Coastal Storm To Bring Light Snow To The Coast This Evening

Hello everyone!

I was hoping to get an update on this evening’s system out last night but other commitments prevented that so here’s a quick rundown of my thinking and why we’re not going to see a significant event.

12Z Weather Balloon Data Showing Very Dry Air In Place. Image Credit: SPC
12Z Weather Balloon Data Showing Very Dry Air In Place. Image Credit: SPC

One inhibitor of large snowfall accumulations will be ample dry air ahead of the storm that will evaporate some of the snow before it can reach the ground. A good inch or two worth of snow could be lost to mid level dry air. Do notice though that the entire atmosphere is plenty cold enough for snow. There is zero threat of any mixing with this storm but there are plenty of other forecast challenges to make things interesting!

10 AM 500mb Winds. Image Credit: Nullschool
10 AM 500mb Winds. Image Credit: Nullschool

Looking at the mid levels, it is fairly easy to see why we’re not in for a big storm and why the highest snowfall amounts will be along the coast. Both disturbances associated with the storm are positively tilted and separate while winds ahead of the system are out of the WSW which pushes the storm ENE. There is some respectable divergence (winds blowing away from each other) over the system but the setup lacks the explosive dynamics needed for a stronger storm that tracks farther west. Development of these dynamics will occur eventually but too late for us. Drive to New Brunswick or Nova Scotia for those.

10 AM Upper Level Wind Map. Image Credit: SPC
10 AM Upper Level Wind Map. Image Credit: SPC

Upper level dynamics are similar to the mid level dynamics: modestly favorable but not explosive. The storm is in the right entrance region of a very strong but low amplitude jet streak. There is only one jet and no jet coupling meaning that we lack the explosive dynamics needed for a strong storm. This is the same story across all levels of the atmosphere and explains why we’re not in for any big snow. However, that doesn’t mean some areas won’t get a moderate storm.

10 AM 700mb Analysis. Image Credit: SPC
10 AM 700mb Analysis. Image Credit: SPC

A look towards the mid levels shows pretty much the same story with one exception. Notice all the WSW flow and the positively tilted troughs but also notice the kink in the flow right over the storm itself. This kink will gradually sharpen and develop along with SE winds ahead of it. The timely development of these SE winds will be crucial to pulling the deeper moisture NW towards our area. If the kink intensifies more quickly, the SE winds will as well, and the moisture will be able to move NW therefore giving us more snow. If the kink lags in development, the opposite will be true.

Kachelmann Swiss Model Showing Expected Impacts This Evening. Image Credit: Kachelmann
Kachelmann Swiss Model Showing Expected Impacts This Evening. Image Credit: Kachelmann

Now that we’ve dug into all the dynamics behind this storm and what some of them mean, we can focus on impacts. This map from the Kachelmann Swiss model shows how the coast will see the moderate snow, the foothills the lighter snow, and the mountains only flurries. Also notice the bands of ocean enhanced snowfall across eastern MA as Arctic air currently over ME is drawn across the warm Gulf of Maine waters on NE winds ahead of the storm. In those heavy bands, over a foot of snow is possible along with blizzard conditions. This will be limited to eastern/coastal MA and no impacts that significant are forecast for the ME or NH coastlines.

HRRR Showing How Snow Evolves Over The Next 18 Hours. Credit: Weatherbell
HRRR Showing How Snow Evolves Over The Next 18 Hours. Credit: Weatherbell

Snow will arrive in the next hour across southern NH and will be falling across the ME coast by sundown this evening. Notice how the mountains see hardly anything while the coast enjoys moderate snowfall. Also check out all the ocean enhancement as bitterly cold air gets drawn into the storm and passes over the warm Gulf of Maine. The snow will move out in the predawn hours as the storm moves farther offshore.

Expected Snowfall Through Tomorrow Morning
Expected Snowfall Through Tomorrow Morning

Here’s my snowfall forecast for tomorrow. This will be a light/fluffy snow that will be easy to move, both by human and by wind. With steady northerly winds, some blowing snow is certainly possible which could create lower visibilities and some drifting in exposed areas though nothing major is expected.

Another cold few days are in store before a brief warmup mid week as low pressure passes to the west.

-Jack

 

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