A strong storm will be moving from Canada into the Great Lakes Sunday and Monday with some light snowfall. This storm is currently being highlighted by blizzard watches and warnings as well as high wind watches and warnings. Many associate a storm that has the blizzard tag with high snowfall amounts, but because the ground is so dry and the air so cold up in the Midwest, blizzards are actually easy to come by.
The National Weather Service defines a blizzard at a particular observation station that has two main conditions occur during a period of 3 hours. The stations must have sustained or frequent gusts at or above 35 miles per hour and considerable falling or blowing so that visibility is reduced to less than one quarter of a mile. Notice how there is no requirement of reaching an exact snowfall measurement. For what it’s worth current forecasts have between 1 and 3 inches for the area with higher totals near 6 inches in the higher elevations.
One of the main features of this storm will be how anomalously strong it is. The way storm strength is measured outside of strength of winds like hurricane strength is to measure the lowest pressure in the center of the storm. For this particular event, it reaches a pressure of 995 mb according to the American Global Ensemble Forecast System (GEFS) model. The 995 mb compared to the climatological sea level pressure on that date of around 1020 mb, a departure from normal of 25mb. However, low pressure systems of that strength are quite common and the climatology is also offset by high pressure systems going through the same area. Wind strength can also be ascertained by the number of contours between the high pressure center’s highest pressure and the low pressure’s lowest pressure. In this case, the GEFS shows the difference between the between the two pressures is 43 mb with the high pressure reaching a pressure of 1038 mb. That indicates some strong winds through the area so that any snow could get lifted in the strong winds.
Winds are strongest aloft because the winds run into the surface of the earth and friction slows them down at the surface. So winds must be brought from aloft down to the surface. The way to do this is to use momentum transfer through up and down motions in the atmosphere, which are called eddies. These eddies are worth considering when lapse rates or the difference between the surface a particular point in the atmosphere (generally surface lapse rates are quantified as the layer between where the pressure is 850 mb, or about 1.5 kilometers into the sky, and the surface) are near or greater than 10 C per kilometer. This occurs during the time of peak heating during the day on Sunday with sustained winds having the possibility of being near 50 knots or about 58 miles per hour and gusts could being near 60 knots or close to 70 miles per hour during the time of strongest lapse rates according both the American NAM and GFS models. This well over blizzard criteria and while the 3 hour period will certainly need to be watched, it doesn’t seem too hard for it to occur based on the forecast models.