ABUNDANT DECEMBER COLD THREATS AHEAD

The first month of meteorological winter is December. Average highs drop into the 20s and 30s across the north, 40s and 50s across most of the south, and 60s and 70s still in place across portions of the Desert Southwest and the immediate Gulf Coast areas. December features the fewest daylight hours of the year with 8-9 hours across northern areas and 9-10 hours in the South.

November began with near to above normal temperatures in place for much of the US. This was interrupted by a cold front bringing a shot of below normal temperatures that impacted the Central US over November 8-11 and the East by November 10-13. The trough that provided this cold shot got stuck across eastern Canada during mid-November due to a ridge developing near Greenland. This kept a cool pattern in place across the Northeast through the third week of November before becoming trending closer to normal. The remainder of the US saw a substantial mid-November warmup occur, with the core of this warmth impacting the Rockies through the Plains and the South. Temperatures routinely became 10-20 degrees above normal in these regions, while the magnitude of the warmth was less potent along the West Coast and near the Upper/Middle Mississippi Valley. This anomalous warmth led to the Rockies and the Plains to see temperatures average 5-8 degrees above normal for November. November should end on a colder note for many in the Central/Eastern US, as multiple cold fronts usher in below normal temperatures to end the month. The overall dry pattern that has plagued the Midwest through the South and East largely continued into November as most areas saw below normal precipitation for the month. Driest anomalies for November favored the Southeast. One area that did see above normal rainfall and mountain snow this November was across California through the Desert Southwest. This was due to a trough bringing multiple systems into Southern California during the November 13-22 time frame. Los Angeles recorded over 4.50″ of rainfall during this stretch. This was only the second time since 1967 that Los Angeles recorded more than 4″ of rain in November. The pattern became more active during the final third of November across the Central/Southern Plains, especially from Kansas to North Texas, which led to above normal precipitation for the month. These late month systems would bring the first real threat of significant snow to the Midwest over the final few days of November.

The chilly end of November will continue into the start of December. A -EPO ridge will form near Alaska, directing Arctic-based air towards the US during the first week of the month. The coldest anomalies should target portions of the Midwest with a fresh snowpack, but cold air will spill across the East during the week with the cold shots impacting the South being shorter in duration. This will contrast with a developing warm pattern in the West. The Alaska ridge should retrograde towards the Bering Sea and far eastern Siberia by the second week of December. This should limit the risks for Arctic air to infiltrate the US. With a ridge building across the South, this should provide a warmer period across the Southern Plains and through the Southeast during week two that should linger into the start of the third week of December. A trough draped over Canada should maintain a cooler overall pattern across the Midwest through the Northeast, but the magnitude of this cold should not be as potent as earlier in the month and there could be brief periods where milder temperatures extend northward. After focusing across the western Pacific during the second week of December, tropical forcing should race towards the Americas/Atlantic Ocean during the third week of December and continue through the Christmas holiday. This should allow for the Alaska ridge to return, providing increasing risks for one or two distinct shots of Arctic air to dive into the Central/Eastern US. The Southeast US ridge should weaken during this time, allowing for cold to dig into the South more efficiently than the cold spells in early December. The most intense cold should favor the Upper Midwest. Temperatures should lean above normal in the West for this period, with warmest anomalies in the Southwest. Cold threats may ease late in December as tropical forcing moves away from the Atlantic and towards the Indian Ocean, with a warmup first likely to occur across the South while the reaction should be more delayed across the Midwest and the Northeast.

An active pattern is on tap to begin December, with two distinct storm tracks to monitor. The further south of these tracks looks to be from the Lower Mississippi Valley towards the Mid-Atlantic, which will primarily fall as rain in the Southeast but could bring wintry precipitation to the Ohio Valley and the northern Mid-Atlantic at times. The other primary storm track will be further north with clipper systems bringing snow chances across the Upper Midwest through the Great Lakes/Northeast. With the cold overhead, lake-effect snow will occur at times off the Great Lakes. The first two weeks of December should also feature multiple systems bringing rain and mountain snow to the Pacific Northwest and the Northern Rockies. The Southwest through the Southern Plains should miss the bulk of the precipitation with these early month systems. The southern storm track may slide further north into the Midwest and the Northeast during week two as the Southeast ridge attempts to expand. This could allow for a system or two to bring more organized snows to portions of the Midwest and potentially the Northeast. With the risks for Arctic air increasing later in the third week of December and continuing into the final third of December, this should shift the storm track back further south. The favored storm track should be from the Lower Mississippi Valley through the Mid-Atlantic, and this could provide the best opportunity for wintry precipitation to impact the South this month. This will also bring increased chances for a winter storm system to impact the Northeast. In addition to any storm systems, the increased cold could lead to significant lake-effect snows to develop off the Great Lakes. As this occurs, the pattern should turn drier across the Plains with weak systems producing localized light snowfall becoming more likely than a larger, stronger system.