While I lived in TX, practically every cold front that came through caused the temperatures to drop sharply and rapidly immediately after the passage of the cold front. But here in the piedmont, the cold often slowly filters in long after the cold front has passed through. Why does the cold air usually lag far behind the passage of the cold front? What different circumstances took place on November 15th to allow the temperatures to uncharacteristically drop immediately afterward?

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Scott Mielock

MIKE MOSS SAYS:      Scott,   There isn't any one single answer to that question, but a few of the factors that play a part in how we perceive the passage of a cold front here include the structure and forward speed of the front itself (some are steep and fast moving, some are shallow and slower moving, etc), the depth of colder air behind the front, and the direction from which the cold air advances. Typically, our fronts arrive from the west and northwest - if they happen to have rather shallow, stable cold airmasses associated with them, there is often a trough and windshift that pushes ahead of the coldest air, which can be trapped and as you noted, filter through the Appalachian Mountains more slowly, so that we note a delay between the shift of winds to the northwest and the arrival of significantly colder and/or drier air. On the other hand, a steep front with especially deep cold air may maintain a close connection with the initial pressure trough and wind shift as some of the cold air passes over the mountains and quickly advances eastward. In some other cases, the initial wind shift may bring in notably drier air , while the coldest air is held up by the mountains. In these cases, a dry passage of the initial wind shift may produce relatively little temperature drop but humidity will fall off quickly. If this happens to coincide with some precipitation into that drier airmass, evaporative cooling may lead the temperature to fall rapidly even though the initially coldest air has been delayed.

Another scenario that often produces a rather sharp temperature reduction with the initial passage of the trough and wind shift is when a cold front advances into central North Carolina from the north or northeast (often called "back door" cold fronts) because there is less tendency toward terrain blocking from that direction. While I'm not as familiar with all the scenarios Texas may encounter (and it may vary depending the part of the state) I would speculate that significant cold frontal passages from the west are less common than those with a strong north to south compnent of motion. As those would originate from the plains states to the north, there would often be little in the way of terrain obstacles to impede the advance of cold air at the surface, whether the colder mass was shallow or rather deep.

I should add that in general, our depictions of frontal boundaries on weather maps can be a considerable oversimplification of the three-dimensional reality of the atmosphere. Even in the absence of terrain features like the Appalachians, there can be situations where a cold front aloft, for example, effectively runs out ahead of a surface boundary, producing precipitation that lowers temperatures, only to be followed by a recovery and then another sharp temperature/dew point drop later, or when two or even three airmass boundaries in quick (or not so quick) succession may cross an area, one producing mainly a wind shift, for example, another mainly a fall-off in moisture, and another that has the largest temperature gradient and the most noticeable cooling. When something like this interacts with mountains as well, it just adds to the ambiguity when it comes to answering the question, "where is THE front?"

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