The boss of the Fed plays big with his speech Friday

Jerome Powell will pronounce Friday one of the most important speeches of his career and try to convince that the US Central Bank knows how to extend the longest period of expansion of the world's largest economy.

Markets will be all ears just like Donald Trump, who attacks the Fed almost daily and accuses him of curbing "his" economy.

Powell speaks Friday at 14:00 GMT for the first time since the end of July rates drop, in front of the gotha ​​central bankers and economists traditionally gathered each year at Jackson Hole, a prestigious mountain resort in the state of Wyoming.

Markets, waiting for confirmation of their expectations on rate cuts, were still nervous Thursday.

For the third time in eight days, the interest rate curve on US Treasuries reversed, a sign often seen as prefiguring a recession in 12 to 18 months.

The Markit manufacturing index moved into negative territory for the first time since the 2009 recession.

On Thursday morning, after already a series of tweets the day before that compared Powell to an incompetent golf player "lacking fingering," President Donald Trump, who is campaigning for much lower rates, reiterated his attacks on Central bank.

"Our Federal Reserve is preventing us from doing what we have to do," he writes indignantly that Germany sells "30-year negative return bonds". He accuses the Fed of "putting the US at a competitive disadvantage".

US interest rates, which the Fed modestly dropped at the end of July for the first time in more than a decade, are between 2% and 2.25% while US growth is 2.1% (annual 2nd quarter) while Germany is close to recession.

But the president blames the Central Bank for having raised the end of 2018 rates too quickly and caused a strengthening of the dollar which handicaps the United States in the middle of a trade war.

For his part, Jerome Powell, who was appointed by Donald Trump in early 2018 before quickly falling into disfavor, advance on a narrow path.

It strives to accompany low inflation and extend the longest US growth in modern history by granting what it called a "mid-cycle adjustment" without promising "a series of drops" in rates.

He is also committed to defending the independence of the Fed and keeping the cohesion of its Monetary Committee, divided in the face of remedies to the prospect of slowing down.

– Mixed signs –

Because the American activity shows mixed signs. It mixes solid consumption with a sluggish manufacturing sector and disappointing business investments that have become chilly in the face of trade tensions.

The next monetary meeting of the Central Bank is scheduled in three weeks, September 17 and 18, and the markets are expecting a further decline in interest rates. But how big?

Esther George, president of the Kansas City Fed, host of the Jackson Hole forum who voted against the decision to cut rates in July, appeared to camp on her positions Thursday.

Asked about CNBC from the station, she said the rate level seemed to be "well placed" at the moment.

She admitted that the economy was facing adverse risks with "the weakening of global growth and the degree of uncertainty associated with trade issues". But it retains a growth projection of 2% for 2020.

Philadelphia Fed President Patrick Harker also said on Thursday at Jackson Hole that he was more inclined to wait to see "for a while" how the economy is evolving. He said the rates "are pretty much where they should be."

He also delayed the importance of the divergence of rates between the United States and their partners: "We are not so much out of sync as that".

The Fed "can not ignore what is happening elsewhere", but it must "first do what is appropriate for the United States," said the official questioned CNBC after the tweets of Donald Trump on the differential of German rates.

After these comments by members of the Fed, the 2-year and 10-year US Treasury yield curve reversed slightly twice but briefly.

The phenomenon, which sees long-term rates becoming cheaper than short-term rates, reflects a rush of investors toward US vouchers as a safe haven given the global slowdown prospects.

Wall Street closed on Thursday because of the index on manufacturing activity and investors preferring not to engage too much, either way, until Mr Powell expresses himself .