The weather pattern for New Jersey this Spring has featured above normal rainfall and unsettled conditions. Rainfall amounts average one to three inches above normal over southern New Jersey and three to six inches over the northern half of New Jersey. In fact, some locations in northeastern New Jersey have even pushed to eight inches above normal over the past 60 days. The amount of rainfall over the region has lead to a constant threat for localized river flooring and urban flooding. So what's behind the heavy rainfall?
Part of the answer which will be discussed in this two part discussion (part two next week) we will look into what is creating a favorable environment for all this rainfall and for the volatile temperatures.
The satellite picture displayed here is of the northern Atlantic and eastern North America. Note the area of blue and green arrows. The circulation is that of an upper level low and trough that has been plaguing the northern Atlantic for the past several weeks. This upper level low and associated trough is a key feature in our weather pattern. Note over the Canadian Maritimes, the upper level winds are converging from the east. Converging air at the upper levels leads to sinking air and sinking air to the surface helps to establish and/or enhance high pressure systems at the surface. Here is where this patter impacts you and me.
You see, that high pressure system at the surface locks in around Maine or the Canadian Maritimes. The circulation around the high pressure system is clockwise, which helps drive an Atlantic marine air mass southeastward from the Gulf of Maine down the coast towards New Jersey and Delaware. This movement of air on the heels of a northeastern and easterly wind down the Northeast coast is what is called a back door cold front.
Note the stationary boundary over New England on this satellite image. This is yet another back door cold front starting to develop and expected to head south this week. There in lies the threat for heavy rainfall.
The pattern is such that this moisture loaded marine air mass drives into the moisture loaded tropical air mass that is brewing over the Gulf Coast, Southeast, and southern Mid Atlantic. As these two air masses interact, overcast skies and showers develop with the potential for some very heavy downpours. However, what really drive the threat for heavy rain is when a disturbance or cold front approaches from the west with a cool, dry Canadian air mass. The third and final air mass interaction significantly lifts all the moisture in the atmosphere leading to rainfall events where one to there inches of rain or more can fall.
The next key is found in the Pacific and will be discussed in detail next week!