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    Fading Fronts

    10/01/2013, 9:00:08 AM

    Tuesday, 11:35 a.m.

    A cold front came charging across the Plains into the Great Lakes and Ohio Valley this past weekend. Ahead of the front, Chicago climbed to 84 on Saturday, while behind the front, it reached only 68 on Sunday. Other cities such as Des Moines and Minneapolis had a similar temperature drop from the warm side of the front Friday to the cool side of it Saturday. The farther east you go across the Great Lakes and Ohio Valley, though, the smaller and smaller that change became to the point of there being no change whatsoever from New York and Pennsylvania on east. Quite simply, the front faded then finally fell apart.

    Now why did that happen? Let's look back at yesterday morning. Here's the GFS ensemble package from the 12z Sept. 30 run, showing the heights and their anomalies at the initialization time:

    There was that pesky offshore storm, but it brought little actual moisture into southeastern New England or the mid-Atlantic coast. There is, however, a much stronger feature over the Northeast Atlantic, one that is contributing to blocking pattern in the Atlantic. Look at the NAO from the past into the near future:

    Normally when you see the NAO so negative, you'd figure on a lot of cool air moving across the country right into the East! But look again at the image again. Note WHERE the blocking happens to be - the higher than average heights run from parts of Ontario and New York state into the far northwest Atlantic. Quite typically the blocking talked about that brings the chill into the East is more centered over Greenland, but this is farther west. In addition, we're not in the heart of the winter season yet, so the same atmospheric setup that brings the chill to the East in the winter season can mean an entirely different thing at the beginning of October.

    What about the next front? It's cruising across the Midwest now, but as fronts go, it's even weaker than the one that fell apart coming into the Appalachians Sunday and Sunday night. A piece of the front will glide across the Great Lakes tonight then across the Northeast tomorrow. There will be a one- or two-day period of cooling across the Great Lakes into New York and New England, but not even close to getting below normal in any location. The front will have virtually no impact on areas near and particularly south of I-70, if not I-80.

    Still another front will come through the northern Rockies tonight and into the northern Plains tomorrow. Here's the 12z Oct. 1 NAM surface forecast for tomorrow morning:

    This front won't even get as far as the central Great Lakes before it comes to a grinding halt, as still another wave of low pressure comes out of the Rockies Thursday afternoon and night and starts pulling warmer and more humid air back onto the Midwest from points south. With the upper-level ridge rebuilding farther west late this week, the jet stream will buckle farther west over the Rockies. Here's the 0z Oct. 1 GFS ensemble package for Friday evening:

    A much deeper storm will head for the Midwest, and with the ridging downstream, the cold front will start to slow down this weekend. What looked a few days ago like a frontal passage Saturday night or Sunday in the East now may be a slowing front coming into the East Sunday night and Monday. And given the expected tenacity of that offshore upper-level ridge, this front will have minimal cooling behind it for the mid-Atlantic and New England before it starts warming again later next week.

    So many fronts, trying to attack a nearly impregnable defense. And it all means warmth into mid-month for most of the Northeast and mid-Atlantic states.

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


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