Banks endured the same bad start to the year as the broader economy did, according to newly available data, and they too blamed this year's prime suspect: Mother Nature.

The American Banker Index of Banking Activity fell to 53.7 in January, from 56.6 in December and 53.8 a year earlier. It was the lowest monthly reading for the index since its debut in June 2012.

Consumer lending activity took a big hit as the polar vortex drove temperatures down into the single digits, produced heavy snowfalls and wreaked havoc on roadways across the country. The reading for applications fell to 43, indicating decreased activity, compared with 50.1 in December. The reading for approvals also declined, to 45.2.

Weather hurts consumer businesses the most, analysts say. Transaction fees could slow because of less shopping and spending, but the biggest wallop — or best place for an excuse — is expected to be in mortgage.

"Residential mortgage lending had nearly come to a halt in part because of this winter's extreme weather," the Federal Reserve Bank of Richmond reported in the most recent quarterly economic survey known as the Beige Book, citing bankers in the district.

Ed Wehmer, the president and chief executive of Wintrust Financial (WTFC) in Rosemont, Ill., echoed those woes in early March.

"This is probably the first time you will ever hear me use weather as an excuse for some facets of our business," Wehmer told attendees at a conference hosted by Raymond James. "I think it has snowed every day."

Wintrust's mortgage business is expected to fall 43% in 2014, because of higher rates and a corresponding slowing of refinancing activity, David J. Long, an analyst at Raymond James, wrote in a research note. However, activity could speed back up in the second quarter.

"As the Chicago market place thaws (literally!), we expect volume to pick up," Long wrote.

Regulation could also be a factor, particularly the qualified-mortgage rules that the Consumer Financial Protection Bureau implemented in mid-January. A number of lenders have expressed concerns about getting borrower applications for mortgages to qualify under the rules.

Bankers tried to remain optimistic about the industry's prospects as the year goes on. The reading for in-market business conditions was 53.7, revealing that bankers are continuing to think positively about the future. The reading for real estate conditions was 52.8.

Index readings above 50 indicate a monthly expansion of activity; readings below 50 point to contraction. For contrary indicators, such as the components that track loan delinquencies and loan-rejection rates, a reading above 50 is considered evidence of deterioration in business activity. The further from 50 a reading is, the stronger the indicated change.

Commercial lending momentum continued, though at a slower pace. The reading for applications slipped to 56.6 from 57.5 a month earlier. In terms of approvals, January's 56.2 reading indicated decelerated activity compared with December's 60.8 mark.

Pricing for consumer and commercial loans remained stable compared with previous months. At 51, the reading for consumer pricing indicated some improvement for banks, while the 49 mark for commercial loans revealed rates that were slightly less favorable.

On the funding side, deposit prices continue to improve for bankers.

Staffing levels among respondents fell for the first time since August, evidenced by January's 48.2 reading.

Weather also influenced other business-related activities by bankers.

SouthCrest Financial (SCSG) in Peachtree City, Ga., noted that its Feb. 28 charter consolidation faced pressure from abnormally harsh weather. Employees' efforts were "Herculean, given the fact that the difficulty of this task has been compounded by two snow and ice storms," Kenneth Maloy, the company's president and CEO, said in a release at that time.

The IBA is a product of American Banker's monthly surveys of bank executives. The latest installment of the diffusion index was based on 320 responses.

The IBA's composite index is a simple average of readings on a range of indicators based on responses to survey questions on topics that include volume and pricing trends in commercial and consumer lending, loan balances outstanding and deposit-account activity.

Respondents are also asked to weigh in on staffing levels at their institutions, as well as business and real estate conditions in markets where they do business. Every effort is made to ensure that the breakdown of companies included in the executive panel is representative of the industry.

The values for individual components of the index are equal to the percentage of responses indicating increased activity plus one-half of those indicating "no change." Component scores are then averaged to arrive at a composite. When calculating the composite, contrary indicators such as delinquencies are scored inversely — the component figure is subtracted from 100.

Robert Barba contributed to this article.

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