After completing this section, you should be able to identify blocking highs, discern between cut-off lows and closed lows, and identify areas that may be persistently wet (or dry) based on the presence of a highly meridional pattern.
In this course, I often preach that becoming a consistently good weather forecaster requires a focus on "the big picture" (the overall synoptic pattern). But, what types of things do forecasters look for in assessing the big picture? The specifics, of course, vary day by day, but I'm going to show an example to introduce you to the basics. A "big-picture" diagnosis starts with a survey of the upper-air pattern, and there's no better place to start than at 500 mb. Check out the short video below, when forecasters could identify an upper-level blocking pattern, and take note of the weather consequences that forecasters can infer from such patterns.
Note that in the video, I referred to the 500-mb lows in this case as "closed lows" instead of "cut-off lows." While some forecasters might use the terms interchangeably, there's actually a big difference between a "closed low" and a "cut-off low." With a true "cut-off" low, the low's circulation drops out of the prevailing westerly 500-mb flow (the low is "cut off" from the prevailing westerly flow). On the other hand, there are traveling 500-mb lows embedded in the prevailing westerly flow that display a "closed" circulation. But, these 500-mb lows are not cut off. They're just plain old closed lows. Generally speaking, closed lows tend to continue their propagation with the prevailing westerly flow, albeit typically rather slowly.
So, all cut-off lows are closed lows, but not all closed lows are actually cut-off lows. However, both closed and cut-off lows can be prone to slow movement, and their slow movement can cause prolonged rainy periods, especially on their eastern flanks. In this case, the stagnancy of this blocking pattern over North America paved the way for protracted, recurrent rains across New England for nearly a week. And, after a week of recurrent rains, flooding was significant.
Recognizing blocking patterns as part of a "big-picture" analysis can help you get an initial basic feel for what type of weather to expect in various parts of the pattern. This understanding may help you in preparing both short-range and longer-range forecasts. Another note of interest is that models tend to break down blocking patterns too quickly, so that's something to keep in mind should you encounter one this semester (and beyond). Finally, I should note that the subtropical high pressure systems that you learned about in your previous studies do not qualify as blocking highs. They typically lie too far south (near latitude 30 degrees) to cause a significant split in the 500-mb westerly current.
While we didn't look at every aspect of the "big picture" here, did you notice just how much information we gathered by carefully diagnosing the 500-mb pattern and recognizing its consequences? Simply by recognizing the blocking highs and closed lows, forecasters knew that weather systems would be slow moving, resulting in some persistently wet and dry areas. When you start making your own forecasts, you should always start with an analysis of the "big picture" pattern, including current observations. Speaking of observations, we need to look more deeply into the observations of critical forecast variables (temperature, wind, and precipitation). Read on.