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The snow and ice storm that turned yesterday afternoon's Atlanta commute into a nightmare sped rapidly northeastward to cause a light but slippery snowfall through D.C., Philadelphia, New York City and Providence overnight into this morning. Here is my morning video:
Meanwhile, the upper air flow will turn from westerly to somewhat more southwesterly in the East. This should help places like Charlotte, N.C., enjoy temperatures in the 50s and 60s this weekend and, at the same time, force the storm causing snow in Chicago to head through the eastern Great Lakes and into New England. This means any precipitation during the weekend in Philadelphia and New York City would fall as rain... although the track of the low pressure area might also keep most of the precipitation to their north. This track forecast was suggested by some models earlier in the week, but now there seems to be more agreement on that idea.
The strength of the Saturday storm will determine how far south the cold air advances after the center goes past each place. This will be a very big deal, because there could be a parade of storms next week. On the north side of each one, there will be snow and ice, while south of each track it will become mild. This is an issue we'll have to watch very closely.
During the last few winters, I have talked about the relationship between pressure and temperature patterns high in the stratosphere (around the 10 mb level) and weather patterns down here where we live. However, before I get to that, this brings up an interesting issue. On an upper air chart, we look for ridges and troughs on maps where the pressures are the same everywhere. This sounds contradictory until you realize that the height of each pressure level varies. In a ridge, you have to go higher in the atmosphere to reach the 500 mb level than in a trough.
So why is it that we use constant pressure values and variable heights to analyze the flow aloft? After all, variations in pressure, reduced to sea level, are used at the ground. "Reduced to sea level" means that the pressure at any given place is adjusted to what it would be at sea level. The result of the calculation is the "barometer reading" we might be familiar with from weather reports. In summary, high in the atmosphere we measure the varying height of a certain fixed pressure level, whereas at the ground we measure the varying pressure vs a single fixed height value: zero.
Why did this happen? Pressure varies quickly with elevation. Yet, the measuring instrument, the barometer, uses a scale that runs from zero (a vacuum) to about 32 inches. This is minuscule compared to the many, many miles you'd have to go up to find zero pressure. If you didn't adjust pressures to sea level, the readings at each place would then constitute a topographic map! In fact, before GPS came along, that's how hikers could use an altimeter to measure how far up they were.
The weather features that vary by only about 2 inches between a strong high pressure area and a strong low pressure area would be totally lost on such a map. So, by converting measurements to what they would be at sea level... elevation zero... what's left on your map are the delicate variations in pressure that define weather systems.
Now, getting back to 10 mb, there is usually a vortex centered somewhere near the north pole. Studies found that when warming high in the stratosphere took place, the vortex could weaken, perhaps split, or in extreme cases even reverse to become high pressure areas. It was found that this occurrence was a signal that a high latitude blocking pattern would form. With the normal flow blocked, the main upper air current would be forced south. This, in turn, would allow cold air to persistently move south and for the prevailing storm track to move south. This could lead to more snowstorms.
This year, there is no blocking, clearly showing you do not need blocking for cold air to advance. However, along the East coast, you have probably noticed that each cold wave has been followed by a spike of warming because storm centers were free to cut to the north instead of being blocked to the south. The next map shows last night's pattern at the 10mb level. There is a strong, but elongated, vortex up there. Compare that to a change that occurred last winter. On the left, a December map shows a vortex, but in January the vortex split and there was temporarily a high, not a vortex, over the pole. Soon thereafter, a blocking pattern developed.
As for why there has been so much cold in the Midwest and Northeast, we can look at a statistical correlation. In El Nino AND La Nina winters, temperatures tend to run higher than average in Great Lakes and Northeast. So, if both of those tend to be milder setups, what's left to explain the cold? Answer: the neutral phase, which we have this winter. Remember though that many other factors can play a role, so just because a certain pattern happens most of the time does not mean it will happen all the time.
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