Coldest Air Mass of Season Likely This Weekend; Snow in the Interior Northeast?

A shift in the weather pattern across North America may bring the coldest air mass so far this fall to parts of the Midwest and East.

It looks like the colder pattern will develop from the upper-Mississippi Valley to the Great Lakes, Ohio Valley and parts of the Northeast by this weekend. In fact, some locations may see their frost and/or freeze of the season and their coldest morning lows since mid-late April.

The first wave will bring cooler air for midweek, but it's the end of the week and this weekend that may present the coldest air of the season so far. Many cities could see their first official readings in the 30s so far this fall. This would result in the first frost and/or freeze of the season for parts of the Upper Mississippi Valley, Great Lakes, Ohio Valley and interior Northeast.

• Chicago: First 30s of the season possible this weekend. Highs only in the 50s Friday-Sunday

The upcoming cold shots will be quite a contrast from the near-record to record warmth the region saw in September.  Burlington, Vermont, and Sault Ste. Marie, Michigan, were among the cities that observed their warmest September on record last month.Things can change, but there is growing evidence that, in some locations, the first accumulating snow of the season may be on the horizon. The area we are watching for this potential snowfall includes a swath from the Catskills and Adirondacks of New York to the higher elevations of Vermont, northern New Hampshire and northern Maine.

Think of a see-saw and the jet stream pattern is kind of like that. It is rarely completely flat (west-east) across North America. A ridge on one side, generally featuring warm weather, is usually offset by a trough, or cooler air, on the opposite side. As we move through mid-October, the jet stream will be driven north through western Canada, thanks to a building ridge across the western United States. This means that warm weather continues across the western states and extends down into the south-central states as well. Conversely, the jet stream tilts south with eastward progression. The polar jet stream will penetrate south across eastern Canada, eventually bringing bouts of chilly air into the Upper Great Lakes and Northeast in the coming week. With time, the cooler air will ultimately become cold enough to support some snow in the region, given a supply of adequate moisture.

Temperature anomalies in the middle levels of the atmosphere (deeper blue/orange correspond to colder/warmer than average temperatures) and the jet stream pattern forecast for late this week.

Forecast Morning Lows

Original Article: http://www.wunderground.com/news/pattern-change-winter-preview-northeast-western-warmth