Prioritize...
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.
Case Study...
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.
Click here for a transcript of the video.
Text on screen: Here’s an example of a blocking pattern that set up over Canada and U.S. Northern Border states as shown on this daily composite of 500-mb heights. There were two blocking highs -- one located over Canada's Northwest Territories and the other simultaneously anchored over Newfoundland in eastern Canada. The result of such a pattern is that the flow across Canada and the U.S. northern border states is not very "zonal" (west-east). Indeed, it's highly "meridional" (north-south). That means the natural west-to-east progression of mid-latitude weather systems across Canada and the U.S. northern border states gets interrupted by the two blocking highs and weather systems tend to move slowly.
So, what formally makes these highs blocking highs? Well, a 500-mb closed high or ridge qualifies as a blocking high if several criteria are met: First, the basic westerly current must split into two branches (like the split in the flow of water around a rock in a fishing stream), and each branch must transport appreciable mass, as seen here. Second, the double-current system must extend over several tens of degrees longitude, so the split must span a substantial west-east distance. Third, a sharp transition from zonal flow upstream to meridional flow downstream must be observed across the split in the westerly current. And finally, the pattern must persist with recognizable continuity for at least five days, although some stricter criteria would require ten days.
These criteria were easily met in this case, so this was indeed a blocking pattern. I should point out that two closed lows formed on either side of each blocking high, which compounded the stagnancy of this blocking pattern. So, what are the weather consequences of such patterns? Well, near closed (or cut-off) lows sometimes we get protracted rains and flooding because of their slow movement. In contrast, the weather underneath the blocking high tends to be sunny and dry. Long-lived blocks can actually create drought conditions under the high.
The stagnancy of this blocking pattern paved the way for protracted, recurrent rains across New England for nearly a week, as these multi-sensor rainfalls over a 7-day period show. A wide area of New England received at least 5 inches of rain, with some areas getting more than 10 inches. Needless to say, flooding was rampant.
The flooding rains in this stagnant pattern were aided by a tropical connection, as suggested by this visible satellite image, indicating a long plume of clouds extending from low latitudes all the way up into New England. The sprawling closed low over the Great Lakes region had helped draw moist Atlantic Air northward in concert with a low-level jet stream, which we can see on this daily composite of 850-mb vector winds. The end result was plenty of moisture convergence in New England, feeding the heavy rain.
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.