Monday Night into Groundhog Day Snowstorm- Measured Excitement

It’s no secret that a big storm is brewing for the plains on Monday and Tuesday. That familiar excitement is floating through the air and the common buzz words like “blizzard”, “large winter storm”, and “f**king snowpocalypse” are beginning to pop up everywhere. OK, maybe not that last one, but it’ll start to appear around the beginning of the weekend if nothing in the weather system shifts too dramatically. For the most part, a lot of the excitement is justified based off of model output and the consistency with which it has shown the Denver Metro Area and Front Range getting significant snowfall. However, while I’m incredibly excited about the upcoming storm and its possibilities, we need to take a more measured and logical approach than the usual hype-train that everyone inevitably gets caught up in when there’s the chance for severe weather. Case in point; the November 17th snow storm where eveitswinterryone was calling for feet of snow up until the day before the event when the forecast actually showed that the majority of the snow would be to the south and east of Denver. The 24 hour forecast did verify pretty well and got a lot of the snowfall amounts correct, but everyone was already so hyped about feet of snow that when Denver got 2″ because rain had switched to snow too late and most of the moisture fell as liquid rather than solid, a mob of Metro area townspeople with pitchforks had formed ready to take off the heads of anyone who got more snow than they did.

While this next storm looks exciting (especially to me since I’ve been following it for about a week now) we’re going to pump the brakes a bit and take a rational look at it because I don’t have ratings and don’t need advertising money. That’s right, I’m only here for you because I love you…. or more importantly, because I love to talk weather! So, the forecast below will follow the standard format that some of you have become accustomed to; background on what it takes to give us a big storm, what is predicted to happen currently in the realm of computer models, and my predictions overall.

Why This Storm System Could Give Us Big Snowupslope

Anyone who has lived in Colorado for a while can tell you about the frequency with which the word “upslope” gets thrown around when winter weather is in the forecast, but a lot of people may still not know exactly what an “upslope” event refers to in meteorological terms. It’s pretty simple overall. An upslope event refers to winds during a storm that move from east to west that go up the slope of the mountains and lifts cool moist air that then turns to snow. It’s really a pretty simple concept and we get it several times a year. It’s Denver’s only real chance of heavy snowfall because of our unique topography with the mountains to the west that blocks a lot of our moisture and creates a rain shadow. Generally these upslope winds are created by a low pressure system that sits near southeast Colorado and flings the moisture against the mountains.

Snow accumulations from upslope storms can be attributed to three primary ingredients, wind speed and direction, atmospheric moisture content, and what meteorologists call “blocking patterns”. We already covered wind direction and wind speed is fairly self explanatory; as wind speed increases, the moisture being brought into the area also increases. Moisture content is a bit more complicated. When we look for big snowfall potential, we want as much moisture as possible through the depth of the atmosphere. For example, below we have two vertical profiles of the atmosphere in Denver where the red line is temperature and the green line is dew point. When the red and green lines are close to each other, there is a lot of moisture in the atmosphere and when they’re further apart, the atmosphere is drier.

On the left, we see today’s atmospheric moisture and winds (arrows on right hand side) which is dry until around 9km up when there is a bit of moisture that could lead to some cloudiness. On the right is the predicted atmospheric profile during the storm on Tuesday. Notice the temperature line and dewpoint line are stacked on top of each other predicting a very moist atmosphere through the lower 6km of the atmosphere and the winds are all heading towards the mountains for that upslope flow. For those that want heavy snow, you really like the right profile!

Finally, we need a blocking pattern for very heavy snowfall. A blocking pattern means that the weather in the US doesn’t change a lot during an extended period of time. This has led to a lot of our largest blizzards here in Colorado; the 2003 blizzard most notably. Unfortunately, it does not look like we will have a blocking pattern for this event, but that doesn’t mean that we can’t get significant snowfall!

So, two of the three parameters are lined up for Tueday’s storm. Not bad!

What Are The Models Saying?


Storm Position at 5pm on Monday. Low pressure (white lines) centered over the Texas and Oklahoma Panhandles, Moisture/Dewpoint (brighter colors are more moisture) being funneled into the area from the Gulf of Mexico, Upslope Winds (Orange arrows- longer is higher speed)


Same Map as before but with moisture replaced with Surface Liquid Water Equivalent Precipitation in inches. 1 inch of Liquid Water is approximately 10″ of snow.

We can see in the above maps that the models are predicting some good conditions to make this a heavy snowfall event. The one thing missing that would make this a truly epic storm system is a blocking pattern to keep it over Colorado and help the snow dump for longer, but currently the snowfall is looking pretty significant and that is consistent with what the models have been saying over the last week. Another thing that we need to be cautious about with this system is the position of that low pressure center. If the low moves at all in its position, we can see more or less snow. Possibly even no snow if it moves to a completely new region, but the models seem to like it over the panhandles so far which will really help us with upslope flow and moisture.

Below is the snowfall accumulation map for the storm from Ben Castellani at BoulderCast that I was graciously allowed to use! Make sure you check out Ben’s site if you live in or near Boulder. It’s a great resource. Obviously we’re seeing a pretty bold prediction out of the latest run of the Global Forecasting System (GFS) with 12-24″ of snowfall for the Front Range in the upcoming storm. Compared to most of the other model runs, this is a pretty big outlier but is a similar prediction (albeit a little high) to what we’ve seen for several days worth of model runs, but I would urge caution to anyone thinking that this a serious prediction. Again, leading up to the November 17th event we were seeing model predicted accumulations in Denver of anywhere from 1-4′ of snow and we only got a few inches. If the low ends up in a different spot than we’re predicting, we could see less snow. If the moisture isn’t as deep or plentiful as we’re expecting, we could see less snow. This is only what the current runs of the GFS are saying and, as we all know, the models aren’t perfect and weather is very unpredictable. Again, measured excitement.


Forecast Precipitation Accumulation for Next Six Days Courtesy of Ben Castellani and BoulderCast

The Final Word

This storm is definitely exciting and isn’t anywhere close to something that we have seen in the last few months. I am more encouraged by this storm than I was during the November 17th storm. The low has a better shape to it and has the ability to bring us more moisture and, while we don’t have a block to keep it here, it does hang around for long enough to give us some significant snowfall. We obviously have plenty of time for this forecast to shift and snowfall amounts to change. In fact, one of the models is predicting that this won’t happen at all, but it seems to be in the minority amongst the 4 or 5 different models that I’ve looked at over the past few days. If we get to late Sunday night and Monday and we’re still looking confidently at 1-2 feet, we can start all the snowpocalypse stuff. But, for now, 4-8″ possibly more, possibly less, and measured excitement.

You can get updates on the storm system with live monitoring as well as daily forecasts with the “My Weather Concierge” app! Forecasts every day from a real person (myself) as well as a “Request Custom Forecast” feature that allows you to get a custom forecast for your event, trip, or personal occasion from an actual Atmospheric Scientist/Meteorologist!

THANK YOU SO MUCH FOR READING! As always, feel free to comment or ask any questions below. I’d love to hear what you thought!

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