Would-be renters looking to move this September are facing a “perfect rental storm” this month with low inventory, nearly instant turnover of apartments, and a spike in prices of up to 20%, according to real estate brokers.

The current rental quagmire is a confluence of larger trends including good economic news, annual high summer turnover, a seller’s market for apartment sales, and an overall loss of rental inventory to co-ops and condos. In the more desirable sectors of the apartment market, renters are now seeing bidding wars, higher prices, and broker’s fees.

Landlords, for the first time in several years, are safely back in the driver’s seat, and experts suggest that if renters have the flexibility, they should hold out until later in the fall.

The chief operating officer at Citi-Habitats, Gary Malin, said he has watched the city’s rental vacancy rate drop, reaching this month what he characterized as a low point since September 11, 2001. Prices, he said, are the same or higher than when the rental market peaked in 2001.

“If there is one complaint with my agents, it’s a lack of inventory and a lot of people looking for apartments,” Mr. Malin said. “With the amount of people in the marketplace, it is a competitive and tough place to do business right now.”

Every fall, thousands of college students and recent graduates start anew with midrange rental apartments. They are joined by second and third generation renters who first came to the city in the summertime, and are looking to move. Companies also generally look to relocate during the summer months when business is slow.

“The first apartment someone usually rents is in the summer. From then on every year, that’s when their lease is up,” the president of Bond New York Realty, Noah Freedman, said. “Every year there is another wave, and it creates a lot of congestion.”

This annual effect has been exacerbated by a trend first noted this spring, where the residential sales market is pushing would-be buyers into the rental market.

“So many people are looking to purchase with not enough supply to meet the demand,” James Fegan of City Realty said. “People are just not finding what they want, so they are renting apartments to give themselves more time.”

Furthermore, several city developers are busy converting rental apartments into co-ops and condos, removing rental inventory faster than it can be rebuilt, according to the Real Estate Group’s Andrew Barrocas.

Mr. Barrocas said one major city landlord rented out 17 apartments over a three-hour period.

And another apartment just put on the market received nearly two dozen inquiries from brokers last weekend seeking to bring clients by for a visit.

According to Larry Rosenberg of rent-direct.com, the number of rental listings for several key segments show about half as much inventory available now as in the winter months. In his Manhattan listings, Mr. Rosenberg said there are 23 studio apartments listed at less than $1,000, about 200 one-bedrooms for as much as $1,600, and 65 two-bedrooms up to $2,200.

The result of the bottleneck on renting apartments: higher prices and less room to negotiate. Mr. Rosenberg said that for two-bedroom apartments in Manhattan’s most desirable neighborhoods, listings begin at $3,000.

In slower times, prospective tenants can negotiate with landlords to whittle down 5% to 7% on advertised rents, Mr. Rosenberg said.

Although sitting on an empty apartment is anathema to landlords, with the high demand and high turnover, they can afford to wait.

Mr. Freedman said that landlords with a larger number of properties gauge the market and adjust prices accordingly. If they receive four applications for one apartment on a single day, he said, they raise the asking price on a similar listing the next day.

On top of a blanket summer increase of prices from 10% to 20%, if a prospective tenant finds himself unwilling to relent on a certain place, a bidding war can ensue, and the landlord can make as much as 20% more than the original listing price.

“I’ve seen apartments get bid up from $1,800 to $2,300 in a day or two,” Mr. Freedman said.

The market is so much in favor of the landlord, industry experts said, that some landlords no longer pick up a portion of the broker’s fees, as is often the practice. The fees, which range from one month’s rent to as much as 15% of a year’s rent, are now uniformly passed on to the tenant.

Low inventory has also made it harder for those who ply the classifieds and online listings in search of no-fee apartments that bypass brokers.

“If [renters] have the luxury of being able to wait until after November, absolutely I would advise them to wait,” Mr. Rosenberg said. “To look at the height of the summer rush, which is going on right now – it is really hard and you need to put in the work getting out there and looking at properties.”