After nearly 60 days of above normal weather on the whole, we have recently entered a new pattern here in the Delaware Valley, one of cooler than normal temperatures. Unfortunately, things have been cooler to the point that we've also seen the storm track suppressed somewhat. This pattern is the result of three factors:
(1) The Buckling of the Alaskan Ridge just off the US West Coast. This has allowed warmer than normal air to buckle northwards into Western Canada and has allowed the jet stream to buckle north into the Yukon. The Pacific Jet, which was the dominant When that happens, the cold air has to move somewhere...so...
(2) A trough forms in the Eastern US. Cold air from Canada gets sent down and our temperatures drop. Depending on the strength of the trough, the position of the polar vortex, etc. we can see some decent storminess from the trough or maybe not. As a result of this current trough, a ridge has built off of Greenland which has allowed the NAO to go negative. Remember, a -NAO does NOT necessarily equate to snow but it does allow for colder air to set up in place.
(3) The subtropical jet, while weak winter, is a factor. Storms are developing and moving along but aren't timing right with the advance of reinforcing shots of cold air. Because the current Texas storm is moving as slow as it is, it will not be able to phase with the advancing cold wave coming down from Canada on Wednesday night and Thursday. Thus, the storm which the GFS model was the showing the other day will not happen. Instead, we get a suppressed and weaker storm which will move off of the Southeastern US Coast before strengthening in the open Atlantic, far away from us.
So, how long will the cold last? If the recent warm pattern lasted nearly 60 days, can we expect a similar lengthy cold shot? Not necessarily. Patterns tend to run in four to eight week cycles so the current cycle could last another three weeks, or another seven. Changes are caused by a number of atmospheric changes in the atmosphere above the Pacific and Atlantic that can lead to changes in your local weather down the line. There have been some hints at some modification in temperatures in about three weeks or so. While we may see temperatures rebound to 'normal' in Philadelphia, the overall pattern for colder weather may hold through at least the middle of February before we can talk about any changes to the pattern.
The next question that's being asked is...WILL IT EVER SNOW? It can but we need a few things to happen. At this point, it's about timing, timing, timing. With a -NAO in place, the weather pattern is colder which generally allows for snow chances to increase. However, you need a combination of a relaxing trough and a storm to develop at the right time to bring snow up this way. There are some hints in about a week and a half of a snowstorm POSSIBLY impacting us (think next Thursday/Friday as a possibility at this point since at least one computer is hinting at a storm) but to give definite specifics on what will happen is just guessworks on anyone's part at this point. Another hint being thrown out by this model is that temperatures in about two weeks might be even colder than what we are forecasting for this Friday. Again, it's two weeks out so confidence isn't high but if this storm does develop and it does snow in Philadelphia, we could see some very cold air in place for the first weekend of February.