The upper level wind flow pattern over Canada continues to feature: 1) high pressure at the surface and an upper level ridge aloft over Western Canada, and 2) surface low pressure and stormy conditions with a moderate to strong upper level trough in eastern Canada.
The Western Canada ridge and surface high tends to block systems from moving in off the Pacific. This allows central and eastern Canada to get cold, as there is no moderating influence from the Pacific air masses. The flow pattern between the Western Canada ridge and the Eastern Canada trough has varied from time to time. It has been northwest to southeast at times, north to south at times and sometimes even northeast to southwest.
The north or northeast to southwest flow is this coldest flow pattern for the Prairies, while the northwest to southeast flow is cold for eastern areas, but not very cold for the West.
This blocking type pattern has also at times limited the inflow of moisture off the Pacific. However, a storm track cutting underneath the Western Canada blocking ridge has allowed enough moisture to be brought inland over the U.S. This then gets transported north into the Prairies. The result is that melted precipitation totals and snow cover have either matched or exceeded normal amounts for the month of December in many cases.
The weather pattern going forward from here continues to show a tendency for ridge west and trough east in Canada, but the pattern does not look as strong as it did earlier in the month. This may mean that some of the moderating influence of the Pacific may reach into the Prairies region at times in the form of more seasonal temperatures and added moisture. However, this is more likely in the west and less likely in the east Prairies. The storm track over the northwest U.S. is not as strong as it was earlier in December, so even with the blocking pattern relaxed we may not see as much moisture as earlier in December.
Also, as we have seen in past discussions, the warmer patterns for the western Prairies have been in the forecast more often than have actually occurred. This suggests a "memory" in the weather pattern that may not allow for much moderation during the medium and long range forecast periods.