The Japanese yen staged its sharpest single-day rally since the late-July coordinated intervention by Tokyo and Washington, surging nearly 2% on Thursday after a month of gradual depreciation. The dollar tumbled from the 160.39 high touched earlier in the week to a low near 155.30, narrowly missing the 155.23 post-intervention trough, sparking intense market scrutiny.

JPMorgan strategists have warned that a decisive break below the 155 threshold could trigger a rapid unwinding of massive yen short positions, potentially fueling a self-reinforcing selloff that pushes the currency further than many anticipate. The bank's analysis, led by Junya Tanase, estimates that roughly 16 trillion to 17 trillion yen (approximately $102.6 billion) in bearish yen bets remain open, raising the risk that "selling begets selling" and drives an outsized appreciation.

JPMorgan calculates that if those short positions were fully liquidated, dollar-yen could theoretically tumble into the 142-146 range. "Recent price action appears to confirm our view that relatively large yen short positions may still exist in the market," the bank noted in a report, though it cautioned that expectations surrounding Japan's Government Pension Investment Fund (GPIF) rebalancing and faster Bank of Japan tightening "seem somewhat excessive," keeping the probability of a sharp break below its assumed 155-165 band low in the near term.

Several catalysts converged to fuel Thursday's yen rebound, amplified by short covering. Speculation that GPIF, the world's largest pension fund, may raise its target allocation to domestic bonds has been interpreted by some investors as yen-positive, even without official confirmation from the fund. Simultaneously, BOJ board member Takata created on Wednesday said a 25-basis-point hike is "not necessarily a done deal," adding that consecutive rate increases are generally possible, signaling a potentially more aggressive tightening path.

Swap markets now almost fully price a 25-basis-point hike at the BOJ's September 18 policy meeting, with roughly 80% odds of another move in December. In the U.S., dovish remarks from Federal Reserve Governor Christopher Waller on inflation progress weakened the dollar, with the dollar index sliding 0.6% to its lowest level since May, narrowing rate differentials in favor of the yen.

Yen short covering and hedging demand from domestic investors magnified the surge, according to Chidu Narayanan, chief Asia-Pacific strategist at Wells Fargo. However, he cautioned that these flows alone are unlikely to drive the yen substantially higher, arguing that a more significant rally would require a hawkish BOJ surprise, fiscal prudence from Tokyo, and broad dollar weakness.

The yen's earlier slide had forced Japanese authorities to intervene jointly with the U.S. in late July, spending a record $96.4 billion over the past month. While intervention temporarily stemmed the decline, doubts about its long-term effectiveness persist. When dollar-yen breached 160 again earlier this week, intervention efficacy came under question, prompting a notably more hawkish tone from Japanese officials.

Japan's top currency diplomat, Atsushi Mimura, told reporters he is "not satisfied" with the yen's current level and pledged to "keep fighting in the FX market," a markedly tougher stance than recent comments. Yusuke Miyairi, FX strategist at Nomura International, noted the significance of Mimura's language given rising market anxiety over intervention risk. "In this environment, the probability of dollar-yen testing 160 has decreased, and the pair is more likely to hover around the lower end of the 155-160 range," Miyairi said.

Traders are also bracing for possible intervention during the upcoming "Silver Week" holiday period, recalling that Tokyo chose to act during a long holiday in April. The break begins shortly after the BOJ's policy meeting. Though BOJ data showed no signs of large-scale official buying on Wednesday, State Street's Tokyo branch manager Bart Wakabayashi observed, "After any sharp move, the first reaction on every trading desk is 'is this intervention?' As we saw last night, I think the market will remain very sensitive and on edge."

With the yen's surge now in focus, attention shifts to the BOJ's September 18 meeting. U.S. Treasury Secretary Scott Bessent has intensified public pressure on Japan to raise rates, and if the central bank disappoints his barely disguised calls, it could surprise traders and trigger renewed yen weakness. Paresh Upadhyaya, strategist at Pioneer Investments, believes the "BOJ appears set to pull the trigger on a hike in September and may open the door for a faster pace of subsequent increases. We are finally seeing some meaningful follow-through on the policy expectations front to complement the earlier intervention."

However, caution persists regarding the yen's sustainability. Hideaki Minami, head of FX spot trading at Mizuho Bank, noted that yen buying may stem largely from overseas investors betting on aggressive hikes, but "it's too early to conclude from today's move that the yen's downtrend has reversed."