The forecast period for our weather discussion was rather quiet, so we decided to focus in on an area we were already interested in as it encompasses the current Weather Challenge forecast city: Grand Rapids, Michigan. Michigan has experienced multiple frontal passages over the past few days contributing to rainfall, pressure fluctuations, and temperature shifts in the region. The GFS 06Z forecast over the next couple days (12Z 1 October to 12Z 3 October) has the precipitation mostly moving out of the region aside from pockets remaining around coast of the Lake Michigan until another disturbance moves through Sunday. On the synoptic scale, we analyzed the jet configuration shown in Figure 1 over North America noting the strong northwesterly meridional flow at 300 mb extending from the Alaska-Canada border through Missouri and Illinois. The ridge-trough pattern in the jet is reflective of the geopotential height pattern across North America displayed in Figure 2, which bears strong resemblance to the Pacific-North American pattern, which is a low-frequency teleconnection pattern known to influence below average heights and temperatures in the eastern United States and above average heights and temperatures over the western United States. A persistent geopotential height dipole is present across the U.S. over the forecast period with a longwave ridge over the western U.S. and a long wave trough over the east, and associated above and below average heights, respectively.
The strong temperature gradient, as evidenced by the gradient in geopotential heights, is what leads to the strong winds in the jet flow aloft. The jet flow pattern set up aloft is advecting cold and dry polar continental air into the upper Midwest, contributing to the cold frontal events observed recently. This flow will persist over the next couple days but will become more disorganized and weaken, shown in Figure 1, as the forecast progresses and the temperature gradient weakens. The advection of the polar continental air into the upper Midwest leads to drier and cooler conditions, but also plays a role in the precipitation we observe and forecast as it moves through the Great Lakes region due to the lake effect.
Lake effect precipitation occurs when cold, dry air moves over a relatively warmer body of water. In the case of our region of interest centered around southwest Michigan, this body of water is Lake Michigan. Figure 3 shows lake surface temperature is forecast to remain around the mid-50 °F range, while the air being advected over Lake Michigan will be approaching the low 40’s°F and potentially enter the 30’s°F over the forecast period. When looking at the precipitable water in Figure 4, which is the amount of water you would find if you condensed all the water vapor in the overhead air column, it confirms that the air mass moving through the region is quite dry and will continue to be so through the forecast period. The westerly surface flow in the area will advect that cold, dry air over the surface of Lake Michigan, which will cause the air to pick up moisture and heat as the specific heat of water is much higher than that of air, causing the air to be warmer with higher moisture content, and therefore less dense, once it reaches the downwind shore.
The decrease in density will already influence the air to rise as it reaches the downwind shore, but other factors can influence upward vertical motion of this air at the surface as well as it reaches the opposite shore. For instance, the surrounding land around Lake Michigan is marginally higher in elevation which can assist some rising motion of the air. More telling, I think, is the role that friction plays on influencing some low-level convergence. Friction changes the speed, and therefore slightly affects the direction, of wind. Since friction opposes flow, the wind will slow and so the Coriolis force will decrease, which will give the wind a directional component toward lower pressure. The force of friction of water on air is much less than that of land. As is shown by Figure 5, there is a slight change in speed and direction as the winds come onto the eastern shore of Lake Michigan where the flow shifts slightly more westerly from a more northwesterly flow and drops from around 15 knots to around 5 knots in the vicinity of Grand Rapids. This speed and directional low-level convergence due to impacts of friction will help contribute to rising motion, which will influence condensation. So, the lake effect potentially plays role in the precipitation seen in the Figure 6 forecast for southwest Michigan and as indicated by the soundings taken on the upwind and downwind sides of Lake Michigan displayed in Figure 7.
