Intel (Nasdaq: INTC), the No. 1 chipmaker, Tuesday reported decent second-quarter financials but spooked investors and Silicon Valley with the dreaded news: third-quarter sales will be worse than expected.

Indeed, said CEO Paul Otellini, revenue could be as low as $13.8 billion as much as $1 billion below analysts' estimates, considering the quarter includes back-to-school sales of new PCs, laptops and smartphones.

To be sure, the forecast by the Santa Clara, Calif.-based Intel could also be wrong. The company said it expects revenue around $14.3 billion plus or minus $500 million.

Still that wouldn't a huge gain from the second quarter's $13.5 billion in revenue, only a 3.6 percent, surely a disappointment as the company starts moving its new Ultrabook chips for customers headed by Hewlett-Packard Co. (NYSE: HPQ), Lenovo Group of China (Pink: LNVGY) and Dell (Nasdaq: DELL), the No. 3 PC maker, the Big 3 in the PC sector.

The Ultrabook is the Intel answer to the chips from ARM Holdings of the UK (Nasdaq: ARMH), which are the heart of the iPad from Apple (Nasdaq: AAPL), the world's most valuable technology company.

Intel's main rival in microprocessors, Advanced Micro Devices (NYSE: AMD), already warned of a second-quarter earnings shortfall. AMD has been the second source for chips for the PC sector.

The Semiconductor Industry Association still maintains its 2012 forecast for a slight gain in chip sales, to about $301 billion worldwide. But Intel, on an annual basis, accounts for about 17 percent of all chip sales, so a bad forecast could be a harbinger of bad weather ahead for the major U.S. chipmakers, headed by Texas Instruments (Nasdaq: TXN), and others, as well as South Korea's Samsung Electronics (Seoul: 005930), the world's No. 2 semiconductor maker.

Shares of Intel dipped 11 cents to $25.27 after hours. That's only slightly below the 52-week high of $25.66. So far this year, they've risen nearly 5 percent.

The broader Philadelphia Semiconductor Index (IndexNasdaq: SOX) fell 1.70 on Tuesday to 351.45. For the year, it's already eased 3.5 percent.

Wednesday could see far more drastic activity.

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