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    Fast Jet Stream Making for an Active 48 Hours

    12/05/2013, 7:49:13 AM

    Thursday, 11:45 a.m.

    To get started today, let me lay out before you three images to give you somewhat more of a visual as to what is happening in the atmosphere. The first of the three is the late morning, color-enhanced water vapor image:

    The three things I want to point out are as follows: 1) the solid area of moisture over northern Ontario and the southern half of Hudson Bay. That's the main storm that is pulling a cold front through the eastern Great Lakes and eastern Ohio Valley right now. 2) the area of moisture stretching from the mid-Mississippi to the central Appalachians. Here we find an upper-level disturbance that is beginning to enhance precipitation with and ahead of the hard-charging cold front, and it's resulting in rare December thunderstorms! Some of these stretch back to Arkansas at this hour! and 3) the much larger area of moisture over the southern Plains and southeastern Rockies. It is this feature that will cause more rain,ice and thunderstorms to break out in earnest over central and North Texas and Oklahoma into southeastern Kansas tonight, a feature that will stream northeastward across the Tennessee and Ohio valleys tomorrow then finally out through the mid-Atlantic and southern New England later tomorrow and tomorrow night.

    That's the 12z Dec. 5 NAM 500mb forecast for 06z tonight, showing the upper-level low associated with the main storm over James Bay, the upper-level trough axis back over the southern Rockies, the upper-level ridge over the southeastern Gulf of Mexico and Florida, ad the broad and fast southwest flow aloft from the southern Plains to New England.

    Finally, the NAM surface forecast for 12z Friday morning, showing the precipitation from the preceding six-hour period:

    Note the two distinct areas of more concentrated precipitation - one over the Northeast with a weak but fast-moving, upper-level disturbance and the rapidly growing area of precipitation over the southern Plains, Mississippi Valley and into the Tennessee and Ohio valleys.

    With the main storm already way to the north of the Great Lakes, the cold air is sweeping in behind the cold front across the Great Lakes and Ohio Valley. At the same time, there is a nice surge of warm air up into the eastern Ohio Valley and mid-Atlantic states, and this clashing of air masses is leading to some thunderstorms of all things in western Pennsylvania southwestward into parts of Kentucky at this hour! That's a highly unusual occurrence for this time of the year, and it shows you how different these two air masses happen to be!

    As the feature coming out of the southern Plains and mid-Mississippi Valley streaks northeastward this afternoon, there will be some additional rain with embedded thunderstorms as the cold front presses east and southeast into the warm air. The northwestern edge of the precipitation shield has yielded some freezing rain and sleet across southeastern Missouri, and there will likely be additional icing issues this afternoon into tonight from southern Illinois into central and northern Ohio, then across northwestern Pennsylvania into parts of upstate New York and maybe northern New England before exiting early tomorrow.

    This feature will push the front off the New England coast and down into the mid-Atlantic states, so colder air will bleed southward behind it, even as the second trough coming out of the southern Rockies and southern Plains fully develops an area of low pressure. By day's end tomorrow, that low will be crossing the Appalachians, with the bulk of the precipitatiion north of the front, and north and west of the track of the storm. Here's the Friday evening NAM surface forecast:

    A lot of this will still be in the form of rain, but, as the colder air digs in deeper and deeper in the low levels of the atmosphere, the rain will begin to freeze on contact with surfaces once surface temperatures slip below freezing. That's almost a given tonight from central and northern Texas into the Ozarks, then into northern and western Tennessee and Kentucky late tonight and tomorrow, even up into central Pennsylvania tomorrow afternoon and interior New England tomorrow night. Eventually the cold air will get deep enough that the rain freezes into sleet pellets. Finally, as the storm moves by, the cold air will get so deep that the entire column is below freezing. At that point, the freezing rain and sleet will change to snow, and a few inches can accumulate in some locations before it dries behind the fast-moving storm.

    The one thing that will hopefully prevent this from becoming a crippling ice storm is the fast movement of the wave. It should keep most places from getting more than an inch of ice. Most, but not all. One of the areas of greatest concern for a damaging ice storm is northern Arkansas and extreme northeastern Texas:

    This will all dart off the New England and mid-Atlantic coast quickly Saturday morning, bringing all forms of precipitation to a swift end. That will allow time Saturday to clean up the mess, clear the driveways and sidewalks and get a leg up on getting any lost power restored. It will have to be quick, though, as the next trough will cone through the southern Plains later Saturday, inducing more rain and ice to develop Saturday night in the Tennessee Valley, and that will spread northeastward across parts of the Ohio Valley and mid-Atlantic states on Sunday as snow and ice.

    Are you tired of winter yet??!!!

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


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