Long-Range Ideas Changing

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If you keep up with long-range model guidance, operational runs of the global models over the past 24 hours have shown a wild change in looks for the next two weeks. They have flipped from the bitterly cold Arctic influence in the South to just a “normal” cold weather pattern for this part of the country. At this point, it is hard to say if it the solutions painted by these operational runs will stay the same or if they will flip back the way they were.

When looking at trends in the long-range forecast, ensembles and global atmospheric signals like the NAO and PNA are less prone to run-to-run flips and flops. Here’s a comparison of ensemble forecasts from Monday to today. The blue bars represent the probability of the highest temperature in a 6-hour period staying below 35┬║ F. The arrows indicate the time frame where the normal highest temperature of the day would occur:

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The ensembles are trending toward a less-cold scenario, but they are not painting a total flip-flop.  Those other two factors are also suggesting the pattern change as well (negative NAO and negative PNA).

The main change in the pattern we have seen in the operational runs starting last night is the development of a split flow in the jet stream. This is the morning run of the European and American (GFS) models showing the forecast upper air pattern next Wednesday at noon:

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Notice how the two lines are separated over the west and flowing together in the east? This often leads to less cold air moving all the way south into Alabama (although Tennessee could still get a good shot of cold weather).

Again, it’s operational vs. ensemble. The operational is a single run of the model; the ensemble is more of an average of multiple runs of the model. If the operational continues to show the split flow out west, then the ensemble will likely come into line with it.

With this in mind, the NAEFS Ensemble is looking pretty good right now. Temperature will be getting back down to normal or just a little below normal by the middle of next week.

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So, Old Man Winter may bring the house…but he may not bring it as far south as initially expected.

This could end up being positive news if you’re hoping for snow, though. The kind of pattern suggested up until today would have been very cold and dry; this slightly warmer, wetter one opens up the door a little for some cold air and some moisture to get together in just the right mix. Right now, there is no definitive expectation of a winter storm on any particular day, but it’s all about the pattern that develops over the next two weeks. Sometimes they produce, and sometimes they do not.

Accurately forecasting a winter weather event more than 5 days in advance is tough. It can be done, but there is a huge margin of error. In other words, no snow for now, but if something comes along that gives some clear evidence of a threat, we will certainly show you the details.

-Jason
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