According to the NAEFS ensemble for the Christmas week we expect below normal temperature over the Pacific NW stretching east to the northern plains, above average temperatures in a zone stretching from TX and Arizona northeast thru the Southern Plains, Midwest and Upper Midwest, and near normal temperatures elsewhere. This is one of those averaging patterns that conceals more than it reveals as a longer term pattern shift takes place over North America, setting the NE up for a wintery period starting next Thursday-Friday that is likely to carry on through the end of the year.

This week is very active weather-wise. A complex low pressure area over the upper Great Lakes will drift north and weaken. Its in phase currently with a frontal system supported by the subtropical jet stream arcing from southwestern TX northeast to the wave apex over southern Ohio connected to a warm front extending east out to the central eastern seaboard. Along the trailing cold frontal boundary an upper system over southeast Colorado this morning will lift and deepen along the front, becoming by Tuesday morning a moderate strength Northeaster centered over NYC. The subtropical front by this time will have cleared most of the eastern seaboard, arcing SW over central Florida and leaving cooler conditions in its wake.

Meanwhile, by Tuesday afternoon ,a new Colorado low will begin to develop just east of the front range. This more potent system will move steadily NNE to lie over northern Michigan by Thursday evening. By that time its very strong cold front will lie from the low centre through western PA, down thru the western Appalachians and will enter the Gulf near Apalachicola. By Friday afternoon the low will be stalled over Georgian bay and very cold air will continue to plunge into the upper Midwest, Ohio Valley and the east from New England to northern Florida. Very strong lake effect snowstorms can be expected Friday eve and thru Saturday south and east of all the Great Lakes. Temperatures will slowly moderate over the deep SE thru the weekend but the NE will remain cold...indeed most of the NE has a white Christmas in store.

Over the west the next in series of low pressure systems will bring strong to very strong wind, rain and mountain snows to the Pacific NW north of central California this afternoon through Monday. Some weak ridge building will follow, with the next system poised to hit the NW Wednesday. Some Santa Ana wind and coastal warning is likely over southern California Thursday as high pressure builds over the Great Basin in the wake of the Colorado low, but the rest of the Pacific coast, the Rocky Mountain States and the Northern Plains will be cool to seasonably cold thru the week with significant Mountain snows.