ZAGREB, Yugoslavia, Sept. 23— As the bright, warm times of summer gave way today to storms that soaked a city returning to life after days of air-raid warnings and worry, a senior official in the secessionist Yugoslav republic of Croatia surveyed both the weather and the future.

"It may be raining outside," Mario Nobilo, an adviser to President Franjo Tudjman, said, gesturing at a downpour that drenched a ceremonial guard and drummers on the cobbled streets around government offices, "but inside, it's political sunshine."

He was speaking as the guns fell nearly silent in Yugoslavia's civil war and a cease-fire declared Sunday seemed, by the recent standards of broken truces and false promises, to be a record -- a day of only sporadic violations. Little Optimism Among Europeans

His optimism contrasted sharply with the prevailing assessments in Yugoslavia and much of Europe where Croatia is being viewed as an underdog after three months of fighting in which Serbian irregulars and Serbian-led federal armed forces have seized big chunks of Croatia's territory, severed the Dalmatian coast from the republic's hinterland and established a bridgehead only 30 miles of undefended highway from Zagreb.

But, as seen and explained from here, the war so far has also produced consequences that appear advantageous to the Croatian cause.

For one thing, Croatian officials like Mr. Nobilo maintain, the conflict has eroded Yugoslavia's confidence in its ability to pursue territorial expansion by military force.

While conceding that such territory has been lost, the Croatians say the fighting has been costly for the Serbians and hardly the cakewalk that the Government in Belgrade may have anticipated. The Croatians note particularly that their forces have captured weapons from blockaded federal army barracks -- at Osijek in the east, at Nasice southeast of here, and in Varazdin to the north. These weapons, they say, could shift the military balance. A Vision of Negotiations

The Croatian officials also argue that the fighting has opened the way for negotiations that, they say, will ultimately bring at least the beginning of recognition for Croatia's indendence, declared June 25. That declaration, part of the unraveling of Yugoslavia's 45-year ethnic experiment, set off the warfare as the federal army moved into increasingly open support of militias drawn from the 600,000-member Serbian minority living among Croatia's 4.5 million people.

If negotiations do start -- still only a long-shot possibility -- the fighting has left the leaders of both Serbia and Croatia with even less common ground than they had before.

Arguably, the two sides are fighting two separate wars in which they have pursued different aims, though with some similar results. The Serbs are fighting for turf and sway over a weaker neighbor and have won no European sympathy for doing so. The Croats are fighting the underdog's campaign for sympathy and respect and have yet made no discernible progress toward their goal of European recognition as an independent country.

And if the fighting resumes, it will be because those two aims are irreconcilable: the Serbian quest for expansion and influence cannot be married to the Croatian demand for independence within the borders before June 25. Differing Approaches

As the war unfolded, particularly over the last four days, the Serbian side deployed the full strength of an army built for conventional warfare against secessionists who, by the account of some Croatian commanders, had delayed too long before mobilizing, securing weapons and training troops.

Yet the Serbian-led federal authorities secured what might be seen as their broad territorial goals -- the expansion of control over areas that have been home to ethnic Serbs and thus part of the "Greater Serbia" -- something central to the dream of Serbian nationalism in a region where nationalist passions have spilled blood for centuries.

In the east, Serbian tanks ring the main centers. South of here at Petrinja, Croatian guardsmen were routed two days ago from a bridge controlling the approaches to Zagreb. The Serbian enclave of Krajina, under Serbian militia control, straddles the gorges that offer access from the center to the coast -- an area most frequently associated with the tourists, sand and sunshine of the Adriatic.

But there have been other visions, as well. Croats, using captured, shoulder-held SAM-7 missiles, have brought down Yugoslav MIG's -- at least two, possibly more, jolting an air force unused to resistance. Serbian soldiers advancing on Petrinja cowered behind tanks, as if they had little stomach for the fray. Correspondents in Belgrade report recurrent rumors of low morale and rebellion among conscripts reluctant to go to the front. A Change by Belgrade

Until Saturday, however, the Yugoslav Army and the Serbian irregulars said they were bent on decisive military action. Then something changed. Maybe, officials here and diplomats in Belgrade suggest, the federal authorities were worried that the war would spill into Bosnia, where advances by the Yugoslav Army in support of militias in Croatia have met resistance that could turn into a messy new fighting front. Maybe Western European governments, fearful of uncontrolled mayhem in the Balkans, put pressure on the Belgrade authorities.