An Alberta Clipper, also known as a Canadian Clipper, is a fast moving low- pressure area that generally affects the central provinces of Canada, parts of the Upper Midwest as well as the Great Lakes regions of the United States. Most clippers occur in winter, but they can also occur outside of the season. Off-season clippers typically occur in November. Alberta clippers take their name from the Canadian province they appear to descend from - Alberta, Canada. However, the storms usually do not actually form in Alberta. Instead, they initially move into mainland Canada via the Pacific Ocean. Then they lose the majority of their moisture through the process of orographic lifting as they come into contact with the Rocky Mountains in the provinces of British Columbia and subsequently, Alberta. The storms arrive in the Canadian plains in a highly disorganized state and with very little moisture. The systems then slide southward into the Dakotas and Minnesota as they get caught up in the anticyclonic flow around the continental polar (cP) high-pressure system that inhabits central Canada. It is ultimately these cold-cored high-pressure systems that send the clipper systems steaming into the United States. It then arcs eastward across the Great Lakes before moving out into the Northern Atlantic Ocean. On this track, a clipper stays hundreds of miles away from the mild waters of the Atlantic or the Gulf of Mexico. This means that the average clipper is moisture-deprived, so it won't drop huge amounts of snow. Instead of two or three feet, it will leave just a few inches on a narrow track that skirts areas such as Milwaukee or Detroit. The storms move into an area at high speed over whatever land they encounter, usually bringing sharp cold fronts and drastically lower temperatures with them. It is not uncommon for an Alberta clipper to cause temperatures to drop by 30°F (16°C) in as little as 10 to 12 hours. Often, the storms bring strong winds with them, which only increases the effect of the already bitter temperatures. Winds in advance of, and during an Alberta Clipper are frequently as high as 35 to 45 mph. These conditions can cause wind chill values to drop into the -20 to -50 Fahrenheit (-30 to -45 Celsius) range. The strong, frigid winds produce true blizzard conditions, due to severe blowing and drifting snow. These conditions are largely due to the intense amount of cold air advection associated with these types of systems. Snowfall amounts with these systems tend to be small, on the order of 1 to 3 inches, as the small amount of moisture and fast movement intrinsic to the system inhibit substantial snowfall totals. However, several factors can combine to produce fairly substantial snowfall totals (6 inches or more). These factors include 1) access to more moisture (which raises precipitation amounts), 2) slower system movement (which increases snowfall duration) and 3) colder temperatures (which increases the snow to water ratio). The southern and eastern shores of the Great Lakes often receive larger than average snowfall amounts from Alberta Clippers during the winter, due to system enhancement from the relatively warmer waters found in the unfrozen lakes. The lake effect can add exponentially to the overall snowfall total of any given clipper. During the winter, Alberta Clippers can occur somewhat frequently, with system intervals on the order of every two to four days common during active periods. November 15th through April 1st marks the period with the highest probability of occurrence for these systems. Clippers are more common than usual during La Niņa years. During this period, the jet stream often dives south across the Great Lakes. During this pervious winter, the Lakes were one of only a few spots in the nation where people actually experienced a white Christmas, largely due to the repetitive formation of Alberta Clippers. Alberta Clippers sail under the push of a northwesterly jet stream. Often piercing outbreaks of polar air, known as the Siberian Express, which continue for days after the low has moved off, follows them. Strong northerly winds and bitterly cold temperatures leave behind dangerous windchills, ground blizzards and days of whiteout conditions where surface visibility is nearly zero. When a Siberian Express follows a clipper over the Great Lakes, major lake-effect snowfalls may strike the lee shores with its cold, gusty northwest winds. A clipper can even produce heavy snow over New England and Maritime Canada; after crossing the Appalachians, it becomes energized by warm Atlantic coastal waters and is reborn into a Nor'easter. Here in Dayton, Alberta Clipper systems tend to bring us only minor snowfall, typically in the 1 to 3 inch range. It is not uncommon to hear reports like this: "Three miles west of Dayton on highway 12, visibility was reduced to less than 150 feet with winds gusting to around 40mph near the city, reducing visibilities to around 500 feet." (Brian Davis - Live Doppler 2x Weather Blog - WDTN Television). A set of forecasting "rules of thumb" has been developed. These include, but are not limited to: 1) A surface low-pressure system should not develop until the upper level system passes I-75. That's good for small snow amounts because it keeps the surface low south and west of Dayton. 2) If a stronger/earlier surface low develops, it is likely to come farther north - shifting the prime snow northward as well. 3) If a trend in the GFS model occurs, with or without a primary surface low, it is possible that we could get a mix of snow and/or sleet. That would obviously hurt accumulations. 4) If the primary vort max supporting this system looks like it will be running out of steam by the time it should be making our snow AND it is too far north - snow production will be at a minimum. The bottom line is that there are many variables when it comes to forecasting for Alberta Clipper systems as they encroach upon the mid-west. The nature of these disorganized systems, coupled with the broad inconsistencies in the topographical regions that they cover in a relatively short period of time typically causes model data to have a large amount of uncertainty. Each forecaster has his or her own forecasting technique, each tailored to each specific forecast area. A skilled forecaster will know when it is time to abandon model output and start to forecast via satellite imagery and sounding data. But never underestimate the delicate balance of art and skill that forecasters must possess to be successful. Alberta Clipper systems demonstrate this metaphor perfectly. The following is a quote from the NWS discussion on 5 February 2007: "WILL HAVE TO MONITOR THIS SYSTEM CLOSELY AS PREDICTING SNOW AMOUNTS WITH CLIPPER SYSTEMS CAN SOMETIMES BE VERY CHALLENGING. ALTHOUGH AMOUNTS ARE EXPECTED ON THE LIGHT SIDE IT COULD CAUSE BIG IMPACTS ON HIGHWAYS AS TEMPERATURES WILL BE VERY COLD (ONLY ABOUT 20F) AND SNOW WILL BE JUST IN TIME FOR THE WED MORNING RUSH HOUR." I include it because it truly exemplifies the forecasting hurdles and variables clipper systems bring to each and every one of us. |