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A Study of Heavy Snowfall Pattern Recognition for Bozeman, MT February 3, 2011

Posted by mikeheard in Uncategorized.
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By Megan Syner – National Weather Service Great Falls, MT
Forecasting winter weather in the northern Rockies is a consistent challenge, given the diverse climates and topography across the region. Over southwest Montana, sparse observational and radar data combined with steep terrain create particular difficulties near Bozeman, one of the largest population areas in Montana. Official observations for Bozeman are taken at the Belgrade/Bozeman airport, situated in an open valley. This is approximately six miles west of the city center, which sits at the base of the Bridger Mountains. Given the terrain differences between the official observation and downtown area, there can be dramatic variations in snowfall amounts.
One method meteorologists utilize to improve snow forecasts is to compare historical snowfall data for several locations with specific weather patterns. Five data points were selected within the Bozeman area including the official climate site at the airport, three cooperative observing stations and Bridger Bowl ski area located in the Bridger Mountains. Snowfall measurements and wind data at various height levels were analyzed for 50 two-day snow events for the period of 1968 through 1995 and from 2000 through 2010 during the months of December through March. Initial findings suggest there are seven heavy snowfall pat-terns;
however, coarse model data and a limited dataset may only partially support these findings.
After the initial findings were identified, the data was re-analyzed with more emphasis placed on moisture, atmospheric stability and the jet stream. With further refinement using weather patterns over a larger scale, the dataset was narrowed from 50 cases to 28 cases that produced meteorologically defined heavy snowfall over a 48-hour period. The three primary patterns can be defined as southwesterly, northwesterly, and easterly flow.

Southwesterly flow predominantly favored heavysnowfall at the Gallatin Field Airport and the Bozeman area where sufficient moisture from the Pacific Ocean and atmospheric energy were common.

Northwesterly flow predominantly favored heavy snow at Bridger Bowl; however even though snowfall amounts were higher, there was lower moisture content in the snow.

Easterly flow, which is fairly common with the “Yellowstone low ”
events, favored heavy snowfall in the immediate Bozeman area, while heavy snowfall was not favored at the Gallatin Field Airport or Bridger Bowl.

Additional research on specific cases is needed for improved analysis and understanding. By identifying conditions for heaviest snowfall potential, forecasters in the Great Falls NWS office can improve snowfall forecasts and provide efficient impact-based decision support services.

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