It’s the first full week of meteorological winter and winter’s cold is already poised to invade nearly the entire contiguous US. Although the cold is not expected to test any records, that almost the entire span of the lower 48 will be simultaneously encapsulated by colder than normal temperatures is unusual. In this active weather pattern however, the cold will not necessarily persist.
A messy upper-air pattern involving a train of relatively small but potent blips in the jet stream has resulted in Canada’s reservoir of cold spilling southward across the majority of the lower 48. Some of the individual systems that contributed to this pattern were responsible for flooding rains on the West Coast and a rare tornado outbreak in the Midwest. Monday afternoon, the jet stream wrapped uniformly around the southern tier of the US without buckling such that there was very little east/west variance in temperature.
The extent of the below normal temperatures is quite remarkable. The 18 UTC December 3 GEFS predicted mean surface temperatures between Monday and Friday to be below normal in every state except Arizona as presented below, courtesy of Tropical Tidbits. South Florida, western New Mexico, and southern Texas will also act as strongholds against winter this week.
Typically the wavelengths of atmospheric systems are short enough to permit warmer than normal temperatures over some areas of the country and colder than normal temperatures over other areas. Note that this plot represents temperatures throughout the week as a whole. Temperatures are expected to warm in the West midweek but not enough to bring the average temperatures for the week as a whole above normal, for example. While temperatures will certainly be cooler than normal this week as a whole, only parts of the Upper Midwest, Central Plains, and Rocky Mountains will average temperatures more than a few degrees below normal.
Climatologically speaking, this cold isn’t that rare either. As demonstrated below by the NAEFS (North American Ensemble Forecast System), a joint forecast system composed of several forecast ensembles from the US, Canada, and Mexico, nowhere is this cold within the fringes of climatological extremes. The plot below courtesy of the National Weather Service presents 850 mb (a few hundred meters above the surface in low-lying areas, at or just below the surface in the Rocky Mountains) temperatures for Wednesday afternoon as a percentile from 1979 – 2009 climatology. The 0th percentile represents a forecast colder than recorded in climatology and the 100th percentile represents a forecast warmer than recorded in climatology.
Note that this hour is the most extreme forecast for this week with respect to climatology and only over North Carolina are 850 mb temperatures expected to be within the coldest 2.5% of those between 1979 and 2009. Elsewhere in the Southeast and lower Mid-Atlantic, temperatures are only forecast to be in the coldest 10% of values in climatology. Throughout the rest of the country, Wednesday’s temperatures will not be particularly cold compared to climatology.
The core of the cold will be attributed to two high pressure systems sliding southeastward from Saskatchewan. The first will quickly slide from the Upper Midwest toward the Northeast through Wednesday night. The worst of the cold will follow the second system from the Northern Plains Wednesday night to the East Coast by Friday night. The funneling of cold from the second high pressure system will have important implications later this week. Just because the intensity of this cold won’t be exceptional doesn’t mean it can’t produce something exceptional.
A storm in the Northern Pacific Ocean is tracking southeastward toward southern California and the Baja Peninsula. Its associated upper-level low pressure system will invigorate a wave of low pressure in the Gulf of Mexico triggered by cold air flowing southward from the Midwest. As recently discussed in the Sunday Storm, the low pressure system is likely to submerge parts of eastern Texas and the Gulf Coast with half a foot of rain while delivering a moderate snowfall for parts of the southern Plains and middle Mississippi River Valley between Friday and Saturday night.
The storm could spread heavy snow much further south and east than typical storms, especially for early December. Uncertainty increases as the storm approaches the Atlantic Ocean, but the antecedent cold become a snowstorm for parts of the South, anywhere from Northern Georgia to the Washington, D.C. metropolitan area. There is also a slight chance the storm brings snow all the way to southern New England. The intensity of the system upon reaching the West Coast and the timing of a yet-to-develop northern jet stream feature in Canada late this week will be key to understanding the extent of the snow. Be sure to stay tuned throughout the week when we know more about this upcoming cross-country snowstorm.