscillation. It's interesting because since then we've only seen favoritism towards PNA or increasing PNA. NAO has been straight as an arrow. AO has been sort of interesting, up and down but mostly positive more negative only recently. Our pattern since Dec 1. has been very symptomatic of these teleconnections phases which represent upper level weather pattern tendencies. When the AO breaks temporarily towards the negative phases like it did around Christmas, arctic air is uncorked. The tough question is what does this say about our future? If major snowstorms are what you seek I know it's been a tough winter with little optimism for the future.

WAITING FOR THE WESTERN TROUGH

For big snow, you need to look for negative PNA. The PNA index during the top 20 biggest snowstorms at the Oshkosh historical site, 17(85%) were negative. Strong correlation. You can get above average snow from positive PNA but it's harder to do. Negative PNA signals a western trough and a storm track across the central US, a pattern that invites the collision of gulf moisture and polar cold. Medium and long range models are signaling at least some western troughiness. The 6-14 day forecast and analogs continue to show weak signals of a western trough as well. The EPO and WPO are about to switch phases. With weak signals I worry pattern changes will continue to get pushed into the future, but if the EPO and WPO are on board I feel more confident saying the pattern will change in early January. Whether that's the development of a full blown western trough or simply a chaotic pattern with northwestern flow flavor other than what we've seen so far; remains to be seen. But it looks like it will get wetter and at least a little bit more chaotic, which is nice. My personal feeling is that after several weeks or months of dry, naturally will flip to the opposite and shock the system. La Nina winters are known for January through March, so just need to be patient and wait on evolution.

Below average heights over western north america in the 6-14 day period will lead to weather pattern changes.

The GFS is showing an interesting system for next weekend into Mon Jan 8. Given the complexity of the pattern it's probably too early to be talking about this. However this system could be the segway between weather pattern regimes.

GFS has shown low pressure system sneaking under the western ridge, phasing and strengthening over Wisconsin next weekend.

Good News, Bad News

The bad news for snow lovers is that if our current pattern dominates through the rest of winter then we will nickle and dime our way to average. There are two ways to reach average. Slow and steady or fast and heavy. The fast and heavy method would require an episode of negative PNA and more chaotic weather pattern.

The good news for snow lovers is that December has nothing on January or February, and that either period has no statistical trend one way or the other. In other words cold December does not lead to cold January anymore than a warm December would.

Arctic air has been steady attacking further west since the beginning of winter. I predict January will be warmer, perhaps at or above average. I think this will mean wetter, but if the snow or rain doesn't come then I believe when arctic air makes another push into the lower 48 in late January or February it will be further west. From that will come the highlights of the 2017-2018 Wisconsin Winter.

I believe the identity of the 2017-2018 winter is yet to be revealed.



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