 Here's an example of a blocking pattern that's set up over Canada and U.S. northern border states as shown on this daily composite of 500 millibar 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 or west to east. Indeed it's highly meridional or north to 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. What formerly makes these highs blocking highs? Well a 500 millibar 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 to 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 rainfall totals over a seven-day period show. A wide area of New England received at least five 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 millibar vector winds. The end result was plenty of moisture convergence in New England feeding the heavy rain.