Heavy Snow Climatology for the Blacksburg, VA CWA

for the Period 1993 through 2005

by Jan Jackson

Data was gathered for all heavy snow events for the Blacksburg County Warning Area, (CWA), during the period of November 1993 through March 2005. Heavy snow events were defined as any storm that brought warning level snow, (4 inches in the piedmont or 5 inches in the mountains), to at least a quarter of the CWA, (approximately 10 counties). There was no minimal coverage needed for any storm that produced 10 inches or more. A total of 32 heavy snow events were identified. The events were then grouped into synoptic categories based on the track of the low pressure system, (storm track), and the dominant lifting mechanism as observed in the data. Seven storm categories were identified, and are described below. All of the individual heavy snow events are listed with the category that they fit into, and each snow event has a link to a map of snowfall amounts from that particular storm. The events in each category were then combined to create a composite map showing the typical storm, (or low pressure system), track for each category. The storm tracks run through 3 or 4 days, and on each of these maps, the heavy snow in the Blacksburg CWA occurred mainly on day 2.

Cyclogenesis - Miller A

Characteristics: A cold high pressure covers most of the U. S. east of the Rocky Mountains. A cold continental air mass is flowing off the continent. There is a current of warm maritime air from a southerly or southeasterly direction in the western Atlantic associated with a more or less well developed warm high pressure, and opposed to the offshore flow of cold air, creating a front between the two air masses. Low pressure generally develops along the front in the Gulf of Mexico or off the southeast U.S. coast, and then tracks northeastward spreading precipitation over the cold wedge north and west of the surface front.

Heavy snow events that fit into the Miller A pattern:

December 20, 1993...January 3, 1994...January 6, 1996...February 16, 1996...March 7, 1996...January 27, 1998...March 15, 1999...

January 25, 2000...January 2, 2002...February 27, 2005...

Cyclogenesis - Miller B

Characteristics: There is an occluding or occluded primary low pressure in the vicinity of the Great Lakes, nearly stationary or moving northeastward. There is a shallow wedge of cold continental air and associated pressure wedge lying between the Appalachian mountains and the Gulf Stream, and the wedge line is oriented southwestward from a cold high pressure centered in eastern Canada. A warm maritime air mass is flowing northward against the sluggish cold wedge, creating and maintaining a stationary, or warm front between the two air masses. This produces precipitation spreading over the cold wedge, partially separated from the precipitation area of the primary low pressure. A secondary low pressure develops near the coastline to the southeast of the original low pressure along the warm or stationary front associated with the cold wedge. Because the cold air mass is shallow, the precipitation type with a Miller B often progresses from snow to sleet, freezing rain or rain. Wintry precipitation may even start off as sleet and freezing rain.

Heavy snow events that fit into the Miller B pattern:

December 29, 1997

Surface low pressure passes to the northwest of the CWA, with a frontal passage

Characteristics: A surface low pressure moving from west to east passes to the northwest of the CWA across WV or PA, and a trailing front progresses west to east across the CWA. Most of the precipitation occurs from convection, (showers or even thundershowers), with the frontal passage.

Heavy snow events that fit into the surface low passing northwest pattern:

March 3, 1999...December 20, 2000...

Closed, slowly moving, upper level low pressure

Characteristics: A closed upper level low pressure system is centered to the southwest of the CWA and is slowly moving east or northeast. Because of the slow movement, precipitation can last for a prolonged period. However, the heaviest precipitation occurs with strong dynamics, which occurs just north and west of the storm track.

Heavy snow events that fit into the closed upper low pattern:

March 19, 1996...March 20, 2001...

Northwest upslope flow

Characteristics: There is a strong northwest to north low level flow of cold air across the Great Lakes and into the southern to central Appalachian Mountains. Low level moisture from the Great Lakes combines with the upslope flow into the northwest facing slopes of the Appalachians and produces localized heavy snow.

Heavy snow events that fit into northwest upslope flow pattern:

February 3, 1995...March 8, 1995...December 5, 1997...March 6, 2001...December 19, 2003...January 23, 2005...

Overrunning

Characteristics: Surface low pressure passes well to the south of the CWA, or is very weak, and most of the precipitation occurs from the warm moist air overrunning the cold air over the CWA. Heavy snow from these patterns often occur in bands associated with a developing warm front aloft.

Heavy snow events that fit into the overrunning pattern:

December 6, 1995...February 2, 1996...January 17, 2000...January 29, 2000...January 19, 2002...December 5, 2002...January 25, 2004...

February 26, 2004...

Alberta Clipper system

Characteristics: A low pressure systems that develops east of the Canadian Rockies in Alberta, Canada. These low pressure systems move southeast into south central Canada or north central U.S. before they track eastward to the coast of North America. Occasionally, these clippers track far enough south to reach the southern and central Appalachians. Clippers bring moderate snowfalls to areas up to a few hundred kilometers north of where they track, and lighter rain or snow to areas to the south of the track.

Heavy snow events that fit into the Alberta clipper pattern:

November 13, 1995...January 11, 1996...January 19, 2000...

Conclusions:

• During the 11 winter seasons from November 1993 through March 2005, all but one of the heavy snow events occurred in the 4 month period from December to March.
• There was one event in November, (Alberta Clipper pattern).
• Not surprisingly, January had the most heavy snow events, with 12. December and March each had 7 events, and February had 5 heavy snow events.

Breakdown by Category:

• The Miller A and B storms, (10), were scattered throughout the 4 months of December through March, but with the majority, (5), in January.
• The Miller A and B storms also produced the heaviest snowfalls, with amounts greater than 20 inches in the mountains.
• Most of the Miller A and B storms also had widespread heavy snow amounts covering greater than half of Blacksburg's 40 county CWA.
• Overrunning was the only other storm category in which most of the storms had widespread heavy snow amounts.
• The Overrunning storms, (8), were confined to the December through February period, with the majority, (4), in January.
• The 2 events in the Surface Low Passing to the Northwest storm category brought fairly widespread heavy snowfall, but it was confined to the area from the Blue Ridge Mountains westward.
• Both of the Closed Upper Low category heavy snow events occurred in March, and were elevation dependant, with the heaviest snow in the highest elevations of the mountains.
• Of the 3 Alberta Clipper heavy snow events, one was in November, and two were in January. The November event brought heavy snow to the mountains, but the two January events brought fairly heavy snow into the foothills and piedmont areas.
• The six Northwest Flow heavy snow events were spread throughout the 4 month period from December to March, with two events each in December and March. These heavy snow events generally covered the smallest area, but often had amounts of 12 inches or more.