Friday, January 24, 2014
The hemispheric circulation pattern is changing next week, but the details are still uncertain. One source of uncertainty is coming from the much hyped polar vortex as it currently has split into two pieces. One vortex resides over the Hudson Bay as it has for much of this winter and last summer, while another vortex has settled over Siberia. The cross-polar flow between these airmasses is decreasing, and they are forecast to become isolated from one another by next week. However, the cross-polar flow is expected to increase near the end of next weekend, and the strength and location of this flow determines how much cold air flows from Siberia into Canada and vice versa.
Additionally, the west coast ridge has also been a dominant and long-lasting feature this winter, and models forecast major changes as Pacific energy is predicted to undercut the ridge. Furthermore, phasing of the undercutting energy with the cross-polar flow around next weekend will keep the west coast ridge evolving and uncertainty high in the medium to long range.
In the shorter term, mountain slopes will warm and valley will stay cool as inversions persist through the weekend. There is a dry grazing wave Friday night, but that won’t have much, if any impact on our weather. A far more substantial lobe of cold air rotating around the Hudson Bay vortex may begin dropping temperatures late Sunday, with light snow possible for Monday and Tuesday.
Forecast uncertainty then increases as another cold wave is forecast to drop down from the north and phase with Pacific energy undercutting the west coast ridge midweek. I’d like to emphasize the timing of the Pacific energy undercutting the west coast ridge is uncertain since the feature has been so dominant this entire winter, but it seems likely to happen. Furthermore, there are additional waves of energy forecast to rotate down from the north through Thursday, and if the west coast ridge undercutting eventually happens as predicted, we should remain in cool and moist northwest flow with accumulating snows, likely significant, through much of the week.
A major wave is then forecast to move through this undercutting flow around next weekend and will likely keep snows going. But the forecast for then will evolve as there is so much uncertainty midweek.