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It is that time of year again when we release our Official Winter Outlook! It’s pretty hard to believe we are already in the beginning of November and in just a matter of weeks, snow will be a common theme! Below we will be going over the science behind this Winter Outlook and of course the outlook itself.
For those that simply want to see the Temperature and Snowfall Outlooks, feel free to scroll to the bottom.
Starting off with the state of the ENSO, last year we had an exceptional El Nino, that led to an incredible amount of warm air through much of last Winter. An El Nino is when sea surface temperatures in the Central Pacific are warmer than average as seen in the image below. Under an El Nino, the sub-tropical jet stream becomes more of a dominant feature which typically leads to increased chances for large storms. Part of the reason the southern half of PA received a massive blizzard last year.
However, this year all indicators are pointing toward a weak La Nina. A weak La Nina is just the opposite with cooler sea surface temperatures compared to average in the Central Pacific.
Above is the Sea Surface Temperature trends over the last year. Notice how it was much warmer than average this time last year was, but gradually over the year, has cooled enough to the point we are in a very weak La Nina, some would even call this a Neutral ENSO currently. Unlike an El Nino, this indicates a less active sub-tropical jet stream, which decreases the massive storm chances, but don’t get upset snow lovers, there are more factors involved…
In order to receive snowfall, a cold air source is needed. Last year, cold air sources were scarce. But, this Winter looks to be colder than average, potentially much colder.
Taking a look at the Arctic Oscillation (AO), it is projected to be negative this year. When the AO is in its negative phase, it allows for the cold air source to sink south into the continental United States, rather than being bottled up in Canada in its positive phase. Below is the current AO. Ever since the beginning of October this has been in a negative phase and will likely remain that way for the majority of Winter. But, the AO is only one player.
The Pacific North American teleconnection (PNA) is another factor. When this is positive, a ridge develops out West leading to a trough over the East. Projections show that this will have fluctuations throughout Winter, however the first half of Winter looks to remain mostly on the positive side. Another ingredient for a quicker start to Winter.
Another factor to look into is the North Atlantic Oscillation (NAO). This is the key player determining often the speed of storm systems. When this is in a positive phase, this results in no blocking over North Atlantic which leads to any storm systems to be a quick mover. This Winter is projected to be in a negative phase, increasing the chances for blocking over the North Atlantic. This means, even though we will have a less active sub-tropical jet stream, whatever storm that does develop, whether it be a clipper or a coastal storm, will move slower than typical systems. Don’t need to be a rocket scientist to figure that one out, the longer the duration of the storm, the more substantial the accumulations are.
Another big factor to determine the weather across Pennsylvania is taking a look at the sea surface temperatures in the Gulf of Alaska. The scientific term we call this area is Pacific Decadal Oscillation (PDO). Anytime the sea surface temperatures are above normal, this also allows for a ridge to take place out West, leading to a trough over the East. Currently, the sea surface temperatures in the Gulf of Alaska are above normal, however in recent weeks, there has been a slight trend to cool off the waters in the GOA. This is critical. Because it is trending at such a slow pace, we believe it will remain above normal leading to ridging out West. But, if the cooling trend speeds up, this could put a damper on many forecasts calling for a colder than normal Winter.
Bringing it home closer to PA, Lake Effect Snowfall was quite inactive last year due to all the warmth. With the expected colder than normal temperatures this year, and a warmer than average lake temperature, this is a recipe for a pretty active lake effect snow year for Western PA.
Current surface temperatures across Lake Erie is nearly 60 degrees! Until the lake completely freezes over (if it does) there is a substantial opportunity for an above average Lake Effect snow year.
After gathering information and observing the state of the ENSO and all teleconnections, we use this info to compare it previous years that were similar. We call these years analog years. The years that are looking pretty similar to this upcoming year are the winters of 1983-84, 1985-86, 1995-96, 2013-14, and 2005-06.
Putting all of these analogs together, we come up with this:
Our top 3 analogs are 1995-96, 1983-84, and 2013-14. If you recall, in 1995-96 was a very snowy and cold winter. Many areas broke record snowfall totals resulting from the blizzard of 1996. Now we aren’t saying records will be broken this year, but the pattern is shaping up similar to that Winter. In 1983-84, similar pattern was shaping up where November was a transition month from warmth to cold, with December remaining colder than average, much like what we are expecting this year. 2013-14 Winter a brutally cold Winter. The sub-tropical jet stream that Winter was not active and yet many areas report above average snow. A good example that you do not need an active sub-tropical jet stream to receive above average snowfall.
This year we are expecting warmth to continue for the first 10-14 days of November before a transition to favorable cold pattern sets up in the 3rd week of November. Expect an early start to Winter straight through December with increased chances of a White Christmas. As we head into January, we may have a thaw for a week or but in general below average temps will persist right through March.
TEMPERATURES & SNOWFALL
While many of you just want to know if there will be more or less snowfall than normal this season, what really determines that is temperatures. Persistent above average temperatures lead to below average snowfall, whereas constant shots of Arctic Air lead to below average temperatures translating to above average snow the majority of the time. This Winter, we predict Pennsylvania will experience slightly below normal temperatures. Of course there will be periods of warmth, but below average temperatures will be the theme. Below is our Temperature Outlook Graphic.
Now the part that you’ve all been waiting for, snowfall. With slightly below normal temperatures expected, will snowfall be above normal? The answer to that is yes. There will be a few common storm types.
We believe most of the activity we experience this season will come from the west. There will likely be an above average amount of Alberta Clippers. Clippers produce light to moderate snowfall and a partnered with cold air. They are often more potent in West and Central PA due to the mountains. Eastern Pennsylvania the majority of the time receives what’s left over from the system, but every so often a coastal low pressure system can develop and dump impressive amounts of snowfall on areas such as Philadelphia and Allentown.
In addition, we will encounter storms sliding in from the Ohio Valley. Moderate to even significant snowfall often falls in a west-to-east fashion during these events. In some cases high pressure to the north will win out and Southern PA receives the majority of the snowfall. Other times warm air dominates in Southern Pennsylvania while the northern part of the state is blanketed with snow.
Of course as usual a few coastal storms are likely to occur, but we don’t think this winter will be packed with “Nor’easters.” Moral of the story, this Winter will bring many light to moderate events that add up over time. If you are located in a more rural area where it only takes three or four inches of snow to make travel difficult, chances are you may be missing a few more days of work this year than usual.
Finally, it’s time to reveal our Snowfall Outlook for the upcoming season!
We predict while the entire state will see above average snowfall, inland areas will be farthest above average.
Don’t see your town/city on the state-wide map? We’ve got you covered! Below are our four Regional Winter Outlooks, click on your area.