Jersey has not had significant rainfall since 3 April and while the UK received plenty last weekend, only 2mm fell in Jersey last Sunday.
"It's looking like the first half of the crop will have a significant yield reduction," said Mike Renourd, technical director for The Jersey Royal Company, estimating the crop could be down by 50%. And forecasters are not predicting any more rain for another two weeks.
The situation was so serious that technical managers at all major supermarkets had visited in the past few days to assess the impact on supply, Renourd added.
"We are having to adjust our sales plans pretty much every day."
The lower yields also meant that retailers were having to rethink their promotional strategies on Jersey Royals this year. "What we would end up doing with each customer sales-wise with 34,000 tonnes would be completely different if we were to get, say 20,000 tonnes."
Jersey Royals are an important summer crop to the multiples and one they heavily promote to outdo their rivals. "It's a key crop and brand it kicks off the UK potato season," said one retail source.
In May last year, retailers were offering an average percentage saving of up to 62.5% on some Jersey Royal lines [Assosia].
Although retailers would still run some promotions, they were likely not to be as hard hitting as when greater volumes had been available, the source added.
"Traditionally, coming up in the next couple of weeks you'd have the big volume coming on line where all the retailers would look to do the big promotions this week, next week and the week after, but clearly that volume is down."
The effect of the current drought the worst since 1976 according to islanders had been compounded by cold weather in January, which delayed this year's crop, said Renourd. "If it had been cold at the start [of the year], it would have been the back end of the season that was affected by the drought rather than the front it really depends on the rain now."
The later planted crop was looking OK at the moment, he added.
In 2008, Jersey Royals were left rotting in fields after a deluge hit the island.
Sales of all Jersey potatoes, not specifically Jersey Royal potatoes, were worth £45.7m [52w/e 18 April 2010 Kantar], up 20.2% on the year before. Volumes were up 32.5% to 37.9 million tonnes.