As we head into December this week, the weather looks to turn a bit colder with even a chance of snow showers. That is right on cue with the start of Meteorological Winter being Thursday, December 1st. With winter upon us, everyone wants to know what will happen. How much snow will we get? Will it be cold or mild? If someone had the definite answers to these questions, they would either be extremely rich or lying. However, with the combinations of many atmospheric signals and modeling we can still make a forecast as to how the winter weather will turn out compared to averages. So, what is it looking like this year?
One of the biggest players on the field that influences our winter weather here in the Ohio Valley is El Nino/La Nina. El Nino is when the waters in the central Pacific Ocean are warmer than normal while La Nina is when they are cooler. In the slideshow (picture 1) you can see the graph of temperature anomalies in the Nino region for the past few years. Notice the moderate La Nina last year that followed a major El Nino before that. The La Nina last winter is likely partially responsible for the large number of tornado outbreaks in the central and south-eastern US and heavy rain in the Ohio Valley this year. In picture 2 you will find one of the forecasts for the next several months. This takes an average of climate models from across the world. It looks like the La Nina will continue this winter but stay weaker than last year. This influence will try to continue an active storm track through the Ohio Valley and possibly bring cooler temperatures.
Another phenomenon in the atmosphere that is often responsible for large arctic air dumps in the eastern US is called “blocking.” This is when areas of high pressure setup over certain high latitude regions funneling cold air into the lower 48 and act as a “block” to keep the cold air in place. Without these, cold outbreaks can still happen but they will be transient in nature instead of persistent. This is also important, but not necessary, for storms to track further south and produce snow in Ohio. Three major regions we watch are in Greenland (North Atlantic Oscillation), the Arctic (Arctic Oscillation), and over parts of Alaska (Eastern Pacific Oscillation.) When the NAO, AO, and EPO are said to be in a NEGATIVE phase, it means pressures are higher and a block may be occurring in the jet stream. So far this fall we have seen a neutral AO and NAO, so that provides little help for forecasting what they will do this winter. However, the past couple winters have featured extremely negative AO and NAO values. If that continues, expect lots of cold air this winter. Because we appear to be a negative decadal phase of the NAO, it does seem likely that it will average at least marginally negative for this winter.
Weather often produces cycles that feed off each other. A great example of this is the repeated rainfall across the Ohio Valley. This excessive rainfall (Cincinnati blew past their all-time highest yearly rainfall already) has led to very high moisture content in the soil. This means plenty of water can be evaporated back into the air and squeeze out more precipitation. The combination of this and the active storm track thanks to the La Nina points towards a very wet winter for much of Ohio. If cold air is around, this could also mean lots of snow.
We also watch the weather pattern in October because there has been some research that suggests this can be a preview to the upcoming winter weather pattern. Obviously it isn’t a perfect correlation, but it is something to watch. This October featured a persistent eastern US trough and multiple cut off lows. Cut off lows are systems that become displaced from the jet stream and often sit over one area for a while. In the winter, this can mean lots of persistent snow. If the October weather pattern is repeated during the winter, this could spell multiple snow and ice events for most of Ohio. Towards the end of month, we also saw a large ridge setup over the eastern US. This suggests that periods of mild air will still occur this winter giving us breaks from the cold. These mild spells will likely pull the average temperature departures closer to normal even with some big cold outbreaks.
We also need to consider that the sun is still in a weak period of activity. This continued low activity time supports cooler temperatures overall.
So you may be wondering what all of these signals point towards for this winter. You may also have just scrolled down to the bottom to see the forecast. Either way, my outlook for this winter is a colder than normal one. I expect temperatures to average a little below normal tempered by a few periods of mild weather. Precipitation is looking above normal, however snowfall is a little more tricky. It may be hard to line up both the southern and northern branches of the jet stream very often for those large snowstorms that head up from the south. However, the colder pattern suggests that multiple clipper systems will be in abundance. Also if any cut off lows sit over the Ohio Valley, these could dump quite a bit of snow. Overall, slightly above normal snowfall looks to be in order.
Be sure to stay tuned to lodeplus.com for detailed discussions and snowfall maps when winter storms threaten this season!
Columbus Temp Departure for DJF 2011-12: -0.5 to -1.5F
SNOWFALL: 28-38 inches (average is 28 inches)