Weather Wisdom

Storm mostly a miss except for areas south of Boston and Cape Cod/Islands

This evening the trend continues keeping to bulk of the storm out to sea and away from most of Southern New England. There is still going to be a period of very heavy snow and wind over Cape Cod, Martha's Vineyard and Nantucket tomorrow and a blizzard warning is now in place across those areas. Since the strong winds will reach northward along the coast a high wind warning is in effect for coastal and southern areas of Plymouth county.

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Although there still has been a large range in snow projections, the gap has closed and Boston itself is likely to receive anywhere from a coating up to 2 inches of snow. There's an outside chance at 3 inches, but I am leaning lower not higher.

For Boston, this storm will really be on the edge of nothing to barely plowable. When storms are this marginal, an inch or two makes a big difference in how the storm is cleaned up, but itís a very small amount in the big picture of this particular storm.

Track too far east
The entire forecast is based on the storm remaining far enough offshore to just giving the area a glancing hit. Those of you on Cape Cod and the Islands will still be plowing and someone may end up with 10 inches of snow, but for most of us the snow totals will be no big deal.

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What if?
I donít like what if scenarios because they bring nothing to the table. That said, some of you might be thinking what if the storm came closer to the coastline? The reality is the storm would have to move nearly 200 miles further west than predicted and this isnít likely at this point. IF however it did, we would be looking at a damaging noríeaster with 1 to 2 feet of snow, widespread power outages and an all around bad situation.
On the other side of the coin, the storm could jut out to sea a bit faster and develop a bit later, this would lower snow amounts further and also make the wind less of an issue, however it would still be very windy.

The reality
The main threat from this storm is the wind. This is especially true over Cape Cod where the blizzard watch is still posted. To punctuate the strength of the storm, note the hurricane force wind warning just off our coastline.
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Tomorrow the commute will be slower in Boston with some snow. Over Cape Cod, where the heaviest snow bands set up, travel could be impossible for a few hours. I expect the snow to begin south of Boston around or just after midnight and spread into Boston and the western suburbs during the early morning hours before dawn.

The storm will move quickly and therefore any snow will end earliest west of Boston, during the morning commute. Along the coast, near Boston, snow stops in the first part of the morning, but doesn't end until late in the afternoon over the outer portion of Cape Cod. These times will be tweaked a bit during the day.

As I wrote, the wind is the big issue from this storm. While some parts of Cape Cod will exceed 6 inches or more of snow, itís the near hurricane force gusts just offshore that are the big story. Off Cape, close to the coast, winds will be strong enough to cause scattered power outages. However, winds do not appear to be forecast at a level to cause widespread power issues off Cape Cod.

If you live over Cape Cod and the Island and even southeastern Massachusetts and extreme south eastern Rhode Island, I recommend preparing for power loss. If it doesnít happen, consider it a good thing.

Milder air coming
Once the storm passes, it remains chilly and windy Thursday before we turn milder for the end of the week and the start of the weekend. Iíll have much more later today and during the evening as the snow develops.

Just how big is the storm?
When we evaluate storms, one of the ways we note the strength of a low pressure center is by its pressure. Storms exist to balance the atmosphere.temp contrast.jpg Their purpose is to mix cold and warm air and try to achieve equilibrium. When a large difference in temperature exists and energy from the jet stream rides over this contrast storms often develop.

Air rises in storms, and as the air leaves the planet the pressure lowers. Wind is the air rushing into the storm trying to fill the void being created by its formation. If the storm is really large the air canít fill the void fast enough and the pressure continues to fall. The faster the pressure drops the more wind we experience as those winds are attempting to balance the rising air.

Normally, on an average day, the pressure of the air above you exerts about 1013 millibars of pressure. In a typical strong storm this pressure can fall to 980 millibars. At that level of pressure a storm would generate a considerable amount of wind and precipitation. The faster the pressure falls each hour during a storms development the greater the intensity.

When a storm loses 24 millibars of pressure in a day the storm is said to have undergone bombogenisis. In laymanís terms, this is akin to the atmosphere exploding a storm in a very short period of time. During the upcoming storm the pressure is forecast to drop 30 millibars in one day. This will create a storm in the ocean with a pressure typically seen in a category three hurricane.

The map below shows the stormís position tomorrow. The black lines around the storm are isobars (lines of equal pressure). The closer these line are together, the greater the wind. Use the key on the right and notice the winds are near hurricane strength around this storm.
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Low pressure isnít the only factor in why a storm produces a lot of wind. If there is a big high pressure area a few hundred miles away this generates even more wind. In the upcoming situation there will be a high with a pressure of 1030 millibars in Canada.
The reason I spent so much time with that explanation is because I want you to understand the scope of this storm. While the heaviest precipitation will fall over the outer part of Cape Cod, far eastern Maine and Nova Scotia the wind field is enormous and has the potential to be damaging.

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