Spring has arrived. But in Manhattan - down on Seventh Avenue, at least - it's autumn.

Although the calendar may mandate tulips and warm weather, Seventh Avenue designers are thinking crisp, cool days and crackling leaves. They traditionally operate on a topsy-turvy timetable, you see, showing spring clothes in the fall and fall clothes in the spring. And nobody would dream of breaking with tradition.Actually, the crazy timetable makes a lot of sense. Creating a collection, writing orders, manufacturing a line and getting the clothes shipped to stores all over the country takes time. A long time. Designers have to start early in the game in order to meet deadlines and answer consumer needs.

Of course, there are dangers in the system.

"Please don't ask me what month it is,"sighs top-selling designer Eleanor P. Brenner. "I work so far in advance I never know!"

Brenner's fall collection might be in the spotlight at the moment - she recently showed the line to fashion editors and press at the Essex House. But even autumn's old news around her sleek, modern showroom and design studio in the heart of Manhattan's garment district. These days she's busy lining up fabrics, making predictions and doing sketches for the spring of 1990.

As for us, we wouldn't dare look so far into fashion's future. We're merely going to move ahead five months or so and give you a sneak preview of the clothing trends that will be on smart ladies' lips and hips in September.

No doubt about it, September will kick off a sensational season. After attending fall market week shows in New York - many were concentrated at the grand old Plaza Hotel and attractive hotel president Ivana Trump sat front and center - that message was clear. But don't expect any big silhouette changes or revolutionary new directions. The "sensational season" we're talking about simply lies in the abundance of wonderful, wearable clothes that will stream into the stores.

Designers, who focused on Chanel and elegant classics in their spring lines, still are following the same road and playing it safe with things they know will have wide general appeal and sell.

Oh, sure. You're going to hear a lot of ballyhoo about skinny knit leggings, bulky quilted jackets, monastic hooded evening robes and sweeping and extravagant blanket wraps that look like flags and cost almost as much as anentire country. Yet, these are just interesting footnotes to the main story. Thecentral theme of the season is much more basic - clean-cut and good-looking sportswear. The kind of things our own New York designers do better than anybody.

Maybe Paris has the edge on haute couture. Maybe Italy has the corner on fabrics and precise tailoring. When it comes to interpreting casual ease and modernity in fashion, though, no one can beat Seventh Avenue's stars when they're at the top of their form. And this fall, that describes them exactly.

Fashion - smart and contemporary American fashion - never has looked better. Designers have considered the fast-paced and demanding lives women lead today and they've produced clothes to suit those lifestyles. Almost everything in the apparel industry seems to have lightened up and become easier and less contrived. Even evening clothes exhibit a new ease for the autumn of '89.

Certainly, wonderful ball gowns will still be around for those gala occasions when only the most elaborate outfits will do. (Scaasi, the designer who has first lady Barbara Bush for a client, has some magnificent ones in "Barbara blue" perfect for parties on Capitol Hill). Dresses that drip with Lesage embroidery and drop-dead designer details will be on the party scene, too. But beyond the customary glitz and glamour there's a new movement in evening attire.

Expect to see many traditional daytime sportswear styles such as turtlenecks, blazer jackets, narrow skirts and pants done up in rich and luxurious fabrics for after five. Expect to see fewer fashions that are dressy and fussy - the kind of clothes that call for spike heels, sheer stockings and a truckload of elaborate accessories.

Fall will usher in an era of calm, uncluttered clothes all around the clock -styles that require little jewelry and often look right with flat shoes, opaque tights and a muffler at the neck. Soigne suits, including spring's umpteen Chanel copies, are giving way to simple sweater sets and knit skirts. Those super-wide pants, so impractical for daytime, have virtually disappeared, replaced by sleeker, racier versions that make more sense.

Nobody has time to fuss with fashion these days and nobody wants to gamble on silly fads - that seems to be the message. Clothes have to work, they have to offer flexibility, they have to provide a messure of longevity to be worth the investment. And make no mistake about it. Although clothes are, generally speaking, less elaborate for fall, they aren't going to cost a penny less. In fact, you're going to have to invest quite a few pennies more. Bad news. The good news is designers are giving you something for your money.

Invest wisely and you can get lasting styles, exquisite fabrics. Cashmeres, mohairs, meltons, tapestries, brocades, suedes and panne velvets fill the market as do chiffons and shimmering silks. You'll also find lots of super colors. Red is going strong. So are spicy shades of curry, citron, pumpkin and rust. There's a whole new range of browns, all rich and appealing. Olive green (not easy to wear) is back and every tone of beige from bisque to khaki is putting in an appearance. Complexion flattering hues such as mauve, stone, celery, powder blue and pewter are in the spotlight and brights are being used together in innovative and exciting ways (such vivid colors as purple, green and yellow can all crop up in the same outfit and often do in the knits of Adrienne Vittadini).

Autumn leaves pale when you look at fall's interesting and often brilliant fashion palette. But there are many other things to look at as well.

To begin with: lengths. You'll be able to find anything and everything from well above the knee to hems that flirt with the ankles. Most designers, however, concur that short is the smartest choice of the season and collections are replete with lengths that hit slightly above the knee.

Oscar de la Renta says short has "an instant youth thing" that long definitely lacks. Bill Blass believes in above the knee except for floor-length slim evening skirts. Calvin Klein declares "long looks dowdy." In spite of these statements, though, everybody seems to agree that the individual has to decide for herself. The customer, they say, deserves freedom of choice and options. (Among designers offering the option of long skirts in their collections are Ralph Lauren and Donna Karan.)

Length really is no issue anymore. The only guideline is to wear what makes you happy. And don't worry.

That same statement applies to pants. Although most designers have moved away from wide trousers in favor of sleek and tapered slacks, some skirt-like pant styles still are in the market. The newer, fresher approach, however, is to look for something slim - maybe a sleek-fitting knit with stirrups. Or Calvin Klein's jodhpurs. Or, if you have the body, you could even go with a cat suit or leotard that fits like a second skin; leggings or heavy-ribbed tights teamed with a long, oversized sweater.

Also being shown with narrow pants for fall: parkas and anoraks (those big cuddly coats Eskimos wear that feature hoods and drawstrings).

The outerwear influence, in fact, is all over the fall market. And clothes you'd normally see on the ski slopes have come down out of the mountains to walk along city streets or ride in limosines. Designers frequently have given sporty gear the luxury treatment. The anorak, for example, appears in everything from shearling and suede to silks and sequins.

So do ponchos, stoles and all those wraps that float and swirl. Designers (maybe they've been to see "Phantom of the Opera" on Broadway ) are absolutely mad for the cape and all its cousins. Almost every top collection is replete with the style.

Equally important this autumn in New York are knits - the sweater has surfaced in a thousand incarnations. It's being shown for evening in a variety of shimmering versions. It's being shown for day in super-soft, costly cashmere. Carolina Herrera likes the look of the sweater so much that she ties colorful cardigans preppy-style around the waists of pretty jewel necked dresses - even a couple of floor-length evening dresses - and adorns the knots with golden pins.

Also essential in the fall fashion picture: the jacket. It's big news for both day and night, a focal point in most designer lines. The prevailing silhouette is oversized and long, resembling a man's comfortable blazer. Charlotte Neuville, a bright new design talent on the New York scene, has done some terrrific menswear jackets taking inspiration from her husband's closet. But not every designer believes in long and oversized. Mary McFadden tops many of her pleated evening gowns with short quilted creations. Oscar de la Renta likes his jackets short and shapely and beaded. Geoffrey Beene likes the bolero and it appeared time and again in his creative collection shown on models who wore slicked back, simple hair styles and no earrings.

Bill Blass opted for minimal accessories as well. He wanted the attention to be on the elegantly-cut clothes in his collection. Outstanding were the animal prints for evening. Gleaming and glamorous leopard spots and shimmering sensuous tiger stripes that would be right at home in a fantasy jungle.

And speaking of fantasy brings us to that designer for the glitterati, Bob Mackie. The zodiac is one of his favorite themes for fall and appears repeatedly, culminating in an embroidered silver zodiac motif on a dramatic velvet cape. Practical? Well, no. Affordable? Don't make us laugh. But, oh, just imagaine sweeping into the Plaza Hotel this autumn in such glorious creation! All heads would turn, the violins would stop playing in the Palm Court, an appreciative hush would fall over the place and a fashion star would be born. Why, someone might even ask for an autograph.

At least, it's something dream about while watering the lawn and weeding the garden during the long, hot summer.