* Outlook for Q3 cocoa grind seen subdued
* Sugar trade focused on harvest weather in Brazil
* Arabica coffee supplies seen ample, analyst says (New throughout, updates prices; adds trade comment, second byline/dateline)
By Marcy Nicholson and David Brough
NEW YORK/LONDON, Sept 19 (Reuters) - ICE raw sugar futures made their biggest two-day tumble in three months on Wednesday, with dealers focused on a global surplus of the sweetener and on the progress of the harvest in Brazil, while a broad decline in commodity markets added to downward pressure.
Arabica coffee also fell as commodity markets extended this week's setback with the Thomson Reuters-Jefferies CRB index falling more than 1 percent to a two-week low as Brent crude oil prices slid to a six-week low.
ICE October raw sugar futures fell 0.48 cent, or 2.5 percent, to end at 18.96 cents per lb, drifting down towards a two-year low of 18.81 cents touched on Sept. 6. The move lower extends the previous session's losses, causing the contract to drop 5.3 percent in the past two days, the biggest two-day fall since June 22.
Dealers anticipate a large global sugar surplus, which is weighing on prices, and are focused on harvesting in Brazil and in the northern hemisphere.
"The weather in Brazil will be a key determinant to watch, to see whether or not production can catch up after a delay at the start of harvesting," Stefan Uhlenbrock, a senior analyst with F.O. Licht, said.
Thomas Kujawa of broker Sucden Financial said, "There are crop reports expected from Brazil later this week and the chat around seems to be they are positive for the bears."
Standard Chartered said in a sugar market report that raw sugar's support near 18-20 cents per lb appears vulnerable.
"We acknowledge that current bearish sentiment is justified to some extent and lower our sugar forecasts to 21 cents in Q3 (23 cents previously), and 22 cents in Q4 (24 cents previously.)"
December white sugar on Liffe fell $11.30, or 2 percent, to finish at $552.00 per tonne.
In arabica coffee, ICE December dropped 3.10 cents, or 1.8 percent, to finish at $1.7440 per lb.
"It consistently runs into resistance around the $1.85-$1.90 level," said Spencer Patton, founder and chief investment officer of Steel Vine Investment in Chicago.
"It's reinforcing to the market that this is the fair value for coffee. It's just sideways consolidation."
Uhlenbrock said he anticipated bearish price risk for arabicas after a rally on investor short covering to seven-week highs last week.
"I do not see the bullish trend continuing. Brazil has had a bumper crop," he said, referring to the world's top coffee producer, where harvesting is now finishing.
November robusta coffee futures closed down $7, or 0.3 percent, at $2,017 a tonne.
Cocoa futures were marginally higher in sideways dealings as traders focused on potential disruption arising from top grower Ivory Coast's reforms and the market awaited the publication of the fixed farmer price for the season starting Oct. 1.
There are fears that an unevenly managed reform, which has overhauled the Ivorian cocoa sector and includes a new regulatory body, will cause chaos in its exports as well as increased smuggling of the beans out of the country.
ICE December cocoa inched up $6, or 0.2 percent, to settle at $2,538 per tonne, while Liffe December cocoa ended up 9 pounds, or 0.6 percent, at 1,642 pounds per tonne.
Traders also looked forward to third-quarter cocoa grind data, a key measure of demand, predicting a subdued outlook.
"The picture is broadly negative," said Eric Sivry, head of agri options brokerage at Marex Spectron in London.
Cocoa merchants in Ivory Coast are threatening to block supplies or resort to smuggling if a prolonged dispute with the country's marketing body over transport costs is not resolved before the start of the new season next month.
Ivory Coast has paid more than 47 billion CFA francs ($93.54 million) into a reserve fund to protect against price fluctuations as it implements a sweeping reform of its cocoa sector, a government spokesman said.
Cameroon's local cocoa grinders purchased 5,082 tonnes of cocoa in August, the first month of the 2012-13 season, the National Cocoa and Coffee Board said on Tuesday. (Editing by Helen Massy-Beresford and Marguerita Choy)