The dominant atmospheric mode of late has limited individual spells of unseasonably cold weather in the United States to at most a few days. The westerly jet stream continues to freely swirl around the high latitudes of the western hemisphere without a blocking pattern in its way (to shunt it south), allowing weather systems to easily move from west to east across the country without lingering in any one particular location for very long.
My post just last Tuesday included weather maps that displayed fierce winter chill over much of the central United States, with temperatures 10 to 20°F below average from the Dakotas all the way to the Gulf Coast and snow in Texas.
In contrast, we are now staring down forecast panels that show 10-20°F above average temperatures in those same places as soon as tomorrow, including the risk for midweek thunderstorms over the Plains.
With a sustained polar-air delivery unlikely anytime soon, a relatively mild weather regime (by late December standards) is expected to remain entrenched across much of the Lower 48 for the next one to two weeks at least.
To be clear, it’s not as though pieces of cold Canadian air won’t occasionally visit the United States in the coming weeks. They probably will. It’s just that they won’t last. This is nothing new, of course.
Just last week, it was ice cold across parts of the Northern Plains, with below zero readings in Montana, the Dakotas, and even Nebraska. But now, that arctic air is long gone. High temperatures will rebound to the 60s tomorrow in Kansas City and to the 50s in the same Nebraska locales that were subzero last Friday. And even the not-as-cold weather in the Northeast over the past few days has already begun to turn around. Those 40s over the weekend in the D.C. area will probably be briefly replaced by 60°F weather on Thursday.
There are signs that as the month rolls on, the peaks in the temperature swings will be warmer than the valleys are cold (relative to climatology) for much of the country. This won’t be particularly unfamiliar to many places in the Northeast and mid-Atlantic, given they have routinely seen remarkably warm bursts in the past month. But it is somewhat of a departure for the middle of the country, which has observed short episodes of really cold weather during the last several weeks.
One of the most likely exceptions to the mild pattern may be in the Southwest, where a barrage of upper troughs associated with cooler Pacific air will continue find a semi-permanent residence there and keep the temperatures below average (but not exceptionally cold) in the weekly mean. But, overall, there is little evidence to suggest an arctic outbreak is coming
This generally mild outlook for the next week to 10 days (at least) owes some of its existence to the state of the tropical atmosphere. While it may seem implausible that our North American weather patterns are intimately tied to those so far away, in reality, the weather in that region of the globe is quite capable of triggering the rearrangement of the jet stream worldwide.
During the next ten days or so, changes in the tropics will try to nudge much of the Nation toward a mild regime; one that is not unlike that shown in the image to the right (where oranges and reds indicate warmer-than-average temperatures, and the blue shading likewise denotes cooler conditions). To be sure, this is in no way a complete picture of what to expect, and actually includes factors not necessarily in place right now. But this map does uncover part of the medium-range forecast uncertainty – in particular, that associated with the way some of the tropical weather systems want to push our pattern.