We talked about the gradual increase in warmth this week...and the likelihood of a warm weekend this coming weekend in past posts. We might be talking 60 on Saturday and Sunday in a few lucky locales if everything breaks "right" around here given the warmth that's incoming later this week.
It's January and such warmth will not last, even in winters where cold is in relative short supply. The pattern eventually nudges less warm. Although rumblings and rumors of arctic invasions are cropping up around the pay universe, the incoming cold after ten days here may not be robust in Philadelphia as some are hoping but nonetheless will be a slap in the face after flirting with 60 degrees this weekend.
Looking at the two big models, there is a pretty consistent theme that the 1/17-1/20 timeframe will be the starting point for this slap in the face. Both big models and their ensemble blends show a colder look in the US, especially in the Midwest and Great Lakes but the GFS also wants to extend this cold into the Northeast as well. The Euro chills us down but not to the extent the GFS does.
In either case, the coldest of cold may be focused over the Midwest and Upper Great Lakes in the 11 to 15 day range. Both models do show the core of colder air focusing initially in the Plains and Midwest but the GFS pushes this east to the coast a bit quicker than the Euro. The European modeling camp wants to keep a bit of a southeast ridge in place and allow for temperatures to not chill as quickly. We get glancing blows of colder weather from the Euro's perspective but the brunt stays west.
As the pattern cools off, the core of cold in the northern hemisphere looks to set up in Western Europe next week. The pattern for next week shows a -NAO setup (high pressure aloft over Greenland) while Western Europe deals with a trough digging across. This would result in a rather cold time of things in France, Germany, and the UK for the week of January 15th. You can also see on the Euro's ensemble map that southeast ridge cropping up along the East Coast, which may "protect" Philadelphia from getting in on a very cold setup while the Midwest and Lakes deal with the most cold.
The key to focus on in all of this is that the warmup this week is not lasting and that winter is on the way back. It may not be as cold as some think around here in ten days or two weeks but it's likely we won't be talking about 60's the weekend after next.