Significant snowfall is still expected Wednesday night into Thursday morning.
A major upper level wave is moving out of Colorado into western Nebraska and South Dakota. This wave is already producing a band of precipitation visible on satellite and radar.
As you can see on the HRRR model simulated 'forecast' radar, this wave will cause an intensification of snow as it races towards Minnesota after midnight. This is why we expect the highest snowfall rates after midnight into early Thursday.
When we discuss waves in meteorology, we're essentially discussing cool fronts aloft at various levels. When a ripple of cool air moves in aloft it creates instability (think: if you have warmer, more moist air and suddenly you cool the layer above it, this forces the air to rise). So, this wave will produce lift out ahead of it. BUT, cooling the air aloft not only creates buoyancy or lift in the atmosphere, it will also help to produce the 'perfect' snow flakes: DENDRITES!
Last night into today, the temperatures aloft have been better for producing snow flakes that accumulate at a 12:1 or 13:1 ratio (snow to liquid). Temperatures will cool almost 10 degrees (F) aloft tonight as this wave moves through, creating the environment for dendrites, or the big fluffy snowflake that piles up quickly at a ratio of up to 20:1 or 25:1... nearly double the rate of the past 24 hours.
NOTE the two forecast vertical temperature profiles below... I point to the area of interest, which is about 7,00 feet up... where the lift and snow will be produced:
So that's the long story, for why snow will pile up tonight.
TWIN CITIES calculations...
Most models are producing ABOUT 0.47" to 0.61" additional (liquid equivalent) precipitation from 7pm Wednesday through 1 pm Thursday... the ratios I've calculated/am forecasting come in at about 19:1... that gives us about a 7" to 12" snowfall range... 7" being the lower 10% chance and 12" being the higher end 10% chance. (AGAIN for the Twin Cities)
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