Brett Anderson

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Severe Thunderstorm Threat

June 4, 2012; 2:22 PM ET

An unusually strong, upper-level storm system dropping into the Pacific Northwest by tonight will collide with hot and increasingly moist air surging northward through the High Plains during the day Tuesday. This will likely lead to an outbreak of severe thunderstorms over portion of the southern Prairies later Tuesday.

The greatest risk for damaging winds, large hail and perhaps a tornado will be from southeastern Alberta to extreme southwest Saskatchewan through Tuesday evening. Storms will be slow-movers, so some spots could get a heavy dump of rain with flash-flooding.

I just looked at the latest data for today and compared it to Tuesday, and while I do expect to see some strong, hail producing thunderstorms late this afternoon and into the evening today over extreme southeastern BC and southern Alberta, the overall threat for damaging storms looks greater for Tuesday farther east.

Why is that?

1. Greater heating and instability on Tuesday.

Note the surge of +30 C degree temperatures into southern Prairies Tuesday afternoon (below).

The lifted index forecast (below), which is a measure of stability, shows a large area of -5 to -9 values into the Prairies. Negative values are unstable. When the values get less than -5, it means that the atmosphere is very unstable, which is what you need for strong thunderstorms.

2. Greater influx of low-level moisture for Tuesday.

The dewpoint (degrees F) forecast for later Tuesday shows a tongue of moist, surface air (55 F and higher) covering southeastern Alberta and southern Saskatchewan. Higher dewpoint air is critical for the formation of large, long-living, severe thunderstorms.

One of the best tools to measure the potential for tornadoes is the Energy Helicity Index, or EHI. The EHI combines two measurements, one that looks at instability and the other low-level wind shear. An EHI value of 1 or higher indicates the potential for supercell thunderstorms, while values generally higher than 2 support tornadoes. The higher the EHI, the stronger the tornadoes. EHI values reaching 5 or higher can signal the potential for EF4 or EF5 tornadoes.

The NAM model EFI forecast for late Tuesday afternoon below shows a few areas in Montana and southern Saskatchewan with an EHI value of 1 to 1.5 (white areas)

3. Pacific trough gets stronger Tuesday.

Image below shows the predicted 500 mb level heights for Tuesday afternoon. The trough (pocket of cold air aloft) is centered from western BC down to southwestern Idaho. The NW-SE orientation of the trough means the trough is negatively tilted, which means the trough is at maximum strength. A NE-SW orientated trough is a positive tilted trough and is generally weak.


Several of the images that I use are available on the professional site.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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Brett Anderson
Brett Anderson covers both short-term and long-term weather and storm forecasts for Canada in this blog for