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    Joe Lundberg

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    Sluggish Upper-Level Low to Deliver Plenty of Rain

    May 9, 2012; 10:32 AM ET

    Wednesday, 11:50 a.m.

    It's a rather innocent-looking feature, but it is one separate from the main flow of the jet stream, and it is not going to be in any hurry to move over the coming days. However, it will have an impact on the weather over a lot of places, much of it resulting in beneficial rain. Here's what the morning water vapor imagery reveals:

    Ahead of it is a much weaker, upper-level feature crossing central and eastern Texas into Louisiana. It is tagging onto the back end of a cold front, and it will temporarily have the effect of drying out the atmosphere from central and eastern Texas over to the Southeast in the next 24 hours. By tomorrow morning, high pressure nosing down from the middle Mississippi Valley will work with a west-northwest flow aloft to foster a nice day Thursday.

    But as all of that transpires, the upper-level low will be rolling eastward across northern Mexico. The latest NAM 500mb forecast for tomorrow afternoon:

    Through tonight, there won't be a lot happening over Texas, weather wise. But as the upper-level low rolls on east, the air aloft will cool, while at the same time tomorrow, it will be getting warmer in the low levels with any kind of sun. In addition to that, winds out of the east and southeast will add more moisture to the low levels of the atmosphere. Add all of this up, and you'll get a rapid expansion of rain and thunderstorms.

    Most of the rain will be limited to Texas, then downstream over Louisiana and Mississippi later Friday and Friday night into Saturday. This feature will be slow to advance farther east, though, basically because of all of the things happening downstream over the Atlantic. One is a strong ridge over the central Atlantic. West of there will be the trough coming through the Northeast tomorrow into Friday morning. Along the East Coast by Saturday, there will be an upper-level ridge, strong enough and with no place to really go to slow the eastward advance of that feature back over southeastern Texas and Louisiana:

    To the north, two other features come into play. One is an upper-level trough coming through the Midwest on Saturday, and another back over the Northwest or northern Rockies. The first of those will send a cold front through the Midwest and Great Lakes Saturday, but it will run into much resistance on Sunday, eventually causing the front to stall. So, with no cooling expected over the mid-Atlantic and the Southeast from this weekend into early next week, the table is set for a lot of wet weather as the second upper-level disturbance moves southeastward and more or less falls into the hole that is dug by the upper level low coming into the South later this weekend.

    Now whether or not this causes the upper-level low to reform or not, as the GFS suggests, remains to be seen:

    The thinking is that this will lead to at least a couple of days of wet weather across the South and into the Southeast, then up the Eastern Seaboard to at least the mid-Atlantic next week. It's the time of the year when you the jet stream is in the process of retreating, so these stronger upper-level features that become disengaged from the main flow of the jet stream tend to move along much more slowly. That, in turn, can lead to an extended period of weather that features plenty of clouds along with some showers and thunderstorms.

    If there's an upside to this, it's that much of these areas are coming out of an extended period of dry weather. So a couple of days of rainy weather would overall be considered a good thing, especially if it can be cleared up by the Memorial Day weekend, which is still two and-a-half weeks away.

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


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