Snowfall forecasts by NBM indicate moderate to heavy snow across the Upper Midwest, Great Lakes, and Interior Northeast States today through tomorrow. More snow although not as heavy extends across the North-central U.S. to the Interior Northeast States later this week. More snow is likely from the Ohio Valley across the Northeast States this weekend. The ongoing snowfall leaves deep snow cover for the Midwest to the Northeast by middle of next week (Fig. 6). The deepening snow enhances arctic air presence peaking in strength early next week across the PJM System.

ECM indicates potential for flooding rainfall during the middle third of December across Eastern Bolivia, Paraguay, and Southwest/Southeast Brazil. The rain may abate in the 11-15-day period. The 24-hour change indicates a significant increase in rainfall for Paraguay and eastward. The culprit is an intensifying upper trough extending from east of Argentina northwestward into Brazil during much of the next 2 weeks.

Climate signals forecasts identify provide clues as to where the weather pattern is heading after mid-December. The West Pacific oscillation (WPO) and Pacific North America (PNA) index is negative after mid-December supporting a shift and expansion of the cold pattern over western North America. Negative East Pacific oscillation (-EPO) spikes in one week as another “polar vortex” cold outbreak strikes the U.S. The North Atlantic oscillation (NAO) aggressively shifts to positive phase during the medium range supporting an eastern North America warm-up and validating the -WPO/-PNA colder support in the West. The question is whether -NAO warmth can eat away at the snow cover validating very warm 11-15-day forecasts by ECM.

Snow cover extends across much of the northern half of the U.S. east of the Continental Divide today retreating slightly in the East during the next 2 weeks while building dramatically over Western Canada to the North-central U.S. where arctic air generation is likely.