|Alberta Clipper Systems as They Pertain to Dayton, OH
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.