The diagnoses of the first snowfall event occurred on Feb. 13, 2007 around Hamilton, Ontario have shown that (1) the GEM regional model forecasted high pressure system (HPS) is over-intensified by 3-4 hPa and fast-moving in southeast direction, blocking the inverted trough traveling to the Hamilton area and resulting in less precipitation in the area of interest. (2) The blocking effect reduce the strength of colder easterly flows over the warmer Lake Ontario, thereby reduce lake-enhanced snowfall and snowsquall activities around the Hamilton area. (3) This blocking effect also decreases the atmospheric instability in the Hamilton area through decreasing the temperature difference between 850-hPa and 1000-hPa pressure surfaces. The temperature inversion between these two levels is forecasted by the GEM regional model but not observed in the NARR analysis. (4) The 250-hPa jet entrance region in the NARR analysis is located in the Hamilton area but not shown in the GEM regional model forecast, resulting in weak vertical motion and less precipitation around the Hamilton area.
The analysis of the second snowfall event occurred on Dec. 3, 2007 around London, Ontario has shown that (1) the GEM regional model forecasted high pressure system (HPS) is over-intensified by 2-4 hPa, blocking the low pressure system traveling to the London area, and reducing atmospheric instability and snowfall in the area of interest. (2) The GEM regional model predicted central MSLP of the cyclones is over-deepened by 2 to 7 hPa, leading to the strong rotation between two cyclones and associated cold northwesterly flows located away from the area of interest. (3) The over-intensified high and low pressure systems affect the movement and location of the cyclones, and decrease lake-effect snowfall and snow squall activities around the London area through reducing the strength of cold northwesterly flows over the warm lake and the upward moisture transport from the lake. (4) The GEM regional model forecasted 700-hPa wind speed is about 22 m/s in the London area whereas the NARR analyzed 700-hPa wind speed is about 28 to 30 m/s.
Suggestions are made for operational forecasters and modelers to improve the predictions of winter high-impact snowfall.