Back in mid November; El Nino energized central Pacific Jet along with the normal dive of the late autumn Polar Jet created a stormy battle across the country as the two patterns competed for dominance. It now appears this will again commence during the second half of December as both air masses and associated jets begin to carve out deeper upper level troughs over the West (were the initial battle will commence) and push northeast in time across the country into the Great Lakes. As time wears on, heights drop across the country and the Polar jet/air mixing with the Pacific Jet becomes more established, basically from the US Southwest to the Northeast. Of course this is a favored storm tracked for storm lovers as the churning of jets and air masses brew storms. This track of storms will bring rain, snow and likely difficult driving conditions depending on each particular storm and available cold, warmth and moisture - and where. Early in the battle anyway, it appears the systems will bring mainly rain to Southeast Lower Michigan but as colder air filters in behind systems, this would set up the region for snow.
As an example of the jet configurations the GFS model 18z 12/09/2015 is projecting, it is easy to see the strong El Nino Pacific enhanced jet gradually becoming mixed with the lowering heights and colder air from the polar region. Note; not only the jet express from the Pacific but also toward the end of the period, the large ridge block developing over Alaska. I would initially take this strong block with a moderate grain of doubt this far out but examining the upper pattern daily for weeks now, suggests at least transitory ridging developing with time toward Christmas in that region as heights lower across the lower 48. One thing that has been trending toward the negative is both the NAO/AO with time which reflects the lower heights..
Check out the loop of the 250MB Jet 18z GFS 12/09/20115 (click on arrow). Of course this will dance around with exact features and storm tracks the next couple of weeks but we are mainly interested in the trend, which shows a definte lowering of heights with time as the NAO/AO appraoch negative territory. How much they move into negative territory will strongly dictate the amount of cold air delivered into the country. Anorther factor I'm watching is the EPO which so far remains positive.
In any event; the second half of December is beginning to look very active and changeable - something holiday travelers will want to watch, especially after mid month.
I'll return with a new blog when specifics become more apparent and affecting the region.
Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian
