The first week of December was quite mild in the I-95 corridor with temperatures averaging 6.5 degrees above normal in Philadelphia, 6.2 degrees above normal at Central Park in New York and 3.1 degrees above normal at Reagan National Airport in Washington, D.C. This week also looks relatively mild with high temperatures likely reaching or even exceeding the 60 degree mark in parts of the region by this weekend. Looking ahead, however, there are signs that the upper atmosphere pattern will change to allow for more normal cold to arrive as we progress through the second half of December.
Upper-level high pressure ridging has controlled the weather scene in much of the eastern half of the nation during the past few weeks and will continue to do so in the upcoming week. The focus of this anomalous high pressure ridging should gradually push northward over the next couple of weeks and this will allow for colder air to “undercut” from the south. It appears as if cold air will spill across the polar region as we progress through the second half of December and push first into the western US with below normal conditions. Later this month, the colder air confined to the western US will be able to push eastward given the expected retreat of the high pressure ridging in the Northeast US. The end result will be temperatures much closer to normal by the time Christmas and New Years roll around.
The 00Z GFS Ensemble 500 millibar height anomaly forecast map (above) shows the movement of the upper-level ridge from its current position as indicated by the arrow well to the north into Canada ten days from now and this will allow colder air to “undercut” and trek eastward from the western US into the eastern US later this month. The forecast maps below show the 5-day temperature anomalies for the periods 12Dec-17Dec (left) and 27Dec-01Jan (right) as depicted by last night's CFSv2 model run and the well above-normal temperatures in the Mid-Atlantic region are replaced by more normal cold conditions by the end of the month.