**1. Introduction**

The United States (US) National Weather Service (NWS) currently defines blizzards as events that have sustained winds or frequent gusts ≥ 35mph (16 <sup>m</sup>·s<sup>−</sup>1) and considerable falling and/or blowing snow that reduces visibilities to ≤ 1 4 mile (400 m) for periods of three hours or longer. These events are recorded within the National Centers for Environmental Information (NCEI) *Storm Data* publication that is reliant on submissions by the Warning Coordination Meteorologist (WCM) at each NWS forecast office. This publication serves as the o fficial archive of storm events for the country [1,2].

Within the contiguous United States (CONUS), reported blizzards are most common over the Northern Great Plains (NGP) including the region centered on North and South Dakota [1,2]. At a county level, the highest frequencies are found along the border of North Dakota and Minnesota, which, topographically, makes up the Red River Valley (RRV) of the North (Figure 1). To some extent, this is impressive considering population-related reporting biases noted for warm-season hazardous weather events such as tornadoes [3–6]. Alternatively, reporting biases could exist by the NWS County Warning Area (CWA), as noted for warm-season hazards such as hail [7] and wind [8].

**Figure 1.** Topography of the Red River Valley (RRV) of the North. Elevation (ASL) is shaded while National Weather Service (NWS) County Warning Areas (CWAs) are denoted by the dark red polygons. Larger water bodies and rivers are highlighted in blue.

Regardless of potential biases in *Storm Data*, the high frequency of blizzards in this region makes physical sense and can be attributed to factors including the topography/land cover, climatology of snow cover, and frequency of high-wind events. A lake plain leftover from the receding Glacial Lake Agassiz 8000 years ago, the shallow Red River of the North flows northward to Lake Winnipeg before eventually emptying into the Hudson Bay [9]. The RRV is largely devoid of trees except within the immediate vicinity of the river and in shelterbelts (tree rows) planted due to agricultural activity. Though the RRV is, on average, only a few hundred meters deep over a 100 km width, there is evidence that winds are enhanced within this region. For example, blowing snow plumes are sometimes seen only within the RRV, typically in regimes with cold-air advection (Figure 2). While there are numerous studies that document topographic influences of valleys on winds in other locations, the authors are unaware of any existing studies for the RRV.

**Figure 2.** False color imagery (generated from I1-I2-I3-M3-M11 bands) from the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi satellite during the daylight (~1:30pm local time) overpass on (**a**) 11 January 2018 and (**b**) 15 January 2018. Snow cover is denoted by pink/red, cloud cover and blowing snow by white, and bare landscape by green (bare ground) or dark (forest) areas. Blowing snow plumes oriented along the RRV are labeled by 'BLSN'.

With a latitude of 45–49◦ N in the center of the North American continent, the RRV is the coldest non-mountainous region within the CONUS [10]. Though the region only receives, on average, 80–100 cm of snow in a year [11], the cold temperatures facilitate an environment that supports an average snow cover extent >85% during the winter months [12]. Snowfall events responsible for this cover have been tied to several meteorological patterns including extratropical cyclones that form due to lee cyclogenesis such as Colorado Lows and Alberta Clippers [13–17].

As the name implies, Colorado Lows originate due to cyclogenesis near its namesake (the state of Colorado). Historically, these types of systems have been associated with a number of impactful blizzards, including events such as the Children's Blizzard on 12 January 1888 [18]. The strength of these cyclones can advect a significant amount of moisture northward, and, as a result, these systems are responsible for the heaviest (and largest scale) snowfalls in the RRV, southern Manitoba, and western Ontario [16]. The more progressive cousin of these systems include Alberta Clippers that propagate rapidly east-southeast from Canada into the upper-tier of the US [19]. Precipitation for these events typically comes in the form of mesoscale snow bands. Overall, snow totals are lower due to lack of available moisture, but these systems can still produce significant winds capable of reaching blizzard criteria [16,20]. While Colorado Lows and Alberta Clippers are colloquial terms for common North American mid-latitude cyclones, blizzards can also be forced by systems that originate in other areas (e.g., Montana). Historically, these events have been given the moniker 'Hybrids' by the Grand Forks NWS Forecast O ffice (NWSFO), and, as such, this term is used herein to describe systems that do not conform to stereotypical patterns but have a defined low pressure center. Depending on the event, snowfall can be meso- or synoptic-scale in nature, with high variability for totals.

While blizzards are often thought of as large-scale events associated with the juxtaposition of winds and snowfall associated with mid-latitude cyclones, the RRV also experiences events known as ground blizzards that are frequently driven by strong winds behind Arctic (Cold) Fronts [16,21]. For these cases, winds greater than 4–7 <sup>m</sup>·s<sup>−</sup><sup>1</sup> impart a force on already fallen snow, rolling it on the surface before being bounced and lofted into the atmosphere [22,23]. As demonstrated in Figure 2, these events can often occur under otherwise clear skies and, in some cases, are confined solely to the RRV, providing evidence of the topographic enhancement of winds.

Historically, the Grand Forks NWSFO has subjectively classified blizzards within their CWA (see Figure 1) into the four aforementioned categories (Alberta Clippers, Arctic Fronts, Colorado Lows, and Hybrids), and has maintained a local database of these blizzards from 1974 to present. These events are identical to the reported blizzards in *Storm Data,* although additional meteorological information is sometimes included within the local dataset vs. what is o fficially provided in *Storm Data*. While *Storm Data* is considered the o fficial dataset for blizzard events, the events are of such importance that the local newspaper (The Grand Forks Herald) has independently kept track of and named impactful events since the winter of 1989–1990.

The purpose of this work is two-fold. First, the climatology of blizzards within the Grand Forks NWSFO CWA will be described for the winters of 1979–1980 and 2017–2018. Besides investigating when and how often blizzards occur, this abbreviated time period will allow for composite patterns to be generated using the North American Regional Re-analysis (NARR) [24]. Given the limitations and known human biases of subjectively defining atmospheric patterns, the second goal of this work is to demonstrate that atmospheric patterns associated with these events can be objectively defined. To do so, a competitive neural network known as a Self-Organizing Map (SOM) [25] will be used.

The e fforts of this work will add to the limited body of literature that discuss blizzards in continental regions such as the Northern Great Plains. The climatology and demonstration of an objective technique to classify these patterns described herein will pave the way for future studies that will seek to identify these events in re-analyses, Numerical Weather Prediction (NWP) models, and climate simulations. This will allow for questions to be investigated that range from best forecasting practices for these events to how blizzards may change in a warming climate.

#### **2. Materials and Methods**

As noted in the introduction, the climatology of blizzard events in this study comes from the publically available NCEI *Storm Data*. For the purposes of this project, only blizzards contained within the Grand Forks NWSFO CWA (Figure 1) were investigated (see Appendix A). These events are referred to as RRV blizzards due to the majority of the CWA encompassing this topographic feature, although a few counties within this region are on the periphery of the valley. To compare events to NARR data, the time period was limited to the winter seasons (October–April, determined by reported blizzards in Storm Data) of 1979–1980 and 2017–2018. Subjective classifications of event types (Alberta Clippers, Arctic Fronts, Colorado Lows, and Hybrids) were made by Grand Forks NWSFO meteorologists using available observations, model, and re-analysis output.
