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    Sunday will be subtle

    By Weatherman

    I'm happy to be able to put forward a remotely optimistic weekend outlook. Some light precipitation (of mixed phase depending on location) on Friday night will give way to a beautiful day. Saturday looks to be a warm and sunny days with temperatures in the 40s in the valleys, and 30s at the summits. But all eyes should be focused on Sunday, which looks like a classic warm air overrunning scenario. In these situations, Vermont typically starts as snow and switches over to straight rain. But northern New Hampshire into Maine can be a very different story where cold air damming hangs tough. Looking toward Sunday afternoon, snow may still be the dominant precipitation over the White Mountains and northeast into Maine. But the temperature profile is going to be borderline. The exact details will depend on to what extent a secondary surface low develops off the New England coast, which will help to reinforce the cold air damming. If this lows fails to develop, we could be left with freezing rain or a straight cold rain. Optimistically so far it looks like the White Mountains will see a 6 inch paste job on Sunday. For those of you savvy on Skew-T diagrams, note the deep layer of overrunning warm air all the way up to 650 mb. That nearly isothermal layer looks to stay just below freezing, and should lead to a pasty snow, but it won't take much of a shift to flip it. Updates will be necessary as the storm approaches.
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  1. This MLK weekend promises to be a great one for the New England skier. We're facing a sizable snowstorm Saturday night through Sunday. Look for widespread accumulations of a foot or more across ski country. Amounts will be highest in the White Mountains and southern Vermont, with less amounts as you move north into Vermont. Cutting to the chase, here are the latest snowfall forecasts from the National Weather Service.

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    I think those amounts look like a decent guess for New Hampshire and Maine, but are overdone in Vermont. Yesterday's guidance was putting on crazy numbers over New England. For that reason I held off issuing a forecast until today when a more realistic picture has emerged. The most stable story so far as come from the European's model ensemble average. As ensemble is the same model run over and over with slightly different conditions and averaging the results, which gives a broad expected value. For a while now the European ensemble has been showing an average snowfall of about 15".

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    As much as I'd like to drone on about the snow, I think that misses the main story of this storm. There will be plenty of cold air pouring into New England which should ensure snow in ski country, but it creates a strong frontal boundary across southern New England and coastal regions. The headline maker in this storm very well may be significant ice accumulation along the coastal front. The both the GFS and European model show this clearly. Below is a model forecast for Sunday afternoon from the GFS, and a total accumulated freezing rain from the ECMWF.

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    Both models show a band of heavy icing across southern New England. This would lead to widespread power outages. However, I think the story may not be quite so dire. Especially considering it is a few days in advance, I think the models are underestimating the cold air at the surface. Surface temperatures just west of Boston look to stay in the teens throughout Sunday. Not only will the preceding air be cold, there are strong signs of this cold air reinforcement in the model vertical profile. Note the winds out of the north and northeast at the lowest altitudes. This should be cold enough and deep enough to yield sleet rather than freezing rain, but we'll need to revisit this as the day approaches.

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    But if you're reading this, you probably care more about skiable snow totals. The easiest statement I can make is that I expect a little over an inch of liquid equivalent precipitation across much of the region. The storm is so progressive (quick moving, positively tilted, nearly open low) that it's hard to imagine much more than this inland of the coastal front. How this translates into snowfall depends strongly on the snow-to-liquid ration (i.e., the fluff factor). A rule of thumb of 10:1 ratio suggests a broadly distributed accumulation of 12-15" consistent with the weather service's forecast for NH and ME. You can see this in the latest mesoscale guidance if you ignore the numbers over southern New England where sleet will likely keep numbers way down.

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    But what about if the snow is fluffier? This possibility is supported by the surface temperatures in the teens. That's plenty cold enough for ratios up to 20:1, assuming that those temperatures are maintained in the snow growth zones aloft. Using the same liquid equivalent precipitation and allowing for the ratios to vary with the surface temperature you forecast accumulations close to two feet in northern New England, and greatly reduced accumulations in southern New England. The fluff factor is going to be the decider in the snowfall guessing game.

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    So what's the realistic expectation? Probably a mix of these two possibilities. The GFS gives a reasonable first guess.

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    Note the starkly difficult accumulations over the Lakes Region of NH vs the White Mountains. What's driving this difference? It's the temperature profile aloft and where that places the dendritic growth zone (DGZ). That's the region of roughly -10 to -20 Celsius. To maximize snowfall, you want a deep region of these temperatures that also has saturated air rising upward. That maximizes the product of dendrites, which are the classic fluffy snowflakes that you are probably imagining. The temperature and humidity profile in the atmosphere can lead to a variety of snowflake shapes.

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    Looking back at that snowfall forecast disparity between the NH lakes and mountains, let's look at the model's vertical profile forecast. First the Lake Region:

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    Note the narrow dendritic grown zone (DGZ) show on the left column with the red dashed lines. The red-purple bars are the air's vertical motion expressed in pressure-based coordinates (omega) where negative omega values mean the pressure following an upward-moving air parcel is decreasing. Even though this region has saturated air and strongly upward lift, the DGZ is narrow and most the prime snow generating region will produce other shapes of snowflakes leading to a lower fluff factor. A similar plot just to the north over the White Mountains will show a slightly colder profile, but a greatly expanded DGZ and therefor higher fluff factor.

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