After a harsh winter in the Central and Eastern United States last year, many are wondering what this winter will bring in terms of cold and storminess. I put this winter forecast together based on a variety of factors including climatology/analogs, and meteorology. Last winter many public, private and amateur forecasters were calling for a mild winter when it actually turned out to be the opposite. The main problem was that a warm pool of water in the gulf of Alaska developed and this promotes ridging in the jet stream over the western USA called a +PNA pattern. This feature didn’t develop until November and December and this completely changed the overall weather pattern for the winter. This feature of sea surface temperatures is called the Pacific Decadal Oscillation and is important for seasonal forecasts as noted above. I think this feature which is still around but slightly cooler will continue to hold and help aid the cold, stormy pattern this winter in the Central and Eastern US. As many busted last year’s forecast, it is important to note why seasonal forecasts are not great and they are vague because we cannot possibly make a deterministic forecast for a day or week months in advance mainly due to non linear chaos in the atmosphere. We can only give an idea of the departure from normal temperatures and precipitation for a 3 month season, DJF in this case. This is considered Meteorological winter and Astronomical winter being December 22nd to March 22nd.
So as for this winter, computer models have a weak to moderate El Niño developing and getting going by the middle of winter. This is an important feature as it influences our weather pattern in the US and activates the subtropical jet stream giving the moisture supply for east coast winter storms. (This will also help California’s drought but will not get rid of it entirely.) It looks to be a “Modoki” El Niño or centrally based. This means the warmest anomalies of sea surface temperatures will be over the south central pacific. Here is the latest US climate model projection of those features in the pacific during DJF. Note the warm anomalies in the south central pacific and the warm pool in the gulf of Alaska.
Generally speaking weak “Modoki” El Niño’s bring below normal temperatures and above normal snow to most places in the Central and Eastern US. The winter of 09-10, “Snowmaggedon” in the Mid-Atlantic region was a weak El Niño. However that was an extremely rare event and I’m in no way saying that’s going to happen again. The point is the potential is there for more snow than the seasonal average this winter as centrally based weak El Nino’s are what you want to see for a snowy winter in the Eastern US.
Another thing to look at is the build-up of snow cover in Siberia during October. There has been some research done on how this affects the Arctic Oscillation. Generally speaking when snow cover builds up in Siberia to high levels in October this leads to a more negative Arctic Oscillation and brings more sustained cold air into the Central and Eastern US during the winter. It has broken records for extent in October so this is a clear signal for a more negative AO this winter. Without getting too technical, when this snow cover builds up early in the season, it really helps build and strengthen the polar vortex so that when the PV breaks apart and drops south this winter it brings in very cold air into the US. This is an important feature for East coast winter storms as if we look at the climatology, most major one’s occur with a -AO. Also, there is the North Atlantic Oscillation which when in it’s negative phase sets up a blocking pattern with a large high over Greenland forcing storms up the east coast instead of heading harmlessly out to sea. During the winter of 09-10, we hit a record low value of the NAO index and we all know what happened that winter. If you have a –AO and –NAO combo, this promotes more favorable conditions for snowstorms in the Eastern US. In the positive phases like 01-02 or the blowtorch winter of 11-12, we had an extremely positive NAO and AO all winter long. However it is important to note, the NAO and AO are not the end all be all as we saw last year, other features are involved. Here is a look at the general AO/NAO setups.
Above is a graphic of the DJF temperature and precipitation anomalies. Note that mild means above normal temps (normal being 1981-2010 Climatology) during this period and cold means below normal temps. Wet means above normal precipitation for the period and dry below normal. Of course a seasonal temperature for a three month period doesn’t please people too much because they care about the daily temperature range more so than the average temperature over a long period of time. The winter of 89-90 is a great example of that. After one of the most impressively cold months of 20th century in the Northeast in December of 89, January of 90 was a top 10 warmest on record and February also was warmer than normal. So don’t think below normal temps means a little below normal steady through, it will vary obviously as weather gets superimposed on top of the seasonal changes. Given this I will add that I think the first half of November and the first half of December will be relatively mild (meaning above normal) for the Central and Eastern US and the bulk of the coldest stormiest period will be late December through February. To see what a typical winter looks like in terms of temperature and precipitation go to the NCDC website, http://www.ncdc.noaa.gov/cag/ and search for DJF average temp and precipitation in your climate region.
Given the setup of the El Nino, PDO, AO, NAO, and Siberian snow cover, I think this winter will has a similar impact on the US as last winter with less overall storms but larger events in the East. Even though it will be cold in the upper Midwest, I don’t think it will be nearly as cold overall as last year. If you live in the Southeast, get ready to turn up the heat!