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The current rash of floods in the Northeast and Mid Atlantic states has many people talking of global warming and its implications. While global warming may have a hand in above normal temperatures the pattern for the residents who live between Washington D C and New Hampshire has been some serious wetness rather than hotter than normal temperatures. The placement of the eclipse grid used by Doc Weather to track climate patterns is a perfect image of what is known as a cascade.
In figure 1 the cascade is illustrated with the accompanying jet stream from the polar regions. In the summer the polar jet stream is pushed far to the north by the expansion of the Hawaii high in the Pacific and the Azores high in the Atlantic. The mean position in the winter is across the middle of the North American continent and the mean position in the summer is across the southern plains in Canada. In figure 1 the eclipse grid with its disturbance diamond (orange) is seen stationed over the eastern Pacific. The curves that make up this diamond most often influence the conditions at high latitudes. Starting in May of this year the area around the diamond began to be influenced by persistent high pressure values (green curves). When this happened a ridge built up over the Pacific Northwest that pushed the jet to the north over the west coast.
In the second image the path of the polar jet coming off of the ridge can be seen. The curves around the disturbance diamond influence the high latitude path of the jet. To the east the next set of curves are seen to be much lower in latitude. These curves influence the middle latitudes. When both sets of curves are influenced to high-pressure values as they are in this chart, the tendency for the polar jet is to raise to the west then descend in a cascade to the middle latitude curves. The smaller ridge of the mid latitude curves keeps the jet to the north as well, but not so far to the north. On the chart the descending jet is seen moving southeastward from western Canada and on into the Midwest. To the north of the ridges a climate response can be seen. This response is in the form of two anomalous low-pressure areas on either side of Hudson Bay. The low to the west of the bay is stationed in between the high latitude jet curves and the mid latitude jet curves. The low to the east of the bay is stationed in the lee of the ridge from the mid-latitude pair. These lows have consistently sent fronts down the cascade form into the mid-Atlantic states and the Northeast this summer, setting up the cascade of thunderstorm there that have led to the recurrent flooding.
In the last chart the polar storm jet is shown completing a transit of the continent and then encountering a high-pressure area stationed as a reflex high for the low latitude jet curves stationed in the middle of the Atlantic. This high influenced the Azores high to move into the center of the Atlantic and to generate high-pressure pulses as the eclipse point was influence by high pressure values. The resulting ridge formation served as an offshore block to the passage of the fronts coming down from Hudson Bay. High -pressure in the tropics sent plumes of moisture up the coast that met with the descending fronts from Hudson Bay generating intense moisture laden front lines that resulted in the persistent flooding. The high pressure cascade scenario in the Doc Weather system is a classic climate pattern when displayed in areas whose climatology can fit into the cascade easily. Unfortunately for the Mid Atlantic states and the Northeast this summer is one of those years when the fit is perfect.