The threat for an arctic outbreak of cold air still looms, although forecasts have backed off on its intensity. While intensity is down, the consensus of a cold air shot has risen.
This is the temperature anomaly forecast from the ECMWF model for 5,000 feet off the ground, with wind vectors superimposed on the anomalies. For this image (valid for the morning of May 12th), we see a large swath of cold air exiting Canada and making its way down through the Great Lakes, plowing into the Eastern US after affecting everyone from central Canada down to Mexico. Wind vectors indicate the deepest cold pocket will be shifting southeast from Michigan/Wisconsin, likely moving towards the eastern Great Lakes in the few days following May 12th. Analysis of temperature forecasts from the European model suggest the northern Plains would easily drop below freezing temperatures, while cities like Chicago, Detroit, Des Moines and Milwaukee may be looking at upper 30s.
The prestigious ECMWF ensemble set, known as the ECMWF EPS (we'll call it the EPS for short), also sees the evolution of this cold wave. In strikingly similar location, the EPS has the coldest of the cold skirting the Midwest and plunging into the heart of the western Great Lakes. At the same time, abundant cool air flows down into the southern Plains and shifts east to allow for a major cool-down across the Eastern Seaboard. Not all that different from the European model alone, the combination of these two forecasts proves to be one that will be hard to beat.
I posted the other day on how the idea of an arctic shot of cold air was having trouble with the models, and that the chance it happens at that intensity was slim. As expected, models have dialed down the intensity of the cold wave, but model uncertainty has been drastically reduced. We could be looking at a nationwide heavy rain event (I hesitate to say severe weather event, because the pattern has been so poor as of late) if current model forecasts go as planned.