For those who have been anxiously awaiting a weather pattern more typical of December, this is the change you've been waiting for. Finally, the jet stream winds aloft that steer our storm systems and separate cold air from warm, are settling southward for a more extended duration. As always, click images to enlarge.
The impact of a southward settling jet stream on temperatures nationwide in the 8 to 14 day period will be a widespread cooling. Though I've indicated near normal temperatures for most of the Eastern United States for the period, this is largely because I'm expecting above-normal warmth at the start of the period to be offset by equal but opposite below-normal cool air for the latter part of the forecast period. You may remember that my monthly forecast for December marked this period as a transition to what I expected to be shots of intense cold in the final week of the month, and I still expect this forecast to hold, as the flip to below normal temperatures at the end of this 8 to 14 day forecast period should only be the beginning of the pattern, which maximizes cold between Christmas and New Year's.
From a precipitation perspective, this transition period should be like most weather regime changes are - chock full of potential storm development. The digging jet stream in the east will tap an open Gulf of Mexico, contributing moisture to the parade of Pacific disturbances racing east across the country. Feeding off the increasing baroclinicity resulting from building cold air, this will increase the propensity for storms in the east, and I've reflected that in the precipitation forecast for the period. What about snow? Certainly there will be substantial snow on the northwest side of the storm tracks, with exact track determining precisely where that axis of snow will fall, but this at least is enough to warrant about a 10% above normal chance of a white Christmas from Pennsylvania through Upstate New York into New England.
Incidentally, as we approach Christmas, here is the percent chance of a White Christmas across the country: