So Cold – But Why?
The biggest reason for the cold across the eastern and central US is a response to abnormal warmth in the western US, western Canada, and Alaska. The ‘Alaskan ridge’, or abnormal warmth several miles above eastern Alaska, Yukon, and the Pacific coast typically leads to an equal but opposite reaction to the east–known as a ‘trough’, or abnormal cold several miles above the eastern and central US. This ridge-trough pattern then proceeds to repeat and encircle the globe (think waves on the ocean), creating areas of abnormal warmth and coldness throughout the Northern Hemisphere.
On average, these temperature abnormalities above the surface are reflected at surface level thanks to the storm track arcing over the ridge and under the trough. Low pressure going through Alaska or western Canada pulls warmer air north into Alaska and prevents much rain from reaching south to California. The storm track then runs southeast through Alberta (hence the name ‘Alberta Clipper’ for systems following the track), into the Plains, and close to the East Coast pulling down cold air dislodged from its formation area in northern Canada/Alaska. To see the effect of removing that cold air, consider that Fairbanks, Alaska has highs of around 15 degrees and no sub-zero lows forecast this week. Compare this to normal highs around 2 and normal lows around -15. In a normal cold air outbreak, plentiful snow cover in the eastern and central US will prevent the ground from absorbing sunlight and re-radiating that energy to warm the air passing overhead. The unique aspect about this cold wave is that snow cover is only near average for early January–instead, the cold air is intense and fast-moving enough to sustain its intensity well into the southern US despite passing over bare ground south of the Ohio River. Where there is snow north of the Ohio River, it’s just that much colder. The temporary snow cover in the Southeast is similar, creating a few nights and days colder there than would occur without the snow.
All of this works well to chill the Plains, Midwest, and Gulf Coast, but the eastern US has one more piece to solidify the deep freeze there. The storm track arcing back north along the Atlantic coast towards Greenland allows any low pressure following the track to feed off of the temperature difference between the cold air over land and warmth along the Gulf Stream. This leads to near-perfect conditions for rapid strengthening of the low pressure allowing it to pull in additional cold air from the Hudson Bay region in Canada as the low pressure heads out over the ocean. Whether or not the low pressure lines up right to produce snow at the coast, the cold air is still getting there.
What about everyone’s favorite wintertime phrase, the ‘polar vortex’? In this case, it appears to be nudged a bit off the North Pole from below by the pattern that is creating the Alaskan ridge–keep in mind that the polar vortex exists more than 10 miles above the ground, while the ridge-trough pattern is most prominent 5-10 miles above the ground. That nudging helps to direct the cold air out from under the polar vortex further south into the US, but the polar vortex itself is still stretched from eastern Canada across Greenland to Siberia.
What Does This Mean for California and the Southwest?
Finally, this setup does no favors to California and the Southwest. The storm track being deflected well to the north prevents rain from reaching those regions, enhancing the fire danger in this normally wet season. These dry areas will need to hope that the Alaskan ridge breaks down to let the storm track run off the Pacific or that there can be low pressure that sneaks south of the ridge, bringing a more active storm track to northern Mexico that can waver into California. Some may recognize bits of La Niña in the results of this pattern (dry California, cold northern US)—we are currently in a weak La Niña state and should remain in one until spring. La Niña may help a bit to set up the pattern, but the fact that we were in a similar state last year should remind everyone that this cold should not be expected from every La Niña.