|Cold Air Damming Explanation|
When a winter storm is approaching, I frequently mention the cold air wedge that forms east of the mountains in my forecast discussion. You may have wondered what that is, and that is why I decided to write this explanation. Cold air damming (or cadding for short), commonly referred to as the "wedge", often occurs in this area during winter storms, and occasionally during all-rain events as well. Cadding occurs when a cold-core (meaning it originated in Canada or the Arctic region and is cold at the center) high-pressure system is located to our north, typically in or near New England. Air flow around a high-pressure system is clockwise, which means that it will send cold air southward, into Virginia and the Carolinas, as the diagram to the right shows.|
You can also see that the low-pressure system that is moving up from the south sends a surge of warm air northward to the west of the mountains. The typical result is a mixture of wintry weather to the east of the mountains where cadding holds temperatures below freezing, but temperatures are much warmer to the west of the mountains. The map below shows a classic cadding situation, with surface temperatures from 7 PM on January 26, 2004 (or 00 zulu/GMT time January 27).
Notice that cadding during this winter storm was particularly strong, with the freezing line (the top of the dark green area) going all the way down into northeastern Georgia. Sometimes cadding only stretches as far as northern South Carolina, it just depends on the strength of the high-pressure system in New England. Temperatures in this area were in the lower to mid 20s. However, just to the west of the mountains, it is much warmer, with temperatures in the lower to mid 50s in central Kentucky. The freezing line to the west of the mountains reached all the way into central Ohio and northwestern West Virginia. Cadding is part of what makes most winter storms so difficult to forecast in this area, because computer models have a difficult time dealing with it, and they tend to underestimate the cadding (in other words, their forecasts are usually too warm.) However, cadding has much less impact on temperatures in the upper atmosphere, where temperatures often rise above freezing during winter storms. Since upper air temperatures usually determine precipitation type, that is why we usually get a wintry mix instead of just snow. Even though cadding keeps surface temperatures well below freezing, the temperatures in the upper atmosphere warm up into the mid or upper 30s, and that situation would cause sleet and/or freezing rain to fall. During the time frame depicted in the map above, although temperatures were in the mid 20s, we were getting freezing rain, because upper atmosphere temperatures were much warmer.
|How Different Precipitation Types Form||Weather Term of the Day|
|For about a 3-month period in the fall of 2007, I included a weather term along with its definition on my website each day. If you missed some of these or would just like to review them again, you can click here (right-click, save target as) to see the list of terms and definitions that I used. Or, click here for the complete glossary from the NWS.|
|Computer Models Explanation|
|The term "computer model" is used often in the Forecast Discussion. A computer model is a sophisticated computer that receives all the current data and outputs a forecast. The forecasts go out to 2 to 16 days and are updated 1 to 4 times daily depending on the model.|