It has been very hard to get a grasp on the long range weather pattern. Every run has a different solution on how the change is going to take place and what magnitude will the change be. Each run places emphasis on one ridge verses one trough. There is also the question of Hurricanes Ignacio and Jimena and how far north they move and the get absorbed into what could be a strengthening upper low in the Gulf of Alaska. These factors keep changing and as a result the models keep changing on the overall long range outlook.
For example here we have the upper flow from last night’s gfs run verses today’s gfs run. Notice the stark difference in the upper flow over Southern Canda as we go from 2 weaker troughs to now one stronger one that splits the difference. The short term implication of this would be that the next cold front will be slower to get to the east coast…perhaps not until late Friday instead of late Thursday. Last night’s run implied the front could stall out and a wave could form bringing perhaps a period of (gasp) rain for us. Now it just pushes it through and done with cooler air for next weekend. The magnitude of the cool air mass is stronger than the one we are getting tonight and Saturday and could last a few days longer. However it makes forecasting from day to day a hair pulling experience which you usually save only for snow events!
This trough takes its time moving out and by Tuesday 9/15/15 it pulls out and the ridge in the east begins to build again.