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    High-Amplitude Pattern Leading to Some Extremes

    10/11/2013, 8:03:13 AM

    Friday, 11:40 a.m.

    The North Atlantic Oscillation, or NAO for short, is in the process of tanking. Here's the latest chart of the NAO:

    This often leads to higher-than-average heights over the North Atlantic, particularly over or near Greenland. Indeed, that is going to be the case this weekend and into the beginning of next week. Just look at the Northern Hemispheric pattern being forecast by the 0z Oct. 11 GFS ensembles for Monday evening:

    In the short term, the building of the upper-level ridge over top of the storm off the East coast has left it devoid of any kind of steering guidance. It has crawled northward from east of Hatteras over the past 24 to 36 hours and has pretty much come about as far north as it can. Now, as storms go, this one is hardly potent in terms of the central pressure. However, when you combine the storm itself with the high that's been to its north over New England and its adjacent coastal waters to the east, a strong pressure gradient has been present, one that has generated very gusty winds along the coast from the Carolinas on north. The onshore flow has also directed warm, moist air inland from off the Gulf stream, and the very slow movement of these rain bands has led to pockets of very heavy rainfall and flooding. Overnight and this morning, that heavy rain has come across the Delmarva Peninsula and southern New Jersey into northern Virginia, Maryland and Pennsylvania. That has led to a expansive area of excessive rain and some flooding, with a few rainfall totals now over 7 inches in southeastern Pennsylvania!

    It's a relatively small area that's being impacted by this storm, but the impact is great. Just a week ago, we saw the same thing with the storm cutting across the Rockies to the northern Plains, with very heavy, wet snow burying parts of Colorado, Wyoming, Montana, Nebraska and the western Dakotas. A similar looking storm is now slicing across the Dakotas, as depicted here by the 12z Oct. 11 NAM surface forecast:

    As it moved across Colorado overnight, it generated winds that gusted as high as 80 miles an hour in the Front Range, causing some power outages. Potent winds will lash the Dakotas the rest of today as the storm passes, with mostly rain falling around the storm.

    Despite the strength of this storm, it will not be able to pull a cold front across the Plains all the way to the East Coast, as you might typically expect. Because of the high amplitude pattern that we find ourselves in these days, the storm will be forced to go around the downstream ridge that is in place over the Northeast. Look at the 500mb forecast for Sunday morning:

    This, this storm is destined for the shores of Hudson Bay, with a cold front that will limp across the Great Lakes and Ohio Valley tomorrow night before falling apart on Sunday.

    We will not have to wait long for the next storm to come along. From the image above, you can already see the next upper-level feature driving toward the southern Rockies Sunday morning, with low pressure to eventually re-form over the eastern Rockies Sunday night into Monday morning. This time around, the storm will have more moisture to tap into, and the storm will head for the western Great Lakes at a slower pace.

    With more moisture to tap into, there will be a much larger precipitation shield associated with it, stretching from the southern Plains to the northern Plains and Midwest. That means a potential for severe thunderstorms in parts of the Plains and Mississippi Valley, not a common occurrence in October. On the cold side of the storm, it will likely cold enough to produce snow in parts of Wyoming, Montana and the Dakotas. It won't be nearly as prolific of a snow maker as the storm a week ago, but snow is likely.

    Once the storm exits, a broad push of much cooler air is poised to sweep across the country from the Rockies to the Appalachians later next week, reaching the East Coast late in the week and to start next weekend. Few, if any, record highs or lows are apt to be set in this pattern, but there will be large areas of unusual warmth ahead of the storm next week, and potentially even larger areas of chill behind it.

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


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