Several issues still need to be resolved before we have a good idea about accumulations.

1) How cold will it be when the bulk of the precipitation falls? In the far western suburbs temperatures may fall below freezing fairly quickly but in the east they may not fall below freezing through the entire storm if some of the warmer model solutions are correct.

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2) How intensely will the snow fall during the storm? An extended period of light snow at temperature a little above freezing, as some models forecast, probably will not accumulate much. But intense precipitation at 32 degrees will.

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3) How quickly and how close to the coast will the low develop? The quicker it develops, the better our chances of getting plastered by a heavy, wet snow. The slower and farther off the coast the low develops, the more likely we are brushed by light snow that may not fall heavily enough to support much accumulations.

The models may not resolve these issues and questions for another couple of days.

Detailed model discussion

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The models have converged towards solutions that re-develop an area of low pressure to our south. The GFS, which predicted the low would form farther north yesterday, caved to the idea of the European and Canadian models last night.

By itself, the fact that models agree the coastal system will form to our south should increase our confidence of accumulating snow. But it’s not that simple.

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Last night’s models varied on how quickly the low would develop and whether a heavy band of snow would form somewhere over our area. The GFS was the most aggressive and suggested the potential for a fairly significant snowstorm over the mountains and our far north and west suburbs with lighter – still measurable amounts – near the city. The Canadian showed a similar heavy snow band, suggesting someone might get at least several inches of snow. The European simulated an extended period of light precipitation with the temperature staying above freezing for the city and points east. If that latter scenario played out, getting accumulating snow in the city would be tough to achieve.

Today’s GFS has backed off on its snowy look though it would still offer some accumulations for our far western suburbs. The forecast valid at 1 a.m. Tuesday (below) shows some of the problems that make this a difficult forecast. Last night’s GFS had the low a little farther south and deepened it a little more than this morning’s run. The location of the low is not far enough south to really pull in the cold air. The temperature in the city is still in the mid to upper 30s in this simulation, suggesting precipitation might begin as rain before changing to snow later at night (see top image in this post). However, there is colder air just to our north in Pennsylvania.

On this GFS run, the low quickly shifts eastward before really getting its act together. That’s a possibility, as the trough is deep but broad. The latter characteristic is usually not conducive to a low rapidly strengthening. More often than not, the low just squeezes out to the east and only gives us light precipitation.

Today’s European model (see below) has a stronger upper disturbance and is snowier compared to last night’s. I’ve chosen the wettest 6 hour period to highlight below but, even after, the model forecasts light snow to continue for an extended duration (we think the duration may be overdone). The model suggests there is potential for a moderate snowstorm for locations west of the city and somewhat lighter accumulations in and around the city.

You can get a feel for how much uncertainty remains concerning getting accumulating snow from looking at last night’s European ensemble members. The figure below shows the percentage of the 50 European ensemble members that predicted an inch or more of snow in the 24 hours ending at 7 p.m. February 9.