(Premium) Intensity of weekend cold still uncertain

After many of us geared up for Spring with warm weather in both the early and middle part of March, the atmosphere reminded us that no transition is as smooth as it may seem. Colder, blustery air has returned to the area, and although a mild stretch is expected during the middle of this coming week, very cold air will again surge into the area by this weekend. As we spoke about several days ago, forecast models have done an excellent job of signaling the overall pattern teleconnections and evolution, with a surge of cold air possible during the first week of April.

Over the past few days, however, the intensity and duration of this cold air has come into question. Forecast models have trended slightly weaker overall with the lobe of the Polar Vortex which will break off and head southward into Southeast Canada and Northern New England. Accordingly, the forecast temperatures in our area have moderated a bit. While the overall evoltion of the pattern still suggests anomalously cold air gets to our region, a more south-to-north gradient appears likely, with the coldest temperatures likely north of New York City and more slight to moderate departures from normal to the south.

Forecast models showing the EPO ridge and incoming cold which is a displaced as a result.

Forecast models showing the EPO ridge and incoming cold which is a displaced as a result.

Much of the uncertainty stems from the evolution of the atmospheric pattern, which begins as a large ridge builds from British Columbia into the Western Arctic. This EPO ridge (Eastern Pacific Oscillation) is quite anomalous, and it dislodges the cold air from the arctic, near the pole, and in Northern Canada, to the south. This aids in the southward movement of a piece of the Polar Vortex, which moves through Central Canada and eventually ends up moving through the Great Lakes and Northeast United States.

The duration and intensity, however, have much more to do with high latitude blocking which is forecast to be present on the Atlantic Ocean’s side of the globe. When blocking exists here, along with the EPO ridging in the Eastern Pacific, the cold air can truly be pushed southward and allowed to amplify. Models over the past few days have trended less impressive with Atlantic blocking. So while the EPO ridge develops, and the piece of the Polar Vortex does move through the Northeast US, it remains generally progressive, swinging through and then moving right back up into Eastern Canada in just a few days time.

Pattern evolution at 500mb with anomalies showing the transient progression of cold next week.

Pattern evolution at 500mb with anomalies showing the transient progression of cold next week.

Such an occurance would likely save us from a string of brutally cold days — but may even open the window for some wintry precipitation. Instead of being a totally suppressive, cold vortex, the polar vortex lobe instead becomes more transient. This could allow for some moisture to surge overhead of the cold airmass, possibly falling as snow.

But, lets digress for now.

It still looks highly likely that there will be a string of below average days late this upcoming weekend into early next week. The change in the duration and intensity only means that we can begin looking forward toward moderating temperatures by weeks end, which forecast models are already hinting at to continue into mid April.