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    Elliot Abrams

    Changes on the Way

    By Elliot Abrams, AccuWeather chief meteorologist
    11/05/2013, 6:49:15 AM

    Tuesday 9 a.m.

    The high pressure area that brought cold, dry air into the Northeast is now shifting offshore, and a southerly flow of milder air is advancing eastward behind it. You may wonder how an area of southerly flow moves east. In one case, we are talking about the wind direction: a southerly flow blows from south to north. However, weather systems move along in response to changes in the three-dimensional atmosphere. With a circular high pressure area, you can find each wind direction on the compass by examining the flow around the high. However, the high pressure area itself is moving (and changing strength in response to various influences), so the wind direction at any one place will change as the high pressure area moves.

    This video outlines the forecast for the rest of the week.

    When the next cold front goes by, it will be followed by another shot of chilly air. Sunshine will take over east of the Appalachians. However, downwind from the eastern Great Lakes and into the mountains, there may be a narrow window of opportunity for snow showers to break out. This panel from the morning run of the the nam-wrf model shows when that might happen:


    590x435_11051529_nam2013-11-05_10-24-05


    Behind most cold fronts in autumn, the air at and near the ground is often warmer than it would be in winter. This is especially true downwind from the Great Lakes. So, even though the 543 thickness line might mark the snow rain line in a heavy winter precipitation situation, it may be too warm for snow showers until you reach around the 528 line.

    When I talk about the 543 line or 528 line, I am referring to the 1000mb to 500mb thickness lines we see on the prog charts (dashed lines). The zero is dropped at the end of both values. Therefore, the line marked 543 means the value is 5430 meters. Thickness is the distance between two pressure surfaces, in this case the distance from where the pressure is 1000 mb (near the ground) and where it is 500 mb (around 18,000 feet up).

    Cold air contracts and warm air expands, so thicknesses are lowest when the average temperature in the layer are coldest. However, for determining the snow-rain line, knowing the average temperature is not enough. If it is very cold at the 500 mb level, it may be mild at the 1000 mb level.

    Some meteorologists use the 1000-850 thickness map to help them identify the snow rain line (which is often near the 300 line or 1300 meter line). The 1000-850 chart is only dealing with the part of the atmosphere between the ground and an elevation around 5000 feet. This relatively low layer of air is often a better indicator of where snow may reach the ground than the 1000-500 layer. In winter, it is always below 0 degrees C at the 500 mb level, but we cannot say the same thing about the 850 or 1000 mb levels. You can create a 1000-850 thickness prog on the accuweather.com pro site. Here is the map for the same time as in the 1000-500 map above. Note the 1300 (blue) line roughly matches the 528 line.

    590x424_11051601_screen-shot-2013-11-05-at-10

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

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