The record-shattering rains behind the flood catastrophe in southeast Texas will continue for several more days and have expanded into southwest Louisiana.

Across this entire region, the National Hurricane Centre is calling for 25cm to 50cm of new rainfall to Friday.

Today Tropical Storm Harvey had drifted to the Texas coastline and strengthened ever so slightly near the warm waters of the Gulf of Mexico. The Hurricane Centre said the storm's peak winds, had increased from 65 to 72 km/h. But it stressed rain, not wind, was expected to be the main hazard of concern.

Rain increased in coverage and intensity over the Houston area today. At Houston Hobby Airport, more than 18cm had fallen today, a new record for the date.

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Flash flood watches expanded as far east as New Orleans, where the Weather Service said 12cm to 25cm were possible to Friday.

Locations near Houston in Harris County had seen up to 94cm of rain, with a county average of 84cm. Isolated areas to the northeast had received up to 1m.

According to the National Weather Service, the forecast of more than 30cm of additional rainfall in the Houston area "would have devastating consequences on the continuing rescue and recovery efforts".

Some areas in southeast Texas could see storm rainfall totals exceeding 1.2m, which would break state records. An analysis by Eric Berger, a scientist who pens Houston weather blog, concluded Harvey is "almost certainly the biggest US flood-producing storm" on record.

The astronomical rain totals had pushed river levels in Southeast Texas near and beyond record levels.

The Weather Service office serving Houston described the rain amounts so far "unfathomable". The 41cm that fell on Houston's George Bush Airport yesterday marks the single wettest day in Houston history, making up nearly a third of the 1.2m the city sees in an average year. More than 61cm fell over the weekend, a record two-day amount.

Over the Houston metro area, so much rain would be expected to happen between just one time every 500 to 1000 years.

A man reaches to take a small dog from a rescue truck at the east Sam Houston Tollway. Photo / AP
A man reaches to take a small dog from a rescue truck at the east Sam Houston Tollway. Photo / AP

The August rainfall in Houston, largely from Harvey, shattered its record for any month by a whopping 34cm.

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This flood disaster has easily surpassed the havoc wrought by the landmark Tropical Storm Allison in 2001, the Weather Service said.

Economic damage in the many billions of dollars is inevitable, according to economists and reinsurers.

As Harvey's centre gradually shifts over the Gulf of Mexico into tomorrow, some minor additional strengthening is possible, before again tracking onshore in the vicinity of Galveston.

The rainfall intensity in the Houston area will wax and wane through to Friday, but periods of excessive, flooding rainfall are possible, especially tomorrow into Thursday. The heaviest rain is expected over the eastern part of the region.