Joe Lundberg

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Plenty of Little Systems Before a Bigger Weekend Storm

December 16, 2013; 10:53 AM ET

Monday, 11:45 A.M.

I must admit that as I got a break last evening from my seasonal job out at the mall last evening and sat down to digest the weather, I was rather surprised by the changes I saw on the maps. And those changes are things that I have had a hard time reconciling ever since! The biggest change was the thrust of arctic air into the Rockies and Plains late this week and this weekend, or a greatly diminished version thereof, and how that impacts the pattern going forward to Christmas.

Late last week it looked pretty certain that a full-latitude trough was going to dig into the Rockies and pull bitterly cold air southward into it, sending temperatures into a downward spiral Thursday and Friday. That arctic air looked for all the world like it would then spread out and cover much of the country in the final days leading up to Christmas.

It doesn't look like that anymore! The question is, why? What has changed? Or, more importantly, why did it change? That I don't have a good answer for.

As I examine it in closer detail, it would appear the answer may lie in the splitting of the jet stream energy this week. While it looked like it would be one giant trough coming east late this week and this weekend, driving a cold front to the East Coast by Saturday, it now seems much more likely for the jet stream to split into two streams, one one diving south into the Southwest Wednesday and Thursday, while the northern branch speeds along across the U.S.-Canadian border.

Let's compare the upper-level charts of this evening from the 12z Dec. 16 NAM forecast to that of Thursday evening:

Remember that image, for we'll be referring to that again in a moment.

Now look at the same chart for Thursday evening:

Note that by this time, the upper-level low appears over Southern California, with the jet stream bending around it, coming back across Mexico and aiming northeastward from Texas to the mid-Atlantic coast. At the same time, the main branch of the jet stream runs southeastward across southern British Columbia to the mean trough position over the northern Rockies, then turns east-northeastward through the Dakotas and over to northern New England. You'll note also there are not too many "kinks" in those height lines, which implies there are no real strong features embedded within this fast jet stream flow.

If you extend that further out in time to Saturday evening, the 12z Dec. 16 GFS 500 mb forecast is suggesting this upper-level low buried over Southern California comes out fairly quickly into the southern Plains, but with a strong downstream ridge over the southwest Atlantic "gumming up" the works, so to speak:

At this point in time there will be a strong contrast in air masses from what arctic air does make it down the Plains and the warm, moist air charging northward from the Gulf of Mexico, and that should be plenty to fuel the development of a strong storm that is destined to head for the eastern Lakes.

Before we get to the big event, though, let's go back to the first image. There you'll find at least two separate features, one moving into the Dakotas, and another crossing the Midwest. Look at the moisture panel from 700 mb, and you'll see there are actually three features:

The first is a batch of clouds with limited snow moving out of the Ohio Valley and over the central Appalachians. Once this feature clears the mountains, there won't be much left of it. Instead, it will act more of a seed to the one right on its heels, a feature that has been generating snow in parts of the Midwest this morning. Of the three, this will end up being the strongest, or have the moist lift and moisture to work with, and when all is said and done, several inches of snow may accumulate in parts of Pennsylvania into New England. Here's our latest snow fall forecast:

There will be little separation between this second feature and one more right behind it, the third one seen in that moisture panel back over the Dakotas. There won't be a lot of snow with that, but there can be a fitful burst of snow that can bring a quick inch or two of snow to places along the track of the upper-level disturbance. Parts of Wisconsin may be able to eke out three or so inches, but, again, that will be the exception to the rule. This final disturbance will then race across the Great Lakes and Ohio Valley tomorrow, then through the mid-Atlantic tomorrow night. The amount of snow will be somewhat secondary to the suddenness of it, and what it can do to travel conditions. I would not expect the snow to last more than a few hours in any given location, and by Wednesday morning it will be gone.

After that, the cold should begin to ease as that upper-level ridge begins to form over the southwest Atlantic and southernmost Florida. There will be another upper-level disturbance that will come flying along in the northern branch of the jet stream. It will cause a relatively weak area of low pressure to move through the Lakes Thursday night and early Friday morning, then across upstate New York late Friday, and through northern New England Friday night. Ahead of the wave, warmer air will move into the Ohio Valley, then the mid-Atlantic and at least southern New England.

Behind the wave, there will be a bleeding of arctic air southward across the Midwest and Great Lakes into the Ohio Valley, then New England Friday night, only for the front to stall somewhere in the mid-Atlantic late Friday night and Saturday. That will set the stage for the bigger storm to come out of the Southwest and southern Rockies this weekend.

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

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Joe Lundberg
Joe Lundberg, a veteran AccuWeather.com forecaster and meteorologist, covers both short and long-term U.S. weather on this blog.