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    Progressive Pattern to Limit Depth and Duration of Cold

    11/01/2013, 8:51:48 AM

    Friday, 11:45 a.m.

    If you're a regular reader, then you're probably well versed in some of the meteorological terminology that is often used to describe the weather, or weather patterns, or climate, or anything related to the weather. One of those terms is called the North Atlantic Oscillation, and it attempts to measure the relative strengths or weaknesses of certain features in the Atlantic Basin, namely, the Icelandic low and the Azores high.

    These two features are regular, or semi-permanent. In other words, they tend to be there in one form or another pretty frequently. When the Azores high and/or the Icelandic low are strong, the North Atlantic Oscillation, or NAO for short, is said to be positive. When the opposite is observed, and the Icelandic low is either weak or completely absent, and/or the Azores high is weaker than usual, the NAO is said to be negative.

    Each phase of the NAO brings with it certain characteristics. For instance, when the NAO is positive, we typically find a pretty strong jet stream flowing in one form or another off the North American continent and across the North Atlantic. That is considered a progressive pattern, allowing features to move from west-to-east with regularity. In contrast, when the NAO is negative, the jet stream tends to have a lot more north and south undulations. At times, upper-level ridges will form in the place of the Icelandic low, and that forces the jet stream to buckle around it. When that happens, colder air from the high latitudes of the Northern Hemisphere is typically driven farther south that usual, leading to arctic outbreaks. It can also mean the potential for significant snows in certain situations.

    Right now, the NAO has trended positive after being negative for much of the past five weeks:

    What lies ahead? Well, if the forecasts are to be believed, then the NAO will remain positive over the next two weeks:

    What does that mean? First and foremost, it suggests that the jet stream ought to be pretty strong flowing away from North America and across the North Atlantic. And that would imply any cold air masses would not have any staying power. Let's unpack that as it relates to the next week across the country, particularly from the Plains on east.

    We know that a strong cold front is marching over the Appalachians into New England and the mid-Atlantic states right now. Behind it, the air has turned cooler, with the coldest air back across the Plains. We also know that a second upper-level disturbance is rotating southeastward across the northern Plains toward the Mississippi Valley, and it will carve out a deep upper-level trough tonight that will reach maturity tomorrow. Here's the 12z Nov. 1 NAM model forecast of 500mb heights tomorrow afternoon:

    Notice that out to the east over the central Atlantic there is an upper-level high in the central Atlantic east of Bermuda. Its presence has forced the lead trough, the one yanking the cold front into the East today, to go way around the high, rocketing toward southern Greenland. In back of the trough is a decent ridge poking up through the eastern Rockies.

    How does this change going forward? Look at the same model, only for Monday evening:

    The trough in the East Saturday afternoon is gone in just over two days, replaced by a building upper-level ridge! To put that into perspective, look at it from the perspective of normal, as viewed by the GFS ensembles:

    Now it will take a little while for the cold air, once it digs its heels in over the weekend, to get dislodged. With surface high pressure settling into the Northeast Sunday night and Monday, it will drain low-level, cold, dry air southward along the Eastern Seaboard up against the Appalachians. Even though the air aloft will quickly moderate, the temperature inversion will be tough to break with the sun angles getting so low as we move into 'solar winter' (the quarter of the year with the lowest sun angles.

    That said, the Plains states will quickly moderate this weekend, especially Sunday afternoon underneath the budding ridge. That warmer air will come east across the Mississippi Valley early next week and should warm things back above normal in the Ohio and Tennessee valleys by Tuesday. That means a three-day run there with temperatures below normal, and that's in. The warming will wait until Wednesday to take hold in the East, but the same will be true there - three days, Sunday through Tuesday, with temperatures below average, then temperatures rebound.

    If I were to extend that discussion out even farther in time, I would talk about how that deepening trough in the West early next week, as seen in the two previous images, will come across the country, but lift up and around the downstream ridge over the East later next week. That means the chilly air mass drilling into the West Sunday and Monday will not fully come to the East late next week.

    In keeping with the progressive nature of the pattern, though, the front will pull another strong cold front across the country, reaching the East Thursday. It will again become a moisture-laden system, one that could easily generate more severe weather as well as heavy rain. There might even be an area of snow on the northwest flank of the storm, though that won't be a widespread feature of the storm. The main thing will be the warming ahead of it, along with the rain and wind and thunderstorms, then a moderating cooldown behind it.

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


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