A fine rain! Truly it was:

Two cues stacked: an exclamation and then "truly it was," announcing the set-piece. The rain gets a full descriptive rhapsody of its own, which is the novel signalling that this is the climax of the plan.

Whispering and scattering, dense and heavy.

Four doubled words that will organise the whole piece. The first pair is light and scattered and whispering; the second is dense and heavy and sinking. The next two lines take them up one at a time and expand each into its own image, which is a tidy bit of construction.

Whispering and scattering, like stars falling from heaven's edge;

The light rain developed: like stars dropping off the rim of the sky. Wide, sparse, falling from an enormous height.

dense and heavy, like the sea's mouth hung upside down, its billows rolling.

And the heavy rain developed: as if the mouth of the sea had been turned upside down. 倒 is read dào here, meaning inverted -- worth marking, because the same character comes back at the very end of this piece with a different sound and a different job.

At first the drops were the size of fists; then it came as if jars were emptied and basins upturned.

The crescendo, and it is measured in household objects: drops the size of fists, then jars emptied and basins tipped over. After stars and oceans, the poem reaches for the kitchen, and the shift in scale is what makes the downpour feel real.

It flooded the ground to a drake's-crown green, washed the high hills to a Buddha's-head blue.

Two named pigments, and this is the sort of detail that rewards a moment. 鴨頂綠, drake's-crown green, is the iridescent green on a mallard's head. 佛頭青, Buddha's-head blue, is the deep blue used for the curled hair of Buddha statues -- a real pigment name, ground from lapis or azurite. So the flooded ground and the washed hills are being described in the vocabulary of a painter's colour box.

In the gullies the water flew, a thousand zhàng of jade; in the ravine-springs the waves swelled, ten thousand threads of silver.

Water flying in the gullies, a thousand 丈 of jade, and the ravine-springs swelling into ten thousand threads of silver. Jade for the falling sheets, silver for the running threads. 丈 stays in Chinese as zhàng, the length-unit -- ten of them make roughly thirty metres, so a thousand is a poetic quantity rather than a measurement.

At the three-forked crossroads it filled before the eye; in the nine-bend brook it slowly levelled.

The flood observed at ground level: the crossroads filling while you watch, the nine-bend brook slowly coming level. 看看 means gradually, as one watches. After all the jade and silver, this is just water rising in a road.

This was the divine dragons' aid in Tripitaka's ordeal — they pulled down the River of Heaven and poured it below.

The close, and here is 倒 again -- now dǎo, meaning to pull something over, where six lines ago it was dào, hanging inverted. One character, two sounds, two jobs, inside a single short poem. What they pull down is the 天河, the River of Heaven, which is the Chinese name for the Milky Way. And the reason is given plainly: 唐僧有難, the Tang Monk is in an ordeal, with 難 at nàn, the counted sense.