WET BLANKET; MILD SUMMER ENDS WITH THUNDERSTORMS.

Byline: Michael Coit Staff Writer

Southern California's cool summer heads out with a monsoonal bang as thunderstorms hit Tuesday and were expected to linger into fall's official arrival Thursday - more weird weather that was a delight to gardeners.

Tropical Storm Hilary swooped into the Southland on Tuesday, bringing fast-moving thunderstorms and humid air. A chance of showers was expected to continue into Thursday.

``We got flooded out. All of our plants got drenched. We won't have to water them for a while,'' said Teresa Holmes, manager of Chatsworth Nursery, as she cleaned up after 15 minutes of heavy rain.

Holmes said strong winds also knocked down a section of the nursery's wrought-iron-and-brick back gate, as well as many boxed trees and displays. ``We got picked on and we weren't prepared. I didn't even have a rain jacket.''

Rain was reported across the region, from Chatsworth to Woodland Hills, and from Van Nuys to Ventura County communities and the coastline.

The rainfall was mostly welcome, though, soaking flowers and plants that already have savored a cool summer.

The ocean-cooling La Nina weather pattern has kept daytime highs about 4 degrees below normal through the summer. A year ago, temperatures were about 3 degrees above normal, said Mark Tobin, a meteorologist for AccuWeather, a weather forecasting company.

Fall should be warmer and a bit more dry than normal thanks to La Nina.

``Our marine layer sticks around for a longer time during the day. So in the summertime that keeps us on the cool side. During the fall and wintertime, those clouds at night will help hold up our temperatures a little bit,'' Tobin explained.

The absence of much sweltering weather this summer allowed homeowners and professional landscapers to water less. Plants also lasted longer.

With autumn's arrival, green-thumbers said this is prime planting time.

``I'm getting a head start. Usually, I plant in October,'' said Barbara Mervau of West Hills.

Pulling a cart through a Canoga Park nursery, Mervau packed in pansies, snapdragons and mums, and several varieties of spring bulbs, as well as irises.

But she said there isn't as much room in the garden as she normally has because many plants, including tomatoes and arugula, have lasted longer this summer. ``Everything looks real good,'' she said.

The patio garden Thelma Marks tends at her Woodland Hills home also is looking good.

``Everything did well this summer. I had to water a little bit less and it was good for a lot of my plants,'' she said. ``The hot sun usually does a job on them.''

Curtis Suchomel, a landscaper based in Simi Valley who also works in the Los Angeles area, said this has been one of the easiest summers during his 15 years in the business.

``It's been great. It's been a really mild summer. You don't need as much watering and it's easier just keeping things alive,'' he said.

The cooler weather actually has been good for business, Suchomel said.

``A lot of times people get put off when the weather is hot. They're afraid to put things in because they think they will die off,'' he said.

``Usually fall is when business gets busy, but being the mild summer that it's been, business has been pretty steady,'' he said.

While fall also usually marks the end of peak water demand, conservation was evident throughout the summer.

Los Angeles Department of Water and Power officials said water demand from June through August was equal to the demand during the same period in 1998, despite the city's population growth over the past year.

DWP officials said residential customers appear to be continuing conservation practices that took root during the six-year drought that gripped Southern California through 1992.

``Part of it was they were forced into changing how they used water during the drought and they have kept to that. Cooler weather only helps that,'' said Rich Harasick, assistant director of water resources.

``The people ought to be proud of themselves for doing that,'' he said.

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Photo: (Color) A downpour raced through the Valley on Tuesday, slowing traffic on the 118 Freeway. More area thunderstorms are expected today.

Gene Blevins/Special to the Daily News