Alberta Clippers are low pressure areas seen frequently in winter, although they can occur anytime from mid October to March. These are called “Clippers” as they are usually very fast moving and can follow one after another. They originate in the Alberta province of Canada and dive southeast into the central U.S.
The upper level pattern required to produce these systems will have an upper level ridge of warm air aloft over the northeast Pacific and far western North America and a low pressure trough and cold air aloft over the central and eastern U.S. This produces a strong jet stream flowing south and southeast into the U.S, and is called a northwest flow. Exactly how far east or west the upper level ridges and troughs are located determines the track of the clippers descending into the U.S. If these features are further west, more of the country is impacted by the clippers, or if further east, the clipper’s influence may be limited to the Great Lakes and Northeast.
The northwest flow is often the result of a strong positive Pacific North American (+PNA) weather pattern.
This is a simplified classic depiction of a Positive Pacific North American (+PNA) weather pattern and some general effects. This is just a generalized description as each pattern’s configuration will be different.
The next few graphics show the formation and life cycle of a “classic” Alberta Clipper. Each jet stream configuration and clipper will be different, these are only generic representation
Conditions come together to set the stage for the clipper’s formation:
With a ridge of mid and upper level high pressure over the eastern north Pacific and western North America and a trough of low pressure over the central and eastern U.S., northwest flow is in place over the Midwest and Plains. Incoming Pacific storms are carried northeast toward Alaska with the trailing front dissipating to the south. Westerly cross-barrier flow (over the Canadian Rockies) begins to develop a lee trough on the high Plains of Alberta.
Here is a close up of where the clipper low will form:
This is a cross section of the air cresting the Canadian Rockies and moving downslope into the high plains of Alberta. Downslope flow begins to warm the air near the eastern slopes (greenish-blue area), with an area of spinning air forming along the low pressure trough on the Alberta Plains.
The Clipper low forms and the frontal systems begin to take shape:
The Pacific storm and frontal system weakens and dissipates over the high terrain of the Rockies. The cross-barrier westerly flow continues and a low pressure area forms where the “spin” began in Alberta. Ongoing downslope flow spreads southward into the U.S., with continued warming of the air mass against the eastern slope of the Rockies. This causes the low pressure trough (lee trough) to transition to a warm front. The new low pressure area forces Arctic air to its north to the west, which will cause the old frontal boundary to strengthen and become a cold front.
The Clipper forms:
This is a cross section of the atmosphere showing the development of the clipper low from the spinning air over the high plains of Alberta.
The Clipper begins to move south into the U.S.
Northwest flow aloft carries the new storm system into the U.S. A now well-defined warm front swings out into the Plains, with very warm and dry air, heated and dried by the downslope flow, causing windy and warm weather to the south of the clipper low. To the north, Arctic air is flowing southward into the northern Plains. Left over Pacific moisture is squeezed out in the cold air causing light snows or light rain to develop along and north of the low pressure area’s path.
The clipper reaches the base of the upper level trough and begins to flow east:
As the Alberta Clipper exits the Midwest. cold air floods southward into the center of the country, depending upon the position of the jet stream. The warm and dry air mass south of the clipper low begins to modify, pick up a bit of moisture and cool off as it moves away from the Rockies and the downslope effect fades. Light rains and snows continue to the north of the low, with heavier lake effect lows possible if the Great Lakes are still open. Further north, the cold front becomes banked up against the Rockies, unable to push west.
The clipper reaches the end of the line — or does it?
As the clipper low reaches the coast, it can have several different outcomes. The low can dissipate offshore, track out to sea, or if jet stream conditions are right, can head up the coast as a major coastal storm. To the west, another Pacific storm approaches, ready to begin the process again. The clipper lows can follow almost one after another, with as little as a day between them; but for clarity, they have been shown much wider spaced here.
Alberta Clippers can follow right behind one and other with repeated light snows to the north and large day-to-day temperature swings to the south of the low’s path. The northwest flow is often the result of a strong positive Pacific North American (+PNA) weather pattern.
This is a generic representation of weather at the surface common with a strong positive Pacific-North American (+PNA) pattern. A deep trough in the central and eastern U.S. keeps a steady flow of cold high pressure areas dropping southward into the central and eastern U.S. The plains will see wider swings in temperatures as Arctic air alternates with warm and dry downslope flow, while areas further east remain consistently colder. This is a dry pattern overall, but there are exceptions. An active subtropical jet stream can undercut the western ridge and bring storm systems through the southern tier of states producing heavy snows for the Plains and Midwest occasionally. The patterns depicted here are just general representations, as each +PNA setup is different.