bitter cold, light snow

Well, 2017 is going out with a bang. The past two days have logged the coldest temps of the year, and Tuesday-Wednesday was the coldest two-day stretch in almost four years (Jan. ’14). We haven’t quite set any records -- yesterday’s high was the second-lowest ever recorded on December 27^th, and Tuesday’s high tied the record (1925, 1892) -- but this is still among the very coldest weather that Chicago experiences. To highlight how “buckled” the jet stream has been, it was considerably colder* in parts of the U.S. (-36 at Int’l Falls, -17 Lincoln, NE, -4 here) than it was north of the Arctic Circle. When 2018 dawns on Monday morning about 1/3 of the entire Lower 48 could be below zero. So yeah, this is a serious cold outbreak.

 

And it’s not done yet. After a reinforcing shot of cold this weekend there is another one in the middle of next week. By the end of next week there are signs of a pattern flip back to more seasonable temperatures. And while the current pattern may be reminiscent of the brutal 2014 winter, there are some key differences (snow cover, La Niña, etc.) that give relatively little confidence in this pattern persisting into mid-January and beyond. In other words, it wouldn’t be surprising if the winter overall ends up closer to average.

 

In the meantime, most of the country will be stuck in the deep freeze. See the map below for one model’s forecast on Monday (New Year’s Day) – below zero temps stretch all the way into Oklahoma with teens and 20s all the way into Texas and toward the Gulf Coast.

 

In Chicago we’re also going to have at least 2-3 periods of light snow. The good news is that none of these are major storms, and airport delays are likely to be minor at worst. But given the frigid temps even a tiny amount of moisture can fluff up into 1-2” of snow that sticks to the roads.** Round #1 will arrive this afternoon and evening, with a dusting to 1-2” of fluffy snow. Round #2 will include some lake-effect snow tomorrow afternoon and evening; 1-2” looks like a decent bet, with the chance for more in areas to the south and/or downwind of the lake. A third round (again with some lake-effect snow) is possible Saturday night into Sunday.

 

For travelers it is worth monitoring a storm here in the Tuesday/Wednesday/Thursday period. There is also some potential for a full-blown Nor’easter blizzard next week as well.

 

 

* Here is your friendly evergreen reminder that wind chill is bogus. Please never use, discuss, or cite wind chill as a number. Even worse is the conflation of the two, as you often see a sentence like “the temperate was xx degrees with the wind chill.” That is beyond dumb. It’s like saying, “I’m eight feet tall with my stilts, and oh by the way my stilts are magical variable-height stilts that constantly shape-shift and no one has ever been able to actually measure their dimensions but meh whatever close enough I’m eight feet tall.”

 

** A typical snow in Chicago has 1 inch of water for every 10 inches of snow. This snow will be so dry that the ratio could be ~30:1!

 

 

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Snow

Most of the area is under a WWA for some snow tomorrow. This would not normally be a big deal, but given that it's Christmas Eve there could be some travel disruptions (especially on the roads) that are worth noting. 

Snow will overspread from the south and west overnight toward dawn. Light snow is likely in most areas from sunrise through early/mid-afternoon. Again, this is NOT a strong storm. There is plenty of cold air in place -- so any snow that does fall will be light and fluffy -- but there is little moisture and little lift. I would take the under on most of the numbers I'm seeing, but far southern suburbs could see several inches (and lake-prone areas in NW Indiana could see a half foot or more). The city and most suburbs could see a dusting up to 2-3 inches; the low end would see flakes in the air but no accumulation, while the high end would certainly be enough to whiten the roads and slow down travel. The northern suburbs could see an inch or two but may well see no accumulation at all. 

Again, I would only be concerned if traveling into NW Indiana or SW Michigan (where a full Winter Storm Warning is in effect). Southbound drives on I-55/-57/-65 are also going to be slow. Within the Chicago area roads are going to be slower but unlikely to see major disruptions. Given the heavy volumes O'Hare and Midway might see some delays but major impacts are hard to imagine. 

As for the all-important White Christmas math...it's a tough call. South of I-55 I would say it's a good bet, starting at 60% and adding 10% for every 50 miles south. In the city it's about a coin flip -- there might not be much accumulation, and what does stick may be gone by Monday morning. The northern burbs are looking iffy at best. 

Beyond this little snow it will be cold but sunny on Christmas, with even colder weather into the middle part of the week. After that a potential storm on Thursday/Friday is worth monitoring. (Note that a legit snow storm before Sunday would be just enough to keep us from the all-time least snowy calendar year on record at Midway! Records there began in 1928-29.)

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Happy Solstice Eve! Mild then cold...and snowy? 2017 recap and snow drought

Happy Solstice Eve! The solstice occurs tomorrow (Dec. 21^st) at 10:28 AM local when the sun’s most direct rays reach their southern nadir and the North Pole is titled away from the sun at its greatest angle. Tomorrow is the “shortest” day of the year (even though we’ve already passed our earliest sunset about two weeks ago, and our latest sunrise is still to come). In the meantime, here is a quick update since I know this quiet and mild weather has given everyone withdrawal symptoms. And don’t worry! It looks like the pattern is about to flip so you’ll probably be able to count on a few more updates as we close 2017.

 

Speaking of 2017, it has been a remarkable weather year in Chicago in several ways. We opened the year with a record-breaking snow drought: after a snowy start to the season the snow shut off around this time in December 2016 and the January-February 2017 period was the first in recorded history to receive less than 1” of snowfall. The calendar-year-to-date total is officially <11” so last year’s snow drought has carried over into this season (at least so far).* 2017 also saw a long string of above-average-temperature months (10, in most locations) and severe flooding in July (northern burbs) and October (western and southern burbs).

 

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This week will remain mild for at least another day or two – highs will be well into the upper 30s with lows near or even above 32F.

 

But by Friday a storm will be approaching Chicago. For now it looks like we will remain in the warm sector, with rain or a rain/snow mix here – ice and significant snow is likely to our west and north. Stay tuned for details, but expect travel delays given the busy holiday travel season and the wide-ranging impact on airports everywhere east of the Rockies as a potential powerhouse develops toward the East Coast by Christmas Eve and Christmas Day. There is a lot of uncertainty about the track of this weekend storm so anyone with travel plans so keep an eye on it.

 

Behind that storm a very cold airmass will arrive by Monday and likely set up for at least a week or two. It is not going to be record breaking, or even close, but it will be cold compared to what we’ve had so far. With the cold will come repeated chances for snow, starting on Sunday. It is far too early to pin down details, but even if we stay on the warm side on Friday it looks like we could see a little bit of snow . The historical odds for a White Christmas are just under ~40% and at this point it’s not looking much more likely than that.**

 

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* The average annual/seasonal snowfall in Chicago is about 37”, with some variation depending on location (especially pertaining to proximity to the lake). But calendar-year snowfall is even more arbitrary since it covers parts of two distinct snow season. And given the variation in official reporting sites (various in the Loop from the 1870s, then Hyde Park, then Midway Airport until 1980 and O’Hare Airport since then) it is possible that <1” was recorded at another site in the area.

** If a White Christmas is defined as ≥1” of snow cover at 7am on Christmas morning, O’Hare’s historical occurrence since 1980 is 37%.

 

 

 

 

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