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    Deep Trough Doesn't Necessarily Mean Bitter Cold

    1/13/2014, 7:18:26 AM

    Monday, 11:55 a.m.

    We have now escaped the clutches of the dreaded Polar Vortex. It was brutal last week, with snow, bitter cold, new record lows, dangerous wind chills - in short, a visit of North Pole weather conditions to the Lower 48. In the vein of some brilliant mind back in 1980 when I passed a physics class (the class dealt with electricity and some other hard stuff I didn't understand - and still don't - and our recitation teacher decided to print t-shirts that had this emblazoned on the front: "I SURVIVED PHYSICS 202"), I think I should print shirts that read: "I SURVIVED THE POLAR VORTEX OF 2014." I could make a small fortune!

    Seriously, that air mass was brutal. One of the reasons why it was so cold is that the source region was frigid. The amplitude of the pattern was such that we at times had cross polar flow, able to drain bitter air from Siberia over the pole and across northern Canada down into the Prairie Provinces then into Plains and Midwest. The upper-level low that represented the coldest air in most of North America rolled by the Great Lakes on Tuesday, and with the source region so cold and a decent snow pack, it was pretty obvious why the bitter cold got as far south as it did!

    Another upper-level trough is about to descend upon the East. Before it reaches full maturity at midweek, it will help push rain across the South this afternoon and up the Eastern Seaboard tonight and tomorrow. And it will largely be rain, too, with maybe some snow on the northwest fringes of the system in the deep interior of the Northeast. Look at the 12z Jan. 13 NAM 500mb forecast for tomorrow morning:

    You can see the disturbance moving into the Southeast at that time, with the rain moving off the East Coast later tomorrow. Note also the next upstream upper-level trough crossing the Plains and Midwest tomorrow morning. There will be a lot less moisture for this feature to work with, but it will squeeze whatever moisture is available out of the atmosphere tonight and tomorrow in the form of snow. Several inches may accumulate in parts of Wisconsin and northern Michigan, with less farther south and west.

    The air behind the first system coming through the East tonight and tomorrow is not cold. Behind the second one it is cold, as a piece of arctic air does get involved. But if you look at the weather map, there's no big surface high, and that's a big difference between what we saw last week and in the weeks before that. Here's the NAM surface map for Wednesday morning:

    You can see from that same image there's another potent system coming over the top of the western ridge into central Canada. Another way to view that is through the GFS ensemble forecasts of 500mb heights and anomalies for Wednesday evening:

    The trough in the East is deep late Wednesday, and another deep trough is about ready to come into the picture. Here's the impact of that on the heights by the end of the day Friday:

    Whatever surge of milder air that comes across the Plains toward the Midwest and Ohio Valley gets erased by the passage of the next cold front, with another surge of very cold air behind it. Here's what the 12z Jan. 13 GFS model 850mb temperature forecast looks like for Friday afternoon:

    -10C to -15C air, but not the -25C to -30C air or lower that we saw a week ago, Furthermore, there is the same kind of surface high coming down behind the front that has the arctic air with it. Colder, yes, but not 20 to 30 degrees below normal kind of cold late this week and this weekend.

    Going into next week, the mean upper-level trough still hangs out in the East in the means. That certainly means it won't be warm. However, how cold will it be? The problem with going overboard on too much cool is that the source region may not be cold at all - especially over western Canada. Indeed, if you look at the Canadian model 10-day temperature anomaly forecast, there's just not much cold air around:

    The heights may well be low, but if you're pulling warmer-than-normal air into the bottom of the trough, rather than bitterly cold air, the results just won't be the same. Therefore, I'd be leery at this juncture of any forecast calling for a return of the polar vortex!

    On a bright note, "Pitchers And Catchers Report" - at least for 28 Major League Baseball Teams - is a sign for spring training camps to open up, and they do so in one month! For Arizona and for Los Angeles, the reporting dates are even earlier because of their trip to Japan to open up the season. In addition, we're now at the bottom of the curve in terms of average temperatures. They'll stay there for the next couple of weeks before they begin their inexorable rise toward summer over the next six months.

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


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