Wednesday Noon:

The Friday storm looks like it may follow a path a little farther south than earlier thought... raising the chance of more serious snow/ice problems for much of Pennsylvania and northern New Jersey. This would also mean less snow reaches extreme northern New England. The Sunday storm now looks like it could be a little farther north than before. However, a lot will depend on how much cold air comes in behind the Friday storm. Here's today's Northeast and Great Lakes video. The Worriers got a headache and agreed to reconvene tomorrow morning.

Wednesday 8:25 AM

A low pressure area moving east from Lake Ontario will be off the New England coast later this afternoon. This map shows the pressure pattern as it was at 6 AM:

On the map, there is a high pressure area northeast of Maine creating a cold flow from Massachusetts northward. Yesterday, there was an east-west belt of high pressure stretching from New England to Illinois. As a strong short wave trough moved in from the west-southwest, it caused barometer readings to drop right in the middle of the high. The resulting low pressure area has been opening up a split between the remaining two high pressure areas as it has continued eastward. The final effect can be seen most easily east of the low pressure area, where you see one set of isobars packed together over coastal New England, then a zone with few lines, then more packed-together lines to the south.

The pressure values are in millibars. The leading 10 in the number is deleted during plotting, so you see numbers like 233 in Massachusetts, 208 near Philadelphia, and 224 at Norfolk, Virginia. If we add back in the leading 10 to the number near Boston, it becomes 10233. But, the pressure is not over 10 thousand millibars (the average on Earth is a little over 1000 mb. On the map, the pressure is shown to the nearest tenth of a millibar, so the correct value is 1023.3 mb. You can now see that the pressure falls on the way to Philadelphia, then rises as you head to Norfolk.

Once you can decipher a few pressure values, you can infer wind direction from the pressure pattern. You would think the air should flow directly from high toward low pressure. However, on the spinning Earth, anything moving in the Northern Hemisphere turns to the right. When a balance is achieved between the right-pulling force and the pressure force (called the Coriolis force), you wind up with the flow running parallel to the isobars. Low pressure is to the left and higher pressure to the right. This is in fact the flow we see on upper air charts. Near the Earth's surface, however, the ground exerts a dragging force. This slows the wind, and the direction then angles in toward low pressure and away from high pressure. You can see this by looking at the arrows I have drawn on the map.

There's one other point I should mention here. If the air moves away from high pressure areas and toward low pressure areas, won't that destroy the highs and fill up the lows? Yes it would if that was the only thing happening, In a low pressure area, or storm, the air rises. We know this because precipitation develops after air rises and cools to the saturation point. In a strengthening low pressure area, the air is being carried away faster aloft than it is being brought in near the ground. This makes the central pressure become lower as the storm advances. The pressure falls as a low pressure area approaches each place and rises after it leaves. This explains something else. The greatest upward motion must be in the part of the storm where the pressure is falling... or the area ahead of the storm. This explains why snow or rain typically begins when the storm is far away to the south or west of you, but it ends shortly after the low pressure center goes by (this assumes the storm is moving from the south or west to the north or east).

Back to today's low pressure area, the easterly wind in New England was bringing in milder air from the ocean, but at map-time there was a sharp contrast between the above-freezing air at the coast and the colder air inland. This map shows the setup as it existed at 7 AM:

The 32-degree isotherm is highlighted, and this marked the boundary between rain on the warmer side and ice and snow where it was colder.

In the wake of the current low pressure area, the Great Lakes and Northeast get a break between storms. However, by tomorrow morning we expect to see a low pressure area over Colorado. It will speed over to Illinois by Friday morning and be southeast of Cape Cod by Saturday morning. Near and south of the storm track, there will only be rain... perhaps even some thunderstorms. To the north as you get into colder air, there will be a transition to ice and then heavy snow. Farther away, the snow will get progressively lighter until you finally get out of the storm's precipitation area. The progs last night suggested mainly snow north of a line from Racine, Wisconsin to Buffalo to Providence. In the heaviest snow band, a storm like this can deliver more than 10 inches in 12-15 hours. Yet another storm will follow, taking a path somewhat farther south. As each one approaches, we will be able to nail down who gets what more effectively than we can right now.

Today's Great Lakes and Northeast Video explains more about the forecast.

Posted by Elliot Abrams on Wednesday, December 17, 2008 8:04 AM
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