It's about time to recall J. P. Morgan's usual comment when asked for a market forecast.

"It will fluctuate," he would say.

Just as no one--apparently--can explain exactly why the nation's money supply expanded at a record rate of $9.8 billion in the week ended Jan. 6, there's no way of telling in advance how the financial and stock markets are going to react this morning to late Friday's money supply news.

It is easy to say that all of the elements are in place for a major selling wave. The national economy is in recession and corporate profitability is slumping badly. Yet, a traditional response by the Federal Reserve to the news of sharp money supply expansion is to cut back on credit and that would make interest rates go up. The recession could become worse and hopes of the housing and automobile industries for a recovery based on lower interest costs would be aborted.

But offsetting the bad news is very good news on the inflation front that may have been overshadowed by the money supply figures last week. Wholesale price inflation, as measured by the index of producer prices, rose just 0.3 percent in December for an annual rate of about 4 percent after a 7 percent rise for all of 1981, 11.7 percent in 1980 and 12.6 percent in 1979.

George M. Ferris Jr., head of the Ferris & Co. investment firm in Washington, emphasized the inflation news in warning investors that too many of them "make investment decisions after reading newspaper headlines" while overlooking more fundamental factors.

"There may be some selling on the news but I'm betting the Federal Reserve is not taking a month-to-month stance and that the recent figures are an aberration in a long-term policy . . . I think the Fed is still committed for the long range to keep down growth of the money supply," Ferris said yesterday.

Any weakness in the markets, he added, would be seen by Ferris stock and bond traders as a "buying opportunity" because in terms of total return over the next three to five years, quality bonds and stocks will be among the most attractive investments.

"We still feel that the American people recognize that the old system didn't work, that real wages are lower than 10 years ago . . . basically, the president still has the support of the people, who aren't blaming him" for current economic woes, Ferris asserted. The recession and lower inflation rates point to lower interest rates for the long term, even if there is a brief rise over the near future, he added.

Virtually all Wall Street and monetary policy experts agree that it is foolish to base investment decisions of any kind on any statistic as volatile and unmanageable as the weekly money supply (the currency in circulation plus checking accounts).

Analysts said, for example, that an earlier-than-normal delivery of Social Security checks on Dec. 31 and year-end bookkeeping by corporations probably distorted total money supply data for the week ended Jan. 6. They noted a similar sharp spurt in money supply at the same time a year ago.

In this murky environment, many investors just sit on the sidelines because they don't know what to do. That's probably a good place to ride out a storm they can't comprehend--and the storm will only intensify this week. Other investors will take the advice of Ferris and jump in now, with equity values at such low levels.

Among developments this week, the likelihood is that some major banks will raise their prime lending rate to top corporate customers from 15 3/4 percent to 16 percent, unless there is some signal from the Federal Reserve that nothing will be done in the wake of the money supply numbers.

But if the Fed does nothing, that will be taken as a signal by some bond traders of a new, easier credit policy. That implies an increase in public and private debts to be financed by the capital markets and a return to higher inflation rates, in the view of some traders.

So bond prices are likely to tumble some more from already low levels as interest rates rise. Long-term city and state bonds last week fell $6.25 for every $1,000 of bonds maturing in 20 years, for example.

In the stock market, the Dow Jones average of 30 industrial blue chips ended Friday at 847.60, down about 19 points for the week despite a recovery late in the week. The Dow is at its lowest level since last November.

Washington business, as measured by an index of 30 local companies, is performing in a similar fashion. Johnston, Lemon & Co.'s index, which continued a consistent rise throughout December when other market indicators headed down, has been on a skid of its own since the new year began.

As of Friday night, the Johnston, Lemon index was at 221.804, its lowest weekly close since early November.

One big factor in the local index is Fairchild Industries, a Germantown, Md., aerospace and communications firm that hit a yearly low on the New York Stock Exchange Friday.

Fairchild closed the week at $11.50 a share after trading as low as $11.38 on a volume for the week of 146,000 shares. The company's shares are now worth about a third of the more than $30 a share it traded at back in 1981--before the Reagan administration decided to cut back sharply on production by Fairchild of the A10 close support fighter for the Air Force.

The Montgomery County company said Friday that 1981 profits fell to $64.3 million ($3.48 a share) from $54.5 million($4.02) in 1980 with fourth-quarter profits off more sharply because of development and start-up costs on commercial aircraft programs. Fairchild also lost seven executives of its electronics and space division--victims of the Air Florida plane crash in the Potomac River on Wednesday.

Another big Washington business trading near a 12-month low is Federal National Mortgage Association, which declined 50 cents a share last week to $7.75 on a volume of 309,000 shares. In the past year, FNMA has traded as high as $12.13 and as low as $6.38. Called Fannie Mae, this company is the nation's largest single owner of home mortgages and, as such, a prime victim of the record interest rates of 1981.

FNMA's board is due to meet on Tuesday and a report on 1981 operations probably will be released in advance, sometime today. A large net loss for the year is anticipated, with losses in the fourth quarter less than those of the previous three months, according to analysts. But with prospects of continued losses ahead, directors of FNMA must decide whether to continue paying a reduced cash dividend or to join Ford Motor Co., whose directors finally voted last week to stop paying out cash dividends on common stock--long after continued losses were evident.

Besides the wholesale price report, there was one other report last week with an element of optimism. Investment Corporation of Virginia, a Norfolk securities firm, said its index of 92 firms in the Middle-Atlantic region (companies in D.C., Maryland, Virginia, West Virginia and the Carolinas) rose 12 percent last year from 173.22 to 194.08. This was in contrast to a 9.2 percent decline in the Dow Jones industrials, a 9.7 percent decline for the Standard & Poor's 500-stock index and a 3.2 percent decline for the NASDAQ over-the-counter index.

Star performers in the Investment Corporation's index included Norfolk & Western Railway (up 27.3 percent), United Virginia Bankshares (up 44.2 percent), Virginia National Bankshares (up 41.8 percent), and Garfinckel, Brooks Brothers, Miller & Rhoads (up 118 percent because of a takeover by Allied Stores.