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Fleeting Chill

January 02, 2013; 10:30 AM

Wednesday, 11:20 a.m.

New Year's Day started bitterly cold in the Midwest, particularly in parts of North Dakota, Minnesota and Wisconsin. One of the coldest spots was Embarrass, Minn., at 24 below zero! A piece of that arctic air mass has moved eastward across Ontario into Quebec and now covers northern New England. Here's the early morning low temperature chart:

Here's the morning pressure and thickness analysis:

Now look at the 850mb analysis:

That's where the true arctic air is this morning. To the south, it's cold, yes, but not out of the bounds of what you would typically expect to see in early January. Still, it's going to lead to a widespread area of the country with temperatures below average today and again tomorrow. The 6z GFS ensemble temperature anomaly forecast for tomorrow:

If you step back from the pattern, though, this is not one that favors long-term cold in the country, especially in the Plains and across the South into the East. You can really make a strong argument for the fact that much of the cold we see now from the central and especially southern Plains and across the South is a result of two main things: 1) a large surface high promoting a strong temperature inversion that inhibits mixing and 2) a massive plume of moisture streaking out of the eastern Pacific and caught in the subtropical branch of the jet stream.

Look at the infrared satellite image from this morning:

That is a huge area of moisture streaming out of the eastern Pacific across Mexico into Texas. In the wake of the cold front that moved off the Gulf coast, the air mass is chilly. Not bitter, but if there's no sunshine and it's raining, temperatures by day will be well below normal, and any cooling at night will only lead to the day as a whole being below average. Really, the biggest departures with respect to normal will be in the southern half of the country, the New England arctic attack notwithstanding.

With the arctic attack really limited to the Northeast, it's only a matter of time before the chill in the pattern is replaced. As mentioned above, temperature inversions are a big part of the problem this week. I've cherry-picked a model forecast sounding for Indianapolis, Ind., for Friday afternoon to illustrate the point:

The vertical line on the left is the forecast dew point, while the one on the right is the forecast temperature with height. Note that in the low levels, the two are relatively close, then they go in opposite directions at about 920mb. That's where the temperature inversion is found, and it is associated with sinking air. With such low sun angles, any place near the high won't have a lot of wind, and with less than 10 hours of daylight, the mixing that does occur is insufficient to break through that inversion and mix the milder air aloft down to the surface. This effect is enhanced over snow-covered ground, which currently includes a lot of places from the mid-Mississippi Valley to New England!

Eventually, though, the high moves on, and the low levels will begin to warm. In fact, now that the arctic air is being drained, more and more milder-than-normal air will move down into the northern Rockies, northern Plains and Midwest from central Canada. We'll end up warming things from the top down, not from the bottom up. Look at the projected anomalies for Monday:

Yeah, it's cold now, but it's not a stormy pattern. Once the split in the jet stream fades within the next week, the chill being kept in place across the South will fade, while at the same time more and more mild air will flow into the northern tier of states from central and western Canada, making it a fleeting chill in the pattern.

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

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