The evolution of D.C.’s cold snap: when do we break out of it?

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.

Hemispheric weather pattern at 500 mb (~18,000 ft above the surface). Solid black line shows the path of storm systems (steering flow), with arrows indicating direction of movement. The high-pressure area (blue H) off the West Coast is routing storm systems (disturbances) into Alaska. As these disturbances slide down the eastern side of the high-pressure cell, very cold maritime Polar (mP) air masses flow in behind the disturbances throughout Canada and, then, throughout the Central and Eastern U.S. The area of low pressure (red L) near the Great Lakes helps to maintain the mP air mass as it drives the steering flow to the south of D.C. Once this northeast Pacific/Alaskan high-pressure cell weakens next week, warmer maritime Polar (mP) air from off the Pacific Ocean will take a direct path into North America and eventually rout out some of the winter-like chill over the Central and Eastern U.S.

Hemispheric weather pattern at 500 mb (~18,000 ft above the surface). Solid black line shows the path of storm systems (steering flow). The high-pressure area off the West Coast is routing storm systems into Alaska. As these disturbances slide down the eastern side of the high-pressure cell, very cold polar air flows in behind the disturbances throughout Canada and then the Central and Eastern U.S. The area of low pressure near the Great Lakes helps to maintain the polar air mass as it drives the steering flow to the south of D.C. Once this northeast Pacific/Alaskan high-pressure cell weakens next week, warmer air from off the Pacific Ocean will take a direct path into North America and eventually rout out some of the winter-like chill.

Model forecast temperature departures from normal for Friday (blue and purple colors show the degree difference colder than normal, while the orange and red colors depict the degree difference warmer than normal). A widespread area east of the Mississippi River is forecast by the model to run 9 or more degrees colder than normal. Note that these cold departures increase to 12 to as many as 18-21 degrees below normal to the southwest of Washington, D.C. (Both maps provided by Penn State University e-WALL)

Model forecast temperature departures from normal for Friday (blue and purple colors are colder than normal, while orange and red colors are warmer than normal). A widespread area east of the Mississippi River is forecast to run 9 or more degrees below normal. Note that these cold departures increase to 12 to as many as 18-21 degrees below normal to the southwest of D.C. (Both maps provided by Penn State University e-WALL)

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.

Visible satellite image from late Wednesday afternoon. As the leading edge of the continental Polar (cP) air mass entered the Mid-Atlantic, our atmosphere became slightly more unstable with the cold air in the upper levels crossing over the warmer surface. Clouds developed in response to the destabilizing atmosphere, but lacked precipitation given the dry nature of the arriving cold cP air mass. (Intellicast.com)

Visible satellite image from late Wednesday afternoon. As the leading edge of the continental Polar (cP) air mass entered the Mid-Atlantic, our atmosphere became slightly more unstable with the cold air in the upper levels crossing over the warmer surface. Clouds developed in response to the destabilizing atmosphere, but lacked precipitation given the dry nature of the arriving air mass. (Intellicast.com)

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.

Illustration of the various air masses and their direction of movement. Continental Arctic (cA) air seeps southward into the U.S. occasionally, but not often, during the winter. Continental Polar (cP) and maritime Polar (mP) air masses are more frequent visitors, and can lower our temperatures in the fall, winter or spring. Maritime Tropical (mT) air can dominate our temperature pattern at any time of the year. (JetStream, the National Weather Service Online Weather School)

Illustration of the various air masses and their direction of movement. Continental Arctic (cA) air seeps southward into the U.S. occasionally, but not often, during the winter. Continental Polar (cP) and maritime Polar (mP) air masses are more frequent visitors, and can lower our temperatures in the fall, winter or spring. Maritime Tropical (mT) air can dominate our temperature pattern at any time of the year. (JetStream, the National Weather Service Online Weather School)

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.

Composite surface pressure pattern for the period September 20-October 5, 2013. Solid black line shows the path of storm systems (storm track), while the arrow indicates direction of storm movement. The large temperature difference (gradient) between warm Pacific air north of Hawaii and cold air in Alaska helped form a strong jet stream that drove mild maritime Polar (mP) air into North America. Note that the storm track lifted far to the north of Washington, D.C., which was warmed by a high pressure heat dome (yellow shading) and flow from the south. (NOAA/ESRL Physical Sciences Division)

Composite surface pressure pattern for the period Sept. 20-Oct. 5, 2013. Solid black line shows the path of storm systems (storm track). The large temperature difference (gradient) between warm Pacific air north of Hawaii and cold air in Alaska helped form a strong jet stream that drove mild maritime Polar (mP) air into North America. Note that the storm track lifted far to the north of Washington, D.C., which was warmed by a high pressure heat dome (yellow shading) and flow from the south. (NOAA/ESRL Physical Sciences Division)

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.

Composite surface pressure pattern from the period October 6-20, 2013. The pattern evolved in the north Pacific, with cold air retreating into the Bering Strait and Aleutian Islands, and warmth building into mainland Alaska and the Northwest Territories of Canada. High-pressure areas developed over these very cold regions and descended southward into the U.S., first into the Rockies and Plains, and eventually into areas east of the Mississippi River. (NOAA/ESRL Physical Sciences Division)

Composite surface pressure pattern from the period Oct. 6-20, 2013. The pattern evolved in the north Pacific, with cold air retreating into the Bering Strait and Aleutian Islands, and warmth building into mainland Alaska and the Northwest Territories of Canada. High-pressure areas developed over these very cold regions and descended southward into the U.S., first into the Rockies and Plains, and eventually into areas east of the Mississippi River. (NOAA/ESRL Physical Sciences Division)

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.

Penn State University e-WALL

Temperature departures from normal on Wednesday morning. (Penn State University e-WALL)

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.

gfs 96h

GFS model forecasts for Sunday morning, October 27 (top graphic) and Wednesday morning, October 30 (bottom graphic). For Sunday, the model keeps high pressure and weak flow in place off the western coast of North America. Washington's cold pattern has yet to depart. Toward the middle of next week, however, storminess returns to northwest North America, as depicted by the tightly packed solid black lines (isobars, or lines of constant pressure). Wind speeds are high where the isobars are tightly packed. This change in the north Pacific pattern will change the properties of air masses in Canada, supporting less in the way of continental Polar (cP) air that can deliver another equally intense, winter-like cold blast to the Eastern U.S. (both forecast graphics provided by College of DuPage Weather Lab)

GFS model forecasts for Sunday morning, Oct. 27 (top) and Wednesday morning, Oct. 30 (bottom). For Sunday, the model keeps high pressure and weak flow in place off the western coast of North America. Washington’s cold pattern has yet to depart. Toward the middle of next week, however, storminess returns to northwest North America, as depicted by the tightly packed solid black lines (isobars, or lines of constant pressure). Wind speeds are high where the isobars are tightly packed. This change in the north Pacific pattern will alter the properties of air masses in Canada, favoring warmer weather for the Eastern U.S. (College of DuPage Weather Lab)

 

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