|Alberta Clipper and Southern Plains-Type|
Snowstorm Development and Forecast Challenges
in Southwestern Wisconsin
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.
Climate Prediction Center Internet Team. 2005. El Nino and La Nina - Related Winter Features Over
North America. Camp Springs, MD: Climate Prediction Center.
Climate Prediction Center Internet Team. 2005. Frequently Asked Questions About El Nino and
La Nina. Camp Springs, MD: Climate Prediction Center.
Craven, J. 2008. Why is it snowing so much in Southern Wisconsin this winter?
Milwaukee: National Weather Service Milwaukee/Sullivan, WI, Weather Forecast Office.
Heidorn, K. 2005. Nor'easters and Alberta Clippers. From The Weather Doctor Website.
Henson, B. 2000. Alberta Clipper. From The Weather Notebook Web site.
Kapela, R., J. Wood, B. Borghoff and M. Kavinsky. 2008. 2007-2008 Winter Season Snowfall -
One For the Ages (Updated 3/10/08). Milwaukee:
National Weather Service Milwaukee/Sullivan, WI, Weather Forecast Office.
Lydolph, P. 1985. The Climate of the Earth. New York: Roman & Littlefield Publishers.
Moran, J. and E. Hopkins. 2002. Wisconsin's Weather and Climate. Madison:
The University of Wisconsin Press.
National Weather Service. 2008. Personal communication. La Crosse: National
Weather Service La Crosse, WI, Weather Forecast Office.
Palmer, C. 2005. Alberta Clippers reinforce cold air. From USAToday Web site.
Robinson, P. and S. Robinson. 2008. Geography 111, Weather & Climate Lecture Slides.
www.unc.edu/courses/2008spring/geog/111/001. Chapel Hill: University of North Carolina.