The national mortgage delinquency rate remained historically low in September and unchanged from levels observed a year ago, signaling that amid labor market weakening and rising housing costs most mortgage borrowers still made their monthly payments.

Real estate market analytics firm Cotality reports that the 3% delinquency rate in September was up slightly from 2.9% in the second quarter. Serious delinquencies (90 days or more past due, including loans in foreclosure) only rose to 1% in September from 0.9% a year earlier.

Pockets of stress in metro-level data have begun to emerge, however.

Molly Boesel, senior principal economist at Cotality, noted in the report that borrowers who fall behind “are struggling to catch up, progressing into later stages of delinquency.”

The share of metro areas with rising delinquencies fell from 70% to 48% year over year in September, yet the portion of metro areas that recorded rising foreclosure rates jumped from 8% to 39% annually that month.

About 1 in every 3,997 properties had a foreclosure filing in September, according to separate reporting from real estate analytics firm Attom.

The so-called “transition rate,” defined as the share of mortgages that passed from current to 30 days past due, fell 0.1% annually in September, to 0.7%, per Cotality data.

“These trends suggest growing challenges for borrowers once they become delinquent,” added Boesel.

Arizona, Nevada and Georgia logged the largest increases in annual delinquency rates in September, each rising 0.2% year over year.

Among 384 metro areas tracked by Cotality, 186 saw higher annual delinquency rates in September. Odessa, Texas; San Angelo, Texas; Farmington, N.M.; and Jonesboro, Ark. led the U.S. with increases of 1.3%, 1%, 0.8% and 0.8%, respectively.