Two days ago, on the premium site, I posted a very detailed analysis of what the pattern may bring for the rest of the month of December and maybe into January. I outlined a couple storm threats, one being on December 17th (now looks to be light rain with maybe a little light freezing rain) and the other December 21-23 (models still bouncing around a lot on that, but looks more likely to be rain than snow). The main story I am following, however, is the return to colder weather by the end of the month. With this post, I want to just outline some of the key points and give you some brief model/teleconnection analysis of that colder turn by sharing with you a brief premium update just recently posted. Again, Premium members enjoy content like this and often much more detailed updates daily, including discussions on long-range forecasts, storm impact assessments, and natural gas market implications. Still, I want to start integrating some briefer updates and showing some additional free content so that this page can continue to be an updated meteorological discussion as well.
LONG-RANGE ANALYSIS: Models continue to shift and bounce around in the long-term. The GFS weather model continues to be the coldest of all the models for the east coast, showing a trough in the Midwest and south here on the 24th that translates to the east coast here for the 27th. This pattern is a classic PNA ridge over western Canada with a trough over the eastern US causing quite cold temperatures for the eastern half of the country. Not all models agree on this, however. The CMC ensembles try and develop a small Midwestern trough by December 24th, but with no negative anomalies yet, as seen here. However, this breaks down and shifts slightly west as a ridge sets up along the east coast, as seen here. The result of this would be warmer conditions for us for the end of December with the heart of the cold centered in the Midwest. A surface analysis of the CMC ensembles showed a pattern, with the east coast mild but the Midwest beginning to feel the brunt of the colder pattern. However, by the end of the run, the cold in the Midwest begins to show that it would translate to the east, a sign that any ridge over the east would be short-lived and part of a transition to a colder pattern as cold would fill the Midwest. The NAVGEM ensembles, which I rarely put stock in, were similar to the GFS, while the ECMWF ensembles were the warmest of all. By December 24th they have the same pattern breakdown and show colder anomalies flooding the eastern half of the country at 500mb, but by the 27th they have the eastern ridge that the CMC ensembles show, with the cold centered in the Rockies and the Midwest. Given that the AO may not be all that negative at this time and the NAO may still be positive, I worry that this may have a little more staying power than originally expected, as this would be the scenario that keeps the region above average into January. Either way, there is colder air in the pattern, and colder snaps would be stronger behind any Midwestern cutters and ensuing cold fronts that would move through the region. The real question, though, is whether this cold would be sustained or just be occasional, and that is what our weather models are still trying to figure out.
On the premium site I will have continual updates on this as the weather models continue to bounce around with this expected pattern, but as I said I also want to start integrating a little more content like this (some of our briefer updates) freely, so make sure to stay tuned for the latest here!