The winter of 2018-2019 is shaping up to be one of the coldest and wettest in a decade. This winter is contingent on a few major drivers, which is the progression of a weak EL Nino, blocking in Greenland and ridging in the Western USA.
Those who live in the South know that more times than not the formation of a weak to moderate EL Nino means a snowier winter. This is very true and I will show some graphics below that explain that very fact. The reason it is snowier is due to the active subtropical jet stream during the EL Nino. I favor a weak to perhaps weakly moderate EL Nino this winter season and this will increase the amount of precipitation. This EL Nino will not be like any other EL Nino and is referred to as a Modoki EL Nino, which means centrally based (See graphic below). This is very significant in that it allows a ridge to form in the Northeast Pacific (-EPO) and will be explained more in the paragraphs below.
Also, we are looking at the blocking in Greenland as a major X factor this winter and this is perhaps the hardest forecast to predict with many variables causing it or keeping it from happening. I favor Greenland blocking through part of the winter but do not think it will remain as a constant all winter. IF this did remain constant then we would be talking of 250-300% above normal snowfall. The last time this value remained constant was in the 1980’s, so it is on the table although not expected. When combined with a +PNA and/or -EPO, then this most likely precedes a developing major winter storm. The biggest benefit to the -NAO, is that the blocking slows down the polar and subtropical jet streams and allows for a phase to occur. This typically provides a major winter storm and blizzards to the Middle Atlantic from Virginia and points north but is not always a win for the Southeast. The +PNA/-EPO in conjunction with the -NAO is what is our best friend, because the +PNA and/or -EPO send energy diving down and this brings the storms systems further south. Combine this with a weak EL Nino and we have a recipe for not one but several winter storms.
The west coast ridging or +PNA would be enough to cause the cold weather and it has happened many times. However, for a cold and snowy winter it takes more than one factor. The +PNA in itself can and has brought some winter events, but for a memorable winter it needs more than one factor to occur. If the +PNA in conjunction with a -EPO occur together then this is a big flashing light to expect some type of winter precipitation across the deep south. If you can get the +PNA, -NAO and -EPO all at one time for several days then the Eastern US will get a major winter storm 9 out of 10 times.
Another piece of the puzzle that appears to influence the weather this winter will be the presence of a -EPO or Negative East Pacific Oscillation. This is when a ridge of high pressure and warm air aloft occurs over the Northeast Pacific Ocean and Alaska. This favors a warmer Alaska but can lead to cross polar flow, which is when the coldest air is tapped from Siberia and driven due south and then east. This was what happened in early 2014 and was responsible for some widespread record cold temperatures. If the -EPO is too strong then it can suppress the winter precipitation further south across Georgia and even Florida. The term “too cold to snow” is somewhat right, because if the air is too cold then it moves all the precipitation further south. Also, you have the dreaded “not reaching the ground” phenomenon, when it is snowing up above but not making it to the ground.
Our analog years are shown below and feature years where a La Nina was preceded by an EL Nino that was centrally based or referred to as a Modoki EL Nino. All these years were characterized by colder than normal conditions and above normal precipitation.
The one thing that seems certain to me is the better than normal chance for below to well below normal temperatures. Keep in mind that we do not need very cold air to get snow and or wintry weather. With the warm winters of the past several years, these below normal temperatures will be a dagger to most people. The coldest period appears to be early to mid-January and early February to early March. However, I do believe that December will turn colder toward mid-month. The maps below show temperatures expected from analog years for December 2018 through March 2019.
We need to look at several factors when trying to determine how much snowfall is predicted to occur. While it appears, we have a better chance this coming winter season for the Mid-South to achieve above normal to well above normal snowfall, all these factors explained above must come to pass. I do favor above normal snowfall for most areas. Snowfall appears to be well above normal in the Blue Ridge of NC and some will be wanting the winter to end early. I want to stress that the Blue Ridge will be the hardest hit relative to normal. The important thing to remember is it only takes “one” big snow to go above normal, but I am expecting at least a half dozen systems to affect the Southeast this coming winter season.
Winter forecasts are made up of several different aspects and it is kind of dependent on many factors. However, if you can get the right timing then it is still possible to have a major winter storm in the right conditions. You must look at all the factors involved and understand that if one does not line up with another then more than likely it will not produce winter precipitation. IF, you are only going to look at one factor then your forecasts will be flawed anyway. What I favor is for all three (+PNA, -EPO, -NAO) of these indexes to line up many times through the winter. With that in mind, expect this winter to produce between 150-200% above normal snowfall in most of the Southeast, with some areas seeing as much as 300% above normal (Refer to map above). To get the % you will simply take the normal snowfall amount and multiply it by the % amount. Some areas in the NC Mountains could end up with 85″ + of snowfall at season end. This will make the ski resorts very happy. The only exception of snowfall will be extreme South Georgia and Florida, where it will be closer to normal. However, I expect for Northern and potentially even Central Florida to see some type of winter precipitation before the winter season ends. See the custom map at the very bottom for my thoughts on the 2018-2019 winter.
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