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January 4, 2018 storm

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Weatherman

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Model trends today have been to the west, which is no real surprise. In this highly amplified flow the models tend to damp down such extremes in the long range. The way we initialize models (data assimilation) there are terms in the equations to penalize extreme solutions and force everything back to average. The result is often suppressing storms in highly amplified regimes like this one.

The track looks to be just offshore with strong snow banding just to the west of the track. This bring heading snow bands to eastern coastal regions of New England from eastern Massachusetts into most of Maine. The White Mountains of New Hampshire into Maine will see a localized maximum as is common in coastal storms. For those of you in Vermont -- tough luck but that's no surprise. Your bread and butter are the lighter snow of upslope wrap-around. I don't mean that there won't be any snow -- there will be some upslope -- but when in these cold regimes it is the light and fluffy stuff that just serves as a top dressing. For skiers, it won't give you any float, it won't serve to reinforce the base, and it won't stand up to skier traffic. It will end up blown into the woods.

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If you look careful at the precipitation map above, you'll note the cellular pattern in the precipitation. This is the result of embedded convection within the snow band. In these intense snow storms, there can be pockets of elevated air that become unstable and rise upward like in a thunderstorm. This is in contrast to overrunning (stratiform) precipitation where moist air rides mostly horizontally from the south up cover cold air. The overrunning results in precipitation. Those same dynamics are at play here, but mesoscale (mid-scale) dynamics are at play that causes that overrunning area to have more energy than the air below it, resulting in overturning.

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A sounding from the model forecast above taken in southeast Massachusetts (where the red dot is) shows this. For the untrained eye, there are three things to see.

  • Note the profile of "equivalent potential temperature" in the lower right. This is the temperature that the air would have if it was brought down to the ground and all the water vapor condensed. This should continue upward if the profile is stable. If it decreased with height, the air will want to convectively overturn and result in precipitation. Note the elevated unstable later around 800 mb pressure level, and a near surface instability area as well.
  • This same unstable layer shows up in the Skew-T sounding in the middle of the page. The diagonal solid dash lines refer to lines of constant potential temperature, which are just subtly different from equivalent potential temperature -- just discounting the events of moisture. The green line is the dew point temperature, and red line is the air temperature. Where the red and green liens meet, the air is saturated.
  • In the saturated layers, note the negative omega values in the bar plot in the lower left -- in dynamics omega is vertical velocity of an air parcel with respect to pressure. Negative omega means decreasing air air pressure and upward motion. Where the air is saturated is becomes buoyant and accelerates upward. Once the instability relaxes the upward motion slows. You can see this upward motions in the negative omega values in the two layers.

The presence of these two unstable layers are likely due to two different mechanisms at play. The elevated, and stronger, unstable layer is the broad scale up-lifting. As snow falls from the upper layer into the lower, it will seed additional precipitation from the lower saturated level. This is a common situation with terrain-based enhancement and is know as seeder-feeder snowfall.

S0o where does that leave us? The responsible NWS is showing a broad moderate snowfall, and that's the appropriate forecast at this time.

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Note that these maps don't accurately show expected terrain enhancements. The White Mountains will once again make out well from this storm. The global GFS guidance picks up on it, and the high resolution 3 km NAM really latches onto it.

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As previously discussed, convective dynamics are at play in this storm, and the global models do not have sufficient resolution to catch this. So give more credence to the NAM forecast in the lower plot.

It's also worth noting that skiers may miss the real news worthy story. You'll hear talking heads on TV tossing around the B-word. That's a result of the strong winds being driven by the intense cold already in place that we're feeling when we go outside. The NAM is showing a taste of that.

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The heavier, wet snowfall in southeast Massachusetts couple with those strong winds could result in power outages. Those would be a big deal in this regime. We'll be returning to extreme cold on the backside of this storm. Those who lose power could be facing freezing pipes within hours of power loss. Temperatures will be diving sub-zero again across New England on Friday into Saturday. If you lose power, watch your pipes!

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So where to ski this weekend? Nowhere? Everywhere? Good luck. It's going to be damn cold. But the White Mountains will have the deepest snow pack in place, with depths in the high terrain approaching three feet. Secondary maximums are evident in the higher terrain of Vermont, but are not quite as deep, especially counting the water content of the snowpack which is helping to pad the New Hampshire numbers.

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Sounds good for ski country. I hope the power outages in the flatlands are at a minimum...

I was up at Loon today. Cover is real good everywhere. This storm if it plays out the way @Weatherman is calling it, EVERYTHING at Loon will be in play, and be good.

Here's to hoping!

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