In the summer of 1998, the atmospheric circulation situation was abnormal, and the characteristics of the polar vortex, the westerlies, the subtropical zone and the tropical system were obviously different from those of the normal year. The main body of the northern hemisphere polar vortex moved to the eastern hemisphere, whose area and even power were less than normal.
There were high pressure ridge and blocking high pressure near Okhotsk Sea and Ural mountain area. The cold air from the polar along the low pressure channel of the two ridges moved from north to south, forming a cold vortex precipitation system in Nen River Basin. After its formation , the cold vortex system failed to move eastward because the Okhotsk blocking high pressure barrier and had to stay 2~5 days or even longer time in Nenjiang Basin, causing the local heavy rain and strong convective weather. The location of the subtropical high inclined 20～30° west longitude and 1～2°south latitude than the normal year, the circulation situation of Songhua River Basin does not adapt to the climates of the summer. The rainfall served as the cold vortex type which often occurs in spring, thus, the rain had been hovering in the Nenjiang basin.
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