Monthly expenses can pendulum as much as income, but the two do not necessarily move in tandem. An analysis of 250,000 bank accounts by the JPMorgan Chase Institute, a nonprofit research arm of the bank, found that roughly 80 percent of households had an insufficient cash buffer to manage the mismatch between income and expenses in a given month.

Few people can comfortably ride out the inevitable financial bronco ride. “Only households that earn $105,000 or more a year are secure against the volatility they are exposed to,” said Diana Farrell, the institute’s president and chief executive. “It’s not just about the unemployed or the poor.”

Middle-income households, for example, saw their monthly expenses deviate by nearly $1,300, the equivalent of a month’s rent or mortgage payment. And one uh-oh expense — usually in the form of a medical, tax or car repair bill — can wreck a family’s balance sheet for a year or more.

Even a single month’s volatility can have a cascading effect. One month, a family copes by using the money earmarked for, say, the utility bill to cover the cost of replacing a busted water heater. The next month, it’s the telephone company that goes unpaid as the family struggles to make up the missed utility bill plus late fees and interest — and so on. Emergencies are not the only source of expense spikes. So are bridal showers, Christmas gifts and outgrown winter coats.

May turned out to be an expensive month for Tomika Waggoner, 44, a nursing home aide in Newport, Ky. Her daughter was graduating from high school, and she needed a few hundred dollars to pay for her cap and gown, commencement fees, a prom ticket and a dress.