Rain and snow have deluged much of California this week, prompting questions about whether the state could dig out of its four-year drought, the worst in recorded history. But certain conditions need to converge on the state before its water crisis ends.

The Sierra snowpack is critical to the state, since much of the water used by Californians throughout the year comes from the slow melting of this snow in the spring. (Learn more about what happens with the snows fail in National Geographic magazine.)

“There is a good chance that if everything goes right over the next four months we could end up with good reservoir recovery,” says Frank Gehrke, who oversees snow surveys for California's Department of Water Resources. “We need much better than average snowpack this year for complete reservoir recovery, and so far we are encouraged.”

Still, for a true end to the drought, these factors need to align:

1. Four More Months! (of Storms)

The rain and snow that have fallen so far is not nearly enough. Most of the rain hitting Southern California won’t refill reservoirs. It may help fill some aquifers, but much of it will simply flow into the sea. It’s the moisture that falls as snow in the mountains that is the most important for water supplies.

The pace of that snow needs to continue during much of the next four months, the region’s wet season. So far, the forecasts look encouraging—thanks to the strong El Niño—but the weather through March could shift.

2. Storms in the Right Place

It’s not just the amount of moisture that falls, it also needs to be in the right place—i.e., over the mountains. It can’t be just along the coast, where too much will wash into the sea. The storms also can’t get shunted to the north.

That’s exactly what happened last year, when a persistent “ridge” of high pressure blocked storms from hitting California. So far this year that hasn’t been a problem, but Gehrke says it remains a “lingering concern” because long-term forecasting remains an inexact science.

Watch: How scientists measure the amount of water in California's snowpack.

3. Below-Freezing Temperatures in the Mountains

Another El Niño year, 1983, happened to be a relatively cold year, and the Sierra snowpack hit record levels. However, in the 1997 El Niño, temperatures were higher, and much of the precipitation fell as rain rather than snow. Much of that water flowed into the ocean via flood control channels, and the resulting snowpack was much smaller, leaving less runoff to fill California’s reservoirs in the spring.

Predictions suggest this year should remain cooler in the mountains, but Gehrke cautions that only time will tell.

4. True Recovery Takes Time

Even if the state’s reservoirs do fill up, effects of the drought will linger. Scientists will have to monitor how much the state’s highly tapped groundwater aquifers have a chance to refill. Also, the state is losing millions of trees to dryness and bark beetle invasions. That, in turn, is driving up wildfire risk.

Because of the damage caused by dry conditions, “it can take more than a single good year of snowpack for a drought to truly end,” says Gehrke.

Watch: Learn more about how snowpack affects water supplies.