The yen's dramatic rebound is intensifying, with strategists at JP Morgan warning that a break below the 155 level against the US dollar could trigger a cascade of forced short covering.

Led by Junya Tanase, the bank's strategists noted in a report that recent price action appears to confirm their view that a relatively large pool of yen short positions may still be outstanding. If USD/JPY falls below 155, they cautioned that "the risk of selling begetting more selling, thereby pushing the yen's appreciation beyond expectations, cannot be ruled out."

JP Morgan estimates that roughly ¥16 trillion to ¥17 trillion, equivalent to approximately $102.6 billion, in yen short positions remain open. Should these positions be fully unwound, the theoretical impact could drive USD/JPY down to the 142-146 range, suggesting that 155 is not just a technical level but a potential trigger point for a concentrated short squeeze.

The warning comes as the yen experiences one of its most powerful rallies since the joint US-Japan intervention in late July. USD/JPY briefly touched 160.39 earlier this week, its highest since that intervention, before rapidly reversing course to a low of 155.30, edging dangerously close to the post-intervention trough of 155.23.

Multiple factors are fueling the yen's ascent. Speculation is building that Japan's Government Pension Investment Fund (GPIF) may adjust its asset allocation, while expectations for faster interest rate hikes by the Bank of Japan continue to gain momentum. Market participants believe these elements have been amplified by speculative yen short covering and increased hedging demand from domestic investors. Should the yen continue to strengthen, additional shorts could be forced to exit, creating a self-reinforcing cycle that pushes the currency higher.

However, JP Morgan has stopped short of turning aggressively bullish on the yen. The bank suggests that market expectations surrounding both the GPIF and the BOJ "appear somewhat overdone," and does not currently view a significant break below its assumed 155-165 range as a high-probability scenario.

Data from the Bank of Japan shows that authorities did not intervene on a large scale on Wednesday. Yet Japan's top currency official, Atsushi Mimura, told reporters that day he was not satisfied with the yen's current levels, emphasizing that "Japan is ready to continue fighting." The remarks have reinforced market vigilance over intervention risk.

Yusuke Miyairi, a currency strategist at Nomura International, believes Mimura's language was noticeably more forceful than previous statements. "In this environment, the likelihood of USD/JPY testing 160 has diminished, and the pair is likely to gravitate toward the lower end of the 155-160 range," Miyairi said.

Swap markets are now almost fully pricing in a 25-basis-point rate hike at the BOJ's meeting this month, with odds of another move in December standing at roughly 80%—indicating that traders are positioning for a faster tightening cycle than previously anticipated.

Brendan Fagan, a macro strategist at Bloomberg Markets Live, pointed directly to the policy signal embedded in the yen's sudden surge: "A 2% single-day rally in the yen against the dollar is the clearest evidence that a more proactive central bank is the best cure for a weak currency."

With exchange rates whipsawing, traders remain on high alert for signs of official yen buying or verbal inquiries about trading prices—behaviors often viewed as precursors to market intervention. Bart Wakabayashi, branch manager at State Street Trust in Tokyo, remarked: "After any sharp move, the first reaction at every trading desk is to ask, 'Was that intervention?'" He added, "As we saw last night, I think the market will remain highly sensitive and highly tense."

USD/JPY was trading at 155.66 as of Friday's writing, putting the yen on track for its best weekly performance since July with a gain of roughly 2.7% against the dollar.

Wall Street strategists are already positioning for further dollar weakness. Bank of America favors shorting USD/JPY with a target of 149, anticipating the yen to strengthen to that level by year-end. TD Securities maintains a "mildly bearish dollar" outlook for the remainder of the year.

Even before this week's dollar decline, speculative investors had begun trimming bullish dollar bets. CFTC data shows that in the week ending August 25, hedge funds, asset managers, and other traders reduced their net long dollar positions to approximately $27.6 billion, down from nearly $50 billion in late July, when bullish positioning reached its highest level since 2014.

Traders are now turning their attention to Friday's non-farm payrolls report, followed by a critical consumer price reading next week. These data points will likely shape market expectations for the Federal Reserve's next moves and, in turn, determine the direction of the greenback.