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    Joe Lundberg

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    Mild Week of Thanksgiving

    November 16, 2012; 10:15 AM ET

    Friday, 11 a.m.

    We're now in the second half of November. Hard to believe that the winter solstice is but five weeks away! Just that long until we start seeing the days getting longer, which is a good thing in my mind. Only about three weeks and change until the earliest sunsets across the mid-latitudes of the Northern Hemisphere, then they'll start getting later, slowly but surely.

    Yet, as you all know, the coldest days are typically much later. It's called the seasonal lag effect. The lowest sun angles do not correspond directly with the lowest average temperatures, as it takes an extra month for the Northern Hemisphere to lose as much of its heat as it is going to lose. In fact, that lag effect is more pronounced over the oceans, where the lowest temperatures are typically found another month later, or in late February into early March.

    The first half of November has been cool in most areas east of the Mississippi. It's also been cold in a good chunk of the Dakotas for the month-to-date. Snow cover in the U.S. is lagging behind, but there is extensive snow cover just to the north in Canada:

    For the better part of the next week, it's hard to imagine the snow cover in the Lower 48 being any greater. There's just no cold air around, as it remains bottled up in Canada for the time being. And so long as there is no blocking downstream, and there's a split in the jet stream, that will remain the case. Here's the latest look at the North Atlantic Oscillation (NAO) going forward:

    While not conclusive, the forecast does imply some sort of blocking tries to return to the fold late this month. And here's a snapshot of the jet stream from the latest NAM forecast for Monday evening:

    The main branch of the jet stream is pretty much across the southern part of Canada, keeping the true arctic air north of it. The weaker southern branch is carrying some upper-level disturbances along, with the net effect of these features to keep it cool in the Southeast through at least the beginning of next week.

    That leads to a period of balmy weather for most of the country deep into next week. To that end, here's the GFS ensemble 7-day means for next week:

    For what it's worth, the latest European weekly forecasts are on the same page.

    If you break it down into individual days, there will be a couple of candidates for record-challenging warmth, especially where there is no snow cover. Pretty much any day from Monday to Thanksgiving Day could be in that neighborhood across parts of the northern and eastern Rockies and across the northern Plains into the Midwest.

    The Southeast and mid-Atlantic will be much slower to see the warmth because of the positioning of the surface high. With it over northern New England, and those rather weak surface lows developing and moving away from the Southeast coast on a couple of occasions, the winds will remain out of an easterly direction, keeping a decided marine influence on the air masses. That will naturally slow the rate of warming, and that's not likely to change much before Thanksgiving.

    Looming over the horizon, though, is that arctic air over Alaska and the Yukon Territory and western Canada. It's not going away any time soon, and there some indications it may want to move south and east toward month's end and into December. The following is a chart from the 6z GFS ensembles showing the forecast temperature anomalies for Saturday, Dec. 1:

    There are several members that just look brutal. Granted, there are many that are still very warm and keep the cold well to the north, if not shrink the cold. One of the things that will have to change to get that kind of cold that some of the members are showing is the snow cover map. And that's very unlikely over the next 7 to 10 days. If it happens, it's going to have to happen the last few days of the month. But sooner or later, I feel pretty confident that at least some of that arctic air will be unleashed on parts of the Lower 48.

    The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com


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