The eyes of the world's financial markets will be on Washington today as the Commerce Department releases its latest monthly report on the U.S. trade deficit -- the report that a month ago sent stock and currency markets reeling five days before Black Monday.

The monthly trade report, a key measure of the nation's fluctuating fortunes in a global economy, is capable of sending new shocks to the markets in this country and around the world where stocks, currency and commodities are traded. Even if analysts have guessed correctly what the deficit will be, there is no predicting how the markets will react.

In the month since the last report, the U.S stock market and its major foreign counterparts have been clobbered, the dollar has fallen sharply against other currencies and the direction of U.S. economic policy has become something of a daily puzzle.

"You are going to have a flurry of trading," predicts Brian Monieson, chairman of GNP Commodities Inc., and a former chairman of the Chicago Mercantile Exchange, where futures contracts on everything from currencies and government bonds to pork bellies are traded.

The release of the trade report has become an "event day," in the market, he said. "Event days supposedly give the markets a signal of which way the economy is going," he said. "You are going to have an immediate reaction to that event."

The Commerce report, covering imports and exports for September, will be released into a vortex of differing aims and expectations at 8:30 this morning, and the first reaction is likely to be felt in financial markets in London, where it will be early afternoon. The New York markets open an hour later, at 9:30.

Many market analysts expect the report to show only a small improvement in the nation's chronic trade deficit. Allen Sinai, chief economist for Shearson Lehman Bros., in a typical forecast, projected an $800 million reduction in the deficit to a level of $14.9 billion.

Stocks plummeted 95.46 points Oct. 14 after the August trade deficit came in at $15.7 billion, down $800 million from July -- a much smaller reduction than most analysts had expected. The dollar also dropped.

"Over the past three months, market sentiment was jarred repeatedly by government reports of surprisingly large shortfalls in merchandise trade," Stephen S. Roach, chief economist of Morgan Stanley & Co. Inc., said in recent congressional testimony.

With the Reagan administration committed then to an agreement with other industrial nations to keep the dollar's value from falling compared with other key currencies, it was "a perfectly natural response" for the market to react to bad news on the trade deficit by bidding up interest rates, Roach said. Higher U.S. interest rates can be used to support the dollar by encouraging foreigners to buy dollars to invest here.

As interest rates continued to rise, Reagan administration officials, including Treasury Secretary James A. Baker III, began to worry that the high rates could trigger a recession. Baker made it plain in a Wall Street Journal interview last week that his first priority is avoiding an economic slump even if it means allowing the dollar to decline.

Some administration officials and a number of economists say the monthly trade figures fail to give an accurate representation of the trade picture, which has shown a clear improvement as measured by the volume of trade instead of dollar values.

As the dollar falls and the prices of imported goods go up, the size of the reported trade deficit also rises. The increase in foreign goods prices relative to those of U.S. produced goods should slow the growth of imports as consumers shift to U.S.-made goods. Similarly, with U.S. exports suddenly cheaper in terms of foreign currencies, the demand for U.S. goods abroad should rise.

Thus, while the monthly trade figures have shown little improvement, numbers included in the Commerce Department's quarterly data on the gross national product, which are adjusted for seasonal variations and inflation, show more of a gain in the trade balance.

Even though the deficit also worsened slightly in the third quarter when measured on that basis, the GNP figures showed a 16.2 percent increase in the volume of exports over the past 12 months on top of an 8.1 percent gain during the previous year. At the same time, the increase in nonoil imports slowed sharply to 2.2 percent from 13.4 percent.

"These volume improvements have been largely masked to date by offsetting price effects," especially higher import prices in the United States, said C. Fred Bergsten, director of the Institute for International Economics.

As much as import prices have gone up, though, since the value of the dollar began its long decline in February 1985, they have not gone up nearly enough to reflect the roughly 50 percent drop in the dollar or to make a large dent in the trade deficit.

"America's trade dilemma is, first and foremost, a problem of excessive imports," Morgan Stanley's Roach said. "Despite almost three years of a massive currency correction, foreign producers have continued to make inroads in our domestic markets." The key reason for this, he said, is that "imports remain relatively cheap" even though their prices have risen at about 8 percent a year -- more than twice the rate of inflation.

Foreign companies have been able to absorb the effects of the lower dollar, Roach explained, because they made such fat profits between 1980 and 1985, when the dollar was strong, that they could match the currency change by cutting their profit margins. Other economists have also noted that some companies have accepted lower profit margins or even losses to maintain their share of the U.S. market until they can cut their costs of production either by buying more components from countries such as South Korea, whose currency has not gone up significantly against the dollar, or by shifting production to the United States.

Roach said foreigners may not be able to live with the low profit margins much longer. "Our estimates suggest that foreign producers are now at the critical break-even point in their pricing of American imports," which could force them to increase prices more rapidly if the dollar continues to fall.

Washington Post staff writer Jerry Knight contributed to this report.