Snow and Sleet Tonight

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The first (real) snow of the season is always an exciting time for us weather people. It looks like for many areas of inland Connecticut we will see our first legitimate snow and sleet of the season tonight. A cold front moved by early this morning after temperatures spiked well into the 50s last night and now a follow up upper level system will swing through Connecticut tonight.

The question will be how quickly do temperatures between 5,000 and 10,000 feet above the ground cool off. As it stands right now they are quite mild – about 45 degrees!

So what are the mechanisms for cooling the air up above our heads? There’s 2 ways to do it in a case like this. One, colder air can be advected in (blown in by the wind, essentially) from the northwest. This is happening, albeit very slowly. If this was the only mechanism for cooling we wouldn’t get the job done in time for snow. The second mechanism for cooling with adiabatic cooling in the atmosphere from lift. As the upper level energy approaches Connecticut from the southwest (what’s bringing the ice storm to Oklahoma, Arkansas, etc.) air will begin ascending over the state. Rising air results in clouds and precipitation but also helps cool the atmosphere.

As usual, we’re straddling the fence between snow and no snow with this one. The NAM, one of our highest resolution models, is the coldest. The NAM can explicitly resolve vertical motion in the atmosphere and can frequently be too overzealous. Exaggerated lift means exaggerated cooling.

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This is for Bradley Airport valid at midnight. I’ve drawn a large arrow to draw your attention to the 0ºC isotherm. The red line is temperature and the green line is dew point. On the NAM the entire column is below freezing by midnight and this is a snow sounding!

Our other models are, predictably, a bit milder though the trend has been for them to come in a bit cooler as we approach the event. The 9z short range ensemble forecast snowfall probabilities show a nearly 100% chance for >1″ of snow northwest of Bradley along with a 50% or so of >4″ of snow.

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For my snowfall forecast I’ve massaged down the 4″ snow probabilities but kept the 1″ probabilities fairly similar to the SREF. The biggest reason for doing so is that the European model remains on the mild side of the spectrum (compared toe the NAM, especially) and I do think there will be some sneaky warm layers that may hold the precipitation type at sleet for a while.

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The heaviest precipitation will fall between 11 p.m. and 4 a.m. and the transition from rain to sleet to snow will occur from northwest to southeast.

One caveat here is that if the thump of lift in the atmosphere winds up coming in a bit more impressive than currently progged and we do flip to snow quickly I would not be surprised if some areas wound up with more than I’m currently forecasting. The models have also been consistently showing a MAUL (moist absolutely unstable layer) near the snow growth region (-15ºC) that could produce an impressive period of short-duration snow. We’ll see how this one plays out!

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