![]() GFS 500 MB Pattern For 00Z (5 PM MST) December 22nd ![]() GEFS 500 MB Pattern For 00Z (5 PM MST) December 22nd Also note these are just two of many different models, this does not include other solutions such as the European. Note there are similarities, but also differences in the magnitude and location of the high pressure ridge. GEFS is on the left, and the deterministic GFS is on the right. This is where the deterministic models are not in agreement, and one needs to look to ensembles for guidance. Below are the 12 UTC (5 AM MST) 13 Dec runs valid 00 UTC (5 PM MST) 22 Dec of just two of these models (GEFS and GFS). Potential Large Pattern Shift: The next step occurs as the ridge rebuilds, but is forced to do so further west - from just off the West Coast northward into Alaska. GFS 500 MB Pattern For 12Z (5 AM MST) December 16th GEFS 500 MB Pattern For 12Z (5 AM MST) December 16th Rockies (not shown), putting a major dent into the 500mb ridge over western North America. One major step in this process occurs early this weekend (graphic below valid for 12 UTC (5 AM MST) 16 Dec), as a shortwave trough slides down the eastern side of the Canadian Rockies (shown below) and then cuts off westward off the U.S. Over the next 7-10 days, the global ensemble predictions systems are suggesting we will see a slight, but significant shift westward, bringing colder air from Canada into the West. In other words, this type of pattern has a tendency to remain nearly stationary, or even retrograde to the west. Over the past week, we've seen it remain nearly stationary. Start of Pattern Shift: These large scale patterns are controlled by processes called Rossby Wave Dynamics. Highly amplified flow (like we are currently seeing) is often associated with lower Rossby wave phase speed. GFS 12Z (5 AM MST) December 13th Pattern Analysis GEFS 12Z (5 AM MST) December 13th Pattern Analysis Here is the current analysis from this morning, 12 UTC (5 AM MST) 13 Dec: An anomalous ridge over the North Pole is making it difficult to establish circumpolar flow, leading to a highly amplified pattern. and Canada, and equally impressive troughs situated along both sides. This pattern has been characterized by an impressive 500mb ridge over most of the western U.S. has been under the influence of a strongly positive phase of the Pacific/North American (PNA) Pattern for some time. Scroll further down for the science behind this pattern change.Ĭurrent Pattern: The western U.S. Check out the CPC 6 to 10 Day and 8 to 14 Day outlooks to see more how this pattern may affect the weather patterns to end the year. It is unclear what exactly the southward extent of this colder air might be, or how long it will remain in place. ![]() This unsettled pattern could bring some increase precipitation potential for some areas of Montana. It is likely that areas west of the Rockies will see their first shot of Arctic air this winter beginning sometime in the December 21-24 time frame. Deterministic solutions are still valuable because they highlight the potential for individual shortwaves that are not apparent in the ensemble mean.Skillful use of the ensembles is the best way to obtain confidence at 7-10 day lead times.Aside from Montana (upslope precipitation), this will continue a dry pattern for the western U.S.The ridge will retrograde westward in 7-10 days, with the coldest air of the season likely west of the Rockies.The past several weeks have been very warm west of the Rockies, due to a persistent western North American ridge.Key Points | End Result l Science l Pattern Shift
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