Winter storm warning issued for Boulder, but snow forecast still complex – BoulderCAST

Nothing is ever easy in Front Range weather! The arrival of the spring, or shall we say winter, storm remains a very difficult forecast, as models struggle to determine where the heaviest precipitation will fall from Wednesday to Thursday, the precise moment when the rain will turn to snow. and exactly where temperatures will bottom out. overnight. All of these factors combine to create a complicated mix of moderate to heavy rain and deep snow tonight through Thursday. Most models would have us believe that a deluge of precipitation is about to unleash the Denver area, but there are a few red flags. We discuss the latest news on this evolving storm which is sure to create all the mess for everyone over the next 36 hours.

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The tendency to “big” precip

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Nearly all weather models now predict 0.75″ or more of precipitation for this event for the entire Boulder and Denver metro, with a preference for higher amounts in the Foothills and south of Denver through the Palmer Divide (1″+). A storm of this magnitude is significant for our arid climate and would account for around 5-10% of our annual precipitation!

Some of the short-range, convection-enabled models are even more bullish with the potential for this storm with over 1.5 to even 2″ in places!

The average and median precipitation amounts for the set in Boulder and Denver are near or slightly above 1″.

By just about every weather model, all systems are opting for a big wet spring storm…

The Riddle of the Upslope

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Sure, it’s good that most models are okay with a significant moisture spill on us over the next 36 hours, but is that realistic? The track of the storm remains worrying far to the south and the upward slope does not really impress. Note the mountain level northerly flow shown in the GFS model forecast below for Wednesday evening at midnight. Straight out of the North to Boulder and even a hint of downhill from Longmont to the Wyoming border!

Winds near ground level are more favorable from the north-northeast, but only slightly. Ultimately, this direction means we can’t really count on an upslope to deliver the goods tonight, except in areas favored by a predominantly northern flow like the Palmer Divide and the southern outskirts of Denver.

When will we turn from rain to snow?

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Even setting aside our hesitations surrounding the modeled moisture deluge, the difficulty of forecasting in Boulder and Denver is compounded by a lack of truly cold air. If all goes according to plan, it will be barely cold enough for lower elevations to turn snowy tonight with temperatures falling near or just below freezing in most areas. There are still some stubborn pattern runs that don’t cool enough for the snow to stick much (peaking at ~33-34°F). This seems more plausible in the presence of northerly winds that will create downward compressive warming from the Cheyenne Ridge across the northern Frontal Range – watch for this from Longmont to Fort Collins to Greeley. This hot air can even ruin part of the party in Boulder. It is to be determined.

According to our latest thoughts, the transition from rain/snow to all snow occurs between 8:00 p.m. and midnight in most places. It might take longer in generally warmer and lower areas like Longmont and Loveland and maybe even Gunbarrel. After the transition, it will be all snow for the rest of the night until Thursday morning. Lighter precipitation will persist into the second half of Thursday and would likely be a mix of rain and snow across the metro area with minimal additional accumulation after the morning hours. The simulated radar precipitation type forecast animation below from the HRRR model shows how the transition from rain to wet snow could occur Wednesday night.

Timeline and impacts

  • Wednesday early afternoon: Cooler temperatures in the 50s with thick but dry cloud cover. It’s going to be a gloomy day!
  • From Wednesday afternoon to evening: The chance of precipitation increases throughout the region, with rain likely beginning in the late afternoon or early evening in the metro area and with snow in the upper foothills.
  • Wednesday mid-evening: Colder air will seep in with temperatures falling into the 30s and snow levels dropping rapidly. The rain will change to all snow over the entire area between 8:00 p.m. and midnight.
  • From Wednesday evening to Thursday morning: The snow persists all night, sometimes abundant. Wet, slushy snow is sure to impact the morning commute, especially in the western and southern suburbs of Denver. The roads will be snow covered in the Foothills. Most of the accumulation will be over by mid-morning with March sunshine and relatively mild temperatures that will melt sleet quickly throughout the day.
  • From Thursday afternoon to evening: Light snow and showers will persist throughout the day, likely mixing with rain at lower elevations at times. Little further accumulation is expected.

Denver meteogram for the next 72 hours

Snowfall amounts

Given all the possibilities for expand or decelerate scenarios tonight, we are expecting a fairly wide range of snow totals in the Denver metro area and higher elevations with this storm. There’s no doubt about it: melting will be a serious issue with warm ground after the spring-like temperatures we had on Tuesday and near-freezing surface air temperatures when the snow falls tonight. . We are particularly concerned about the northern flow keeping things above freezing northeast of Boulder and north of Denver, where it could be difficult to get a lot of snow. However, for most of the Denver and Boulder area, 2-10″ of wet snow can be expected (highest in the south and southwest). In higher terrain where snow will start earlier and melting will be much less of an issue, 6-12″ is our forecast range with locally higher amounts possible (if not likely) in parts of Jefferson County and possibly being near the top of the Palmer. To divide. Our snowfall forecast map for the event is shown below. The gradient in snowfall amounts is expected to be most dramatic in eastern Boulder County with very high uncertainty in this region as well!

The National Weather Service has issued a winter storm warning for the west Denver metro area (including Boulder and Golden), the Jefferson County foothills and the Palmer Divide. Technically, Longmont is also included in the warning, but that’s kind of laughable. The rest of the Front Range from Denver to the Wyoming border is under a winter weather advisory at this time (but may upgrade to a warning at some point). These winter highlights run until noon on Thursday.

The next big one?

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Regardless of how tonight’s mess ultimately plays out, Mother Nature may already have an encore performance underway for early next week as some models consistently show a Major The Four Corners Low hits the central and southern Rockies from Sunday night through Monday. Still at five days, it’s obviously quite uncertain at the moment, but confidence is rising little by little Something will occur early next week, perhaps of the same caliber or even greater than the storm knocking on our doorstep right now.

This type of moisture infusion into storms is usually only possible during the transitional seasons (spring and fall). The pitifully hot and dry excuse of a fall in 2021 offered us nothing of this magnitude in northeast Colorado. We’re excited to expect some big, juicy storms to hit the Front Range again as we enter the wettest time of the year!

That’s all for the moment. We expect things to be much wetter and muddier at this time tomorrow!

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BoulderCAST Team

This post had major contributions from at least two members of our team. The more meteorologists the better, right?

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