Just as New England transitioned out of our extended dry pattern with a couple of soaking rain storms, working to take a bite out of the drought conditions for Southeastern New England, it appears as though we're heading back into another stretch of dry weather, at least for a week or so. In fact, on-air last Friday on NECN I mentioned that I thought it was unlikely we'd see any rain across New England this week. As it turns out, we may see some drizzle on parts of Cape Cod Wednesday morning, but no appreciable rain is forecasted. How do meteorologists look that far out, and why did I make such a bold statement so far in advance?
The first step is to understand that there is always uncertainty with a forecast - that is, I certainly couldn't say with 100% confidence last Friday that no rain would fall all week in a changeable New England fall season. What I was looking at, however, is the jet stream. The jet stream is a feature that I mention often, including today, in my discussions on the New England Weather Analysis Page, and plays a heavy hand in determining weather not only for New England, not only for America, but for the world at large. First discovered by bomber pilots flying over the Pacific Ocean toward Japan, using Mount Fuji as a fix for heading and distance. These pilots found that, when flying at certain high altitudes, they were making no progress or gain whatsoever on Fuji, and in fact were, at times, losing ground on their fix. They were forced to lower their altitude to gain on Fuji and move toward their targets. It wouldn't be long after that meteorologists would probe this "jet stream" of air further, and discover that this corridor of wind located high in the sky flows around the globe as if a river of air.
In present day meteorology, we know the fast winds of the jet stream can occasionally flow at over 200 mph, and being such a fast current of air, steer storm systems across the globe. Also known as the "Westerlies" because the air current generally flows from west to east, the jet stream develops along the barrier between cool air to the north and warm to the south, and therefore acts as a thermostat for the atmosphere, carrying cool air southward when it dips south, and allowing warmth to rise north when the wind corridor rises northward. A dip in the jet stream, known as a "trough" (think of the shape of a trough for farm animals, much like a U), also can result in disturbed weather, as cold air aloft can aid in development of clouds and precipitation, while a bump in the jet stream, known as a "ridge," not only brings milder air but also tends to produce high pressure, or fair weather.
In these two images, notice the shift in jet stream winds, from the first image, representing the jet stream winds for today, Monday, September 17. In this image, the blue contouring represents wind speed, and the wind direction tends to flow along the black solid lines. 
This level of the atmosphere is somewhere near 30,000 feet, where many commercial jetliners fly, and also can help to explain why travel to the west coast can sometimes take much longer than travel from west to east - often it depends on whether you're fighting the jet stream winds! Notice also that the current jet stream features a trough in the Western U.S., a ridge in the nation's midsection, and another trough of cool air along the Eastern Seaboard. In time, however, the jet stream pattern is forecasted to change, reaching the pattern indicated in this second image by Monday, September 24, a week from this writing. Notice in this second image that the pattern is much easier to define: a trough in the west and a ridge in the east. 
In fact, the Eastern U.S. ridge is large and well-pronounced, implying warmth and also implying that the storms, steered along the jet stream corridor, will miss New England well to our west and north, sliding into Canada and leaving the Northeastern United States in a dry and warm pattern, and in fact, with such a well-defined, or "amplified," pattern, I have to believe we may come close to breaking record warm temperatures Monday afternoon! This is the pattern I saw shaping up last week, and the reason I was willing to mention we may not see any significant rain at all in the coming week.
Can this forecast change? Of course! Occasionally, the timing and intensity of disturbances traveling along the jet stream can reconfigure the pattern of these Westerlies, but by and large we're looking at a major weather pattern shift that seems unlikely to fail to transpire.
