Host Jay Leno gestures during a panel for his upcoming television series The Jay Leno Show at the Television Critics Association Cable summer press tour in Pasadena, California August 5, 2009. REUTERS/Mario Anzuoni

So far, so impressive. After only a handful of fall premieres, broadcasters already have two reasons to celebrate: Fox's Glee and the CW's The Vampire Diaries debuted to strong numbers last week, suggesting that perhaps the coming weeks won't be a repeat of last fall's disappointing returns.

Wednesday night's conceptually risky Glee (7.5 million viewers) was the network's highest-rated scripted fall series premiere in three years, and Thursday's Vampire Diaries (4.9 million) was the CW's most-watched series premiere.

Both scored well in the networks' internal intent to view tracking polls leading up to their premieres, which should give other shows that rank higher on that list some hope. Fox's animated spinoff The Cleveland Show is currently at the top, followed by CBS' crime-drama spinoff NCIS: Los Angeles, NBC's comedy Community, ABC's sci-fi drama V and ABC's mystery series FlashForward.

All of NBC's new shows are on among the top 10 in the poll, which bodes well for the network's mission to climb from fourth place this fall. But fans said there were more likely to watch NBC's new dramas and comedy than the network's most crucial gamble, The Jay Leno Show, which premieres Monday night.

How many viewers will watch Leno is the biggest guessing game in town. Most agree the talk-show host's premiere will draw a crowd (after all, Conan O'Brien taking over The Tonight Show attracted plenty of curiosity in May). The question is where Leno will settle during the coming weeks.

NBC has defined success as Leno doing as well or better than he performed as host of Tonight, which would mean averaging 5.2 million viewers. But if NBC winds up with significantly lower average ratings at 10 p.m. with Leno and 11:35 p.m. with Tonight under O'Brien, will anybody outside of NBC consider the maneuvering a success?

What would be ironic is if NBC rebounded this fall thanks to its new scripted programing, while its heavily promoted, drama-replacing weeknight talk show struggled.

In the coming weeks, each network faces unique challenges.

-- Fox is hoping the charm of Glee doesn't wear thin and can continue posting strong numbers. The other sensitive hour on the schedule is the second season of Fringe diving into the Thursday night maelstrom. If both shows can hold firm numbers and there's no surprise drops from Fox's returning shows, its executives can put their feet on their desks and wait until Fox's midseason hits return.

-- NBC is tinkering with so much of its schedule that the entire lineup is a wild card. The network particularly needs its Thursday night comedy lineup to continue functioning after softening last season. If promising new comedy Community catches fire, Leno pulls his weight and at least one of the network's new dramas, like Trauma, sparks interest, the network could claim its back on track.

-- Vampire Diaries viewers returning for a second bite is of crucial concern to the CW, and the network also hopes that Thursday's ratings surge signals a rising tide that will lift the show's dramas this week.

-- CBS executives aren't likely to lose much sleep during the coming weeks. The network only needs one new hit to declare victory, and Tuesday's NCIS: Los Angeles is considered as close to a sure thing that exists in network TV. Legal drama The Good Wife on Tuesdays also could surprise, and most expect such scheduling moves as putting The Mentalist on Thursdays and shifting Monday's The Big Bang Theory to after Two and a Half Men to pay off.

-- ABC's biggest nail-biter is its new Wednesday night comedy block. Modern Family will win over critics and Cougar Town has viewers curious, but the chances of ABC instantly creating a successful block is like trying to roll snake eyes four times in a row. Thursday's FlashForward is considered the network's keystone new drama, but November's V might be its dark-horse surprise.