The State of the [Arctic] Atmosphere: Sunday, November 9, 2014

You’d most likely have to have been living under a rock for the past few days if you’re unaware of the MAJOR change of weather we’re about to experience in the next 24 hours. Indeed, Brendan laid the groundwork for the pattern change this past Tuesday on the blog. Not much has changed in the overall big picture he painted for us then, though we obviously have a much better handle on the details.

Let’s get some expectations-setting out of the way right off the bat: the cold this week will be fierce, long-lasting, and will rival (if perhaps not break) existing records, particularly record low highs by mid-week. Most models suggest that by the time we drop below freezing mid-to-late afternoon Monday, we may not recover above the freezing mark again for another 7-10 days! The duration of this cold will be remarkable. Lastly, though this cold will be profound and extended in nature, it may not offer that much in the way of snow for those hankering for high totals, but it will bring us -- for sure -- at least some measurable snow!

As of press time (early evening Sunday), the Arctic front in question is draped across southern Montana, far northeast Wyoming, and western South Dakota. Prior to its arrival, we can expect increasing clouds with breezy and unseasonably mild conditions overnight. In fact, we’re likely to set a new record-warm low Sunday night/Monday AM: lows will only fall to near 50℉, with the record warm minimum for 11/10 being 47℉ from 2007.

That mild weather comes to a screeching halt on Monday as our Arctic front approaches the area by during the morning hours. Before its arrival, and dependent on the exact hour the front moves in, we could see temperatures well into the mid- (to perhaps upper-)50s, briefly by late morning Monday. By early afternoon, however, temperatures will begin to drop dramatically as the front itself races through the region. We may see a brief period of rain showers by early-to-mid afternoon, before temperatures crash below freezing any precipitation quickly turns into snow, with a slight chance of some freezing drizzle possible at the outset.

To a sense for how fast temperatures will drop off as the front passes through -- take a look at today’s 12z Canadian model for snapshot 2 meter temperatures at 11am Monday. It warms Denver to near 60℉, while 20s and 30s push into northern Colorado.

Weather5280 Models

By 6pm, temperatures will probably already have dropped into the upper 20s, with areas of scattered light snowfall possible. Count on very blustery conditions, with west winds 15-25mph before noon shifting to out of the north by mid-afternoon, with sustained winds of 25-35mph possible and occasional gusts near 40mph by early evening. Temperatures will continue to drop precipitously as the evening wears on, and we’ll likely wake to AM lows in the mid-teens by Tuesday morning.

Latest guidance suggests folks in Denver proper could pick up anywhere from 0.5 to 3” of snow by Tuesday morning, atop a possible layer of ice as initial precipitation briefly melts on contact before freezing.

An isolated snow shower cannot be ruled out during the day Tuesday, but it will be primarily dry after the early morning hours, with partly to mostly cloudy skies and high temperatures struggling to reach 20℉; wind chills probably won’t escape the mid-teens for much of the day! In short, after Sunday’s high near 70℉, and early-day highs in the 50s Monday, it will be the equivalent of severe weather whiplash.

A reinforcing shot of even colder air will push into the region late Tuesday into Wednesday. As a result, after a relatively dry Tuesday (during the daylight hours, at least), snow showers may redevelop by the Tuesday night -- or, more likely, Wednesday morning. Lows Tuesday night will drop to the low teens, or perhaps as low as about 10℉ by Wednesday morning.

On Wednesday, expect overcast skies with scattered snow showers. “High” temperatures Wednesday will occur during the first half of the day and may flirt with the current record-cold high for the date of 9℉ from all the way back in 1916! Periods of light snow may continue Wednesday night -- though with definite lulls at times -- into early Thursday. Our overnight lows Wednesday night into Thursday morning will be brutal -- only in the single digits, with wind chills near (or even occasionally below) 0℉ possible at times!

Josh Larson | Weather5280

While models still differ considerably for possible snow totals with this second push of cold air late Tuesday into Thursday, we do have the potential for another 2-4” of snow during this period, with very high snow to liquid ratios on the order of 30:1 or higher. There is also some disagreement regarding whether continued scattered snow showers will last into the day Thursday or not -- some models think so; others disagree. What is clear, however, is that highs may not make it out of the mid-teens again on Thursday, with one model even suggesting we might not make it out of the single digits!

Beware the (relative) ‘warm-up’ that many outlets are forecasting for Friday. Though the day won’t be as brutally cold as Wednesday or Thursday, I think it’s highly unlikely we make it out of the 20s for a high. And lest you hope we may warm up next weekend, I’m sorry to inform you that most models suggest another reinforcing shot of brutally cold, Arctic air is possible Saturday into Sunday, which might bring us the coldest readings of the entire next 10 days, along with another chance for accumulating snow late Saturday into Sunday.

A note about how extraordinary this cold is for this time of the year -- early-to-mid November. The ECMWF model forecasts that by Thursday morning, when we expect air temperatures in the single digits above zero in Denver, 850mb (lower-level) temperatures will be nearly 4 standard deviations below normal for the date. To translate those statistical terms into English: the model believes that temperatures would be warmer than what we are currently forecasting ~99.9904% of the time on any given November 13th. In theory, the magnitude of cold would be something we’d expect for this time of year somewhat less often than one in a hundred years!

Needless to say, the entire team will be closely tracking this week’s exceptional cold and chances for snow, and expect numerous updates throughout the week both on the blog as well as on Twitter -- @weather5280.