* Index fund roll weighs on spot contract

* Fiber hovers near technically oversold, 14-day RSI at 31.8

* Traders eye monthly U.S. government supply-demand forecast

NEW YORK, June 9 (Reuters) - Cotton futures dipped to 1-1/2-week low on Monday, under pressure after weekend storms across the United States showered key cotton regions in Texas, the top-producing state, with much-needed rains.

The front-month July cotton contract on ICE Futures U.S. fell to 84.07 cents a lb before closing down 0.23 cent, or 0.3 percent, to settle at 84.55 cents a lb.

A fourth straight down session kept the spot contract near technically oversold territory, with a 14-day relative strength index of 31.8.

Rains showered the world's top exporter this weekend, including areas of Texas that have been plagued by a multi-year drought. Forecasts of more rain to come further weighed.

The July contract was also under pressure from the rolling of closely-watched index funds.

The December ICE contract, which represents the new crop, was also under pressure from the favorable U.S. weather, closing down 0.71 cent, or 0.9 percent, at 77.29 cents a lb.

"We've seen very beneficial rains in production areas. Texas hasn't received rains like this in multiple years," said Sterling Smith, a futures specialist with Citigroup in Chicago.

A monthly U.S. government forecast was expected by many to call for higher domestic output in the 2014/15 crop year due to the Texas rains.

Front-month prices have been buoyed by worries over tight nearby supplies, but have come under increasing pressure from expectations of more supplies to arrive in the new crop year that begins on Aug. 1.

Speculators brought their bullish bet in fiber to the lowest since December 2013, weekly U.S. government data showed on Friday.

Total open interest fell to 181,422 lots on Friday, down from 184,263 previously and at the lowest since April, exchange data showed on Monday.

(Reporting by Chris Prentice; Editing by Marguerita Choy)