National Weather Service United States Department of Commerce

Frigid Airmass to Bring Significantly Colder Temperatures; Snow Continues Downwind of the Great Lakes

A frigid cold airmass will bring significantly colder weather across the eastern two thirds of the country early this week. Record low temperatures are expected in the Southeast by Tuesday morning. Moderate to heavy lake effect snow will continue downwind of the Great Lakes through Monday. Several inches of snow accumulation is expected, including in portions of the Chicago Metro Read More >

Automatically generated image showing areas of energy helicity index.

 

Thumbnail of an automatically generated image showing areas of energy helicity index. Thumbnail of an automatically generated image showing areas of energy helicity index. Thumbnail of an automatically generated image showing areas of energy helicity index.
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EHI 

The Energy Helicity Index (EHI) is a number which represents the combination of instability and storm relative helicity.  Our calculation uses mixed layer CAPE (surface to 3000ft average parcel) and 0-3 km storm relative helicity.  Values greater than 2 or 3 have been correlated to cyclonic supercells with increased tornadic potential.  Negative values are indicative of an environment favorable for anti-cyclonic (left moving) supercells, but tornadoes associated with anti-cyclonic supercells are extremely rare.

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