Cyclonic storms affecting Southwestern Wisconsin in the winter generally follow two main storm tracks (Moran). The first storm track starts in Western Canada, near Alberta. These storms, known as "Alberta Clippers" or "Clippers", drop into the Dakotas and track eastward across the Great Lakes (Henson). The other storm track starts in eastern Colorado or New Mexico, drops south into the Texas/Oklahoma Panhandle region, and then curves northeast into the Upper Midwest (Lydolph). These Southern Plains storms also are known as "Panhandle Hook" -type storms, or "Panhandle Hookers" (Robinson). A third winter storm track (a "Gulf Low") starts in the Gulf of Mexico and also tracks toward the Upper Midwest. However, these storms generally track too far east to significantly impact Southwestern Wisconsin (Lydolph; Moran). The sensible weather and regional impacts from Clipper and Southern Plains storms are very different. In addition, recent research suggests that the development of a strong La Nina can intensify the impact of Southern Plains storms in the region. This paper will examine the forecast challenges of Clipper and Southern Plains-type winter storms in Southwestern Wisconsin. Alberta Clipper-type storms are the easier of the two storm-track types for meteorologists to forecast and for residents deal with. This is due to two main factors: their speed and their lack of significant moisture (Moran; Palmer). Clipper-type systems affect Wisconsin most frequently in mid-winter, and form on the lee side of the Canadian Rockies, either through lee-side cyclogenesis or the re-strengthening of a Pacific storm that has survived the track over the Rockies (Heidorn; Lydolph; Moran). Once formed, these cyclones are carried by the jet stream into Montana and the Dakotas (Heidorn; Robinson). Being so far from a source of moisture, these storms simply can't produce much snow. As a result, snow totals from Clippers tracking through Southwestern Wisconsin are light, and generally on the order of one to three inches (NWS La Crosse; Palmer). The most significant impacts of these rapidly-moving systems are strong winds (often 65 km/hr [40 mph] or more) and a blast of Arctic air that usually sweeps into the region in their wake (Heidorn). While these brisk winds can lead to reduced visibilities (through blowing snow) and bitter wind chills, the overall impact on the region is limited. These systems generally are easy to forecast in terms of storm track, snowfall amounts and post-frontal weather. At the other extreme are storms moving up from the Southern Plains, both in terms of forecasting difficulty and potential impact to the region. Southern Plains storms develop over eastern Colorado or New Mexico, and are most common in late fall/early winter or late winter/early spring (Lydolph; Moran). These storms drop south into the Texas and Oklahoma Panhandle region, are picked up by the jet stream, and directed toward the Upper Midwest and Ohio Valley (Craven; Moran). As Lydolph notes, the relative warmth of the Great Lakes in winter also aids in guiding these cyclonic storms north toward the Upper Midwest and then east down the chain of lakes. In addition, since these lows develop so close to the Gulf of Mexico, they entrain copious amounts of Gulf moisture and transport this moisture north with them (Lydolph; Robinson). Probably the biggest challenge in accurately forecasting Southern Plains storms is figuring out the exact storm track, which, in turn, has the biggest impact on resulting snowfall totals. Also, since these storms move more slowly than Clipper-type systems, they can drop heavy snow for a longer time. As a result, the impacts of these systems generally are felt over a larger area for a longer time (NWS La Crosse). As these systems move into the Upper Midwest, warm air and precipitation wrap cyclonically into the cold sector behind the low. Precipitation type changes from rain to all snow or a mix of snow, sleet and/or freezing rain (NWS La Crosse). If the low tracks closer to Southwestern Wisconsin, the band of heaviest snow often sets up just north and west of the region, closer to the Twin Cities. If the low tracks farther south, Southwestern Wisconsin often is in the bulls-eye of the heaviest snow band (NWS La Crosse). Heavy snow generally is the biggest problem when Southern Plains systems move into Southwestern Wisconsin (Moran). Unlike the one-to-three inches of snow resulting from a Clipper-type system, a half-foot of snow - or more - can result from a Southern Plains storm (Moran). The development of El Nino or La Nina conditions also seems to have an effect on Southern Plains storms affecting Southwestern Wisconsin (CPC). Recent research by Jeff Craven, Science and Operations Officer at the National Weather Service office in Milwaukee, Wisconsin, shows a correlation between strong La Nina winters (such as the winter of 2007-2008) and more intense storms moving out of the Southern Plains, both in terms of snowfall generated and numbers of storms tracking through the region (Craven). Normal La Nina episodes usually result in near normal winter temperatures and precipitation in Wisconsin, while the Ohio Valley tends to see more moisture and, potentially, higher snow totals (Craven). According to the Climate Prediction Center, moderate to strong La Nina episodes usually result in a more meridional jet stream over the U.S. and Canada, along with colder and stormier conditions across the northern states. The strong 2007-2008 La Nina contributed to a stronger than normal 500 mb jet stream, with an amplified trough of low pressure over the Rocky Mountains and a ridge of high pressure over the eastern states (CPC; Craven). This, in turn, helped focus and intensify Southern Plains systems moving toward the Upper Midwest (Craven). In addition, lower than normal surface barometric pressure along the Southern Plains storm track helped intensify cyclonic storms moving up from the Plains (Craven). As Craven notes, "Low pressure systems that track between Chicago and the Ohio River Valley tend to produce the heaviest snows in southern Wisconsin." When enhanced by a strong La Nina, this can lead to significantly heavier snow totals in the region. A further contributor to the increased severity of Southern Plains storms during the 2007-2008 La Nina winter was the increased Gulf Moisture brought north by the storms. This led to higher than normal 1000-500 mb precipitable water (PW) values across the Upper Midwest (CPC). This wetter than normal area extended into South Central Wisconsin, including Madison, Milwaukee, and the eastern edge of the Southwestern Wisconsin CWA (CPC). Finally, the 600 mb -12° to -18°C optimal snow growth isotherm along with the 850 mb -2° to -8°C optimal heavy snow isotherm were both shifted north to sit across Southern Wisconsin throughout meteorological winter (Craven). This slight shift north in higher PW values and optimal snow conditions contributed significantly to the stronger and more numerous Southern Plains storms affecting Southwestern Wisconsin during the winter of 2007-2008 (CPC; Craven). The resulting snow totals during the La Nina-enhanced meteorological winter of 2007-2008 are remarkable. Madison, WI, broke their all-time seasonal snowfall record, with more than 90 inches of snow (Kapela, et al.). Milwaukee had their fourth snowiest winter (84.5 inches), while La Crosse had their second snowiest winter (57.5 inches) (Craven; Kapela, et al.; NWS La Crosse). The forecast challenges posed by winter storms moving through Southwestern Wisconsin are numerous. While the light snow and blast of Arctic air from Clipper-type storms are relatively easy to forecast and generally seen as a nuisance, the heavy, long-duration snow of Southern Plains-type storms can be deadly. When a strong La Nina enhances these Southern Plains storms, as was the case during the 2007-2008 meteorological winter, it can lead to stronger, more numerous storms - along with a more challenging, and frustrating, winter for Southwestern Wisconsin meteorologists and residents. Sources Cited Climate Prediction Center Internet Team. 2005. El Nino and La Nina - Related Winter Features Over North America. Camp Springs, MD: Climate Prediction Center. www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/nawinter.shtml. 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