We are on the leading edge of a fantastic period of winter weather in the northeast with many powder days inevitable following the snowfall MLK day and Tuesday. The pattern, as discussed in previous posts will be dominated by upper air ridge in British Columbia which will be accompanied by a healthy southern branch of the jet. The medium range models have had an especially difficult time with this because the interactions between the Polar Jet and Southern Jet are very difficult to pin down more than a few days in advance. For the last few days models have in fact indicated a very dormant southern branch which I have taken with a grain of salt primarily because of the ENSO state. The last cycle of model runs just released was finally beginning to turn on this issue and show the potential for two partially significant to significantly significant events over the next 10 days. This not surprisingly would result from the southern branch.
1) First chance comes on Friday as the newly established baroclinic area over the southeast coast will be the focusing mechanism for the development of a coastal wave of low pressure. There is already an Alberta Clipper which is expected to impact the region that day so the question becomes whether the former and latter mentioned systems interact constructively or destructively. The result can vary by region so its difficult to know or make any determination. Certainly we can establish that there is a very good chance for new snow leading into the weekend of the 20th and 21st.
2) The southern branch could then make a more definitive impact in the period between the 23rd and 25th. The ensembles have shown that during this period the eastern trough weakens somewhat only to subsequently restrengthen for the last few days of the month. This is probably the reason why we have yet to see any indications of a phase and a major storm. Since the mean trough position is in the east however the threat is certainly there and will certainly be worthy of some speculation in the days to come.
Winter should make some big amends over the next few weeks.