What has proven to already be a messy storm will only get messier heading into Friday, as Boulder will experience its most significant precipitation event  since the Boulder Flood in September 2013, alongside a potential historic snowfall event unfolding in the Foothills.

The first wave from the storm arrived a bit early this morning, giving Boulder a quick shot of snow before changing over to rain by the lunch hour. Snow accumulations from this morning range from 1 to 3” over most of the Plains, with Boulder recording 3”. Most of that has already melted, as the temperatures have fluctuated between 33 and 38 degrees throughout the day.  There was a lengthy break in the action, but a plume of precipitation has returned to the region. The surface low is trending ever-so-slightly north in the models compared to previous runs, meandering its way into southeast Colorado overnight tonight, increasing the easterly flow over NE Colorado (20-30 knots at 700mb). As surface temperatures just 70 miles east of Denver are already in the mid 40s, this will advect in warmer air to the region at low levels.

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700mb temp and wind valid at 3am MT Friday. Notice the easterly winds of 30-40 knots in NE Colorado blowing warm air towards Boulder, along with “abundant”Gulf moisture.

Rain and snow will be the trend through the evening, with heavy snow falling in some of the more intense bands due to dynamical cooling. The warmer air will limit accumulation for most areas  below 5500 feet, but 1-3” of wet snow overnight will be likely before the eventual changeover to all rain before sunrise. Friday will see temperatures climb into the 40s with rain, lots of rain, throughout the day for the lower elevations, with liquid precipitation totals of 1.5+”.

Thursday 6pm MT atmospheric sounding from Denver International Airport. Note the very moist layer extending up to 500mb (dew point curve on left hugging the temperature curve on right), coinciding with easterly flow. The air is just too moist for the surface temp to drop below freezing and the overnight warm air advection will not help.

On a more wintery note, the Foothills  have been getting consistent snowfall all day long, with most areas above 6500 feet already recording 6 to 12” of snow as of 8pm MT. The action is just getting started in this region. The models have heavy precipitation, all of which will be snow, continuing for the next 16 hours, with the snow potentially lingering into Saturday. Considering the duration of the storm,  the steep lapse rates, abundant upslope and moisture, the forecasted snow total of 15-30” by Saturday morning is still right on track.

Ben Castellani

Ben grew up in western Pennsylvania and holds both a bachelor's and a master's degree in meteorology. He currently works on a programming language tailored for scientists at Harris Geospatial Solutions in Boulder.

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