The Front Range Storm

If you are looking for oddities in the snow climate of the western United States, the area near and immediately east of the Continental Divide is a good place to start.  

First, let's talk about the snowiest month of the year, as illustrated in the plot below from Brian Brettschneider.  Most of the western United States and Canada west of the Continental Divide sees their snowiest months in December, January, or February.  However, near and east of the Continental Divide, the snowiest months are late in the cool season and in many areas in February, March, or even April.  

Snowiest month of the year (now featuring Canada). pic.twitter.com/nA2GvHhsGE

— Brian Brettschneider (@Climatologist49) March 5, 2021

Focusing on the Front Range region of Colorado, based on average snowfall, the snowiest month in Boulder is March (16.1").  The least snowy month from November to April is actually January (9.7"). 

One of the reasons for this is that the late winter and spring are the peak period for the development of lee cyclones over southeast Colorado.  Such storms can tap into Gulf Moisture which is transported northward and eastward toward the Front Range.  

The models have been forecasting such a cyclone to develop tonight and tomorrow, resulting in a prolonged period of precipitation in the Front Range area over the weekend.  Below is the GFS forecast for 1200 UTC 14 March (0600 MDT Sunday) showing the cyclone centered over southeast Colorado with heavy precipitation encircling the system to the east and north.  Precipitation is heaviest north of the low center where strong frontal forcing and upslope flow over the high plains impinges on the Front Range.  

In these situation, there is often a band of colder, terrain-channeled flow near but upstream of the windward slope of the Front Range. Below is an example from a paper by Larry Dunn showing easterly flow over the plains to the east but, northernly flow near the Front Range (Boulder indicated by BOU). 

Source: Dunn (1987)

The boundary between those two flows, often referred to as a blocking front since the terrain channeled flow is a result of topographic blocking, is often the locus for enhanced vertical motion, resulting in heavier precipitation near and downstream (in this case west) of the blocking front.  

Source: Steenburgh (2014)

For the last couple of days, the GFS and members of the Global Ensemble Forecast System (GEFS), which shares components with the GFS, have been spitting out some insane snowfall numbers for the Front Range of northern Colorado.  Below is the downscaled NAEFS forecast plume from 0000 UTC 10 March.  The NAEFS is comprised of members from the GEFS and the Canadian (CMCE) ensemble.  The mean for Boulder for this weekends storm was around 2" of water and 20" of snow, but most of the GEFS members were above this, resulting in a mean closer to 25" of snow, whereas most of the CMCE members were below this, resulting in a mean closer to 15" of snow.  

The GFS is not included above, but it has been on the high side of the ensemble, putting out some insanely big numbers.  In contrast, the European has been more along the lines of the Canadian.  

A glimpse into the thinking of meteorologists dealing with this spread in forecasts is provided by the snippet below, taken from the National Weather Service Boulder Forecast Office Area Forecast Discussion issued 819 PM 11 March:

Source: NWS Boulder

The latest downscaled SREF has most members leaning toward a modest storm for Boulder, with most members in the 5-12" range, but a couple much higher than that.  

This is a storm that illustrates both the challenges of weather prediction, but also the challenges of forecast communication given the range of possible outcomes from modest to historic.