With the Washington, D.C. area and much of the Eastern U.S. in the midst of an unseasonably cold stretch, we turn our attention upstream, where changes in the Pacific Ocean tell the story of the current cold snap and provide clues as to what’s next. The past few weeks have seen an evolution in the weather systems crossing the Pacific, and all of us are now feeling the effects of that pattern change. It’s a change that will be relatively short-lived, however, as the weather models portray a warming trend after the weekend.
This cold snap is due in part to the atmospheric circulation pattern in the Pacific, which, as the largest ocean basin, stores the most heat and energy. Thanks to an energetic series of storms crossing the North Pacific, the jet stream – a fast-moving current of air along which waves of energy travel – has lifted far to the north into Alaska. Cold continental air descends down the “slide” from Alaska and the Northwest Territories of Canada into the United States, including the East Coast.
The air mass descending from the high latitudes is called continental polar, or “cP” for short. It is labeled as such because of the source region and characteristics of the air. A cP air mass is dry and stable, and also very dense given its extremely low temperatures. When these air masses move over warmer surfaces – which happens as they travel south through Canada and the U.S. and encounter decreasing amounts of snow cover and a higher sun angle, among other factors – they undergo a process of modification, warming in the lower layers of the atmosphere and losing stability. Cloud cover and, sometimes, precipitation accompany the leading edge of the cP air mass.
In our warm spells, such as we experienced early in October, the dominant air mass is maritime polar, abbreviated “mP.” The air flows into the U.S. from off the northeast Pacific Ocean, and can wring out a lot of moisture over the mountains of the Northwest. Because mP air masses are warmer to begin with, and cover plenty of land from the West Coast to the East as they modify, they don’t tend to drop our temperatures much. For example, after a period of strong warmth directed into our region from out of the South, a cold front crossing our area with mP air in its wake would perhaps cool temperatures from 10 degrees above normal to near normal or a few degrees above.
Now, the D.C. area can also fall under the influence of a mP air mass from the nearby Atlantic Ocean; think back to two short weeks ago when we received our drenching rains. mP Atlantic air is a less frequent visitor, though, as weather systems move west to east and deliver new air masses from the north and west most commonly.
Going back to that hot early-fall period, the Pacific pattern was aligned favorably for mP air masses to ride east into North America and downstream into the Eastern U.S. The jet stream was very strong, feeding off of the large temperature difference (or gradient) between cold Alaskan air and much warmer air over the open ocean closer to Hawaii. Storms forming along and near the gradient entered the Western U.S. and forced hot air from the South to move north and east into areas downstream of the Mississippi River. A high pressure heat dome developed within these areas, producing consecutive days of 80- and 90-degree temperatures in late September and early October.
During the ensuing period (Oct. 6-20), features in the Pacific evolved. Cold air sprawled across Alaska pulled back west to over the Aleutian Islands and Bering Strait, forcing oceanic warmth to push northward into northwest North America. The cP air mass then had a passageway from the Polar region to the U.S., initially sinking into the Rockies and Plains, but penetrating the Midwest mid-month and finally reaching the East in the last few days.
As of Wednesday morning, the cP air mass had revealed itself in the form of temperatures 10 to 15 degrees below normal throughout much of the Midwest, Ohio/Tennessee Valleys, and mid-Atlantic.
Though the frosty winter-like air has arrived and will stay for awhile, model guidance suggests it will not stick around in this more intense form forever. The northeast Pacific will become more active again with fast westerly flow replacing benign weather along the western coast of North America. That should act to draw in milder mP air from off the ocean and, by the middle of next week, warm temperatures toward normal or a few degrees above (mid-upper 60s) across the D.C. area.