At this point in a typical year, as the seasons officially turn from winter to spring, snowpack would still be accumulating across the Mountain West.

But this winter wasn’t typical, even before a heat wave this past week. It was the warmest on record for six Western states. Snow cover is the lowest level on record for the Colorado River Basin, and across much of the rest of the West, there are record or near-record low amounts of snow.

That alone would create a challenging year for water managers, who rely on slow and steady snowmelt to feed streams, rivers and reservoirs and meet spring and summer demand for irrigation and drinking water. While rainfall runs off quickly and can more readily evaporate from soil, snowpack serves as a valuable and lasting source of moisture and accounts for a majority of water supplies across the region, as much as 80 percent in some areas.

The intense heat wave threatens to make water management all the more challenging.

Much of the thin snowpack was already “ready to melt” before the heat set in, said Jon Meyer, assistant state climatologist at the Utah Climate Center. “This is the nail in the coffin.”

It’s unusual to see the whole West like this, said Leanne Lestak, an associate senior scientist at the University of Colorado Boulder who specializes in mapping snow and how much water it holds.

In early March, Ms. Lestak and her team found that vast majority of the Western United States had less than two-thirds of the amount of snow typical for this time of year, with few exceptions. In Arizona and parts of Nevada, New Mexico and Oregon, snowpack was less than a quarter of what it would usually be.

“The situation is pretty dire,” Dr. Meyer said.

The heat wave is also increasing the already-elevated fire risk across some drought-stricken areas. In Nebraska, drought set the stage for the largest wildfire in state history, which broke out last week and has not yet been contained.

The conditions that led to this year’s low snowpack are unusual, too. Snow droughts often develop from dry weather patterns that starve the West of any significant precipitation during the winter, said Dan McEvoy, a climatologist at the Desert Research Institute and Western Regional Climate Center.

But in many places, it wasn’t necessarily a dry year, he said. Instead, temperatures have been so warm that precipitation has fallen as rain, rather than snow, even at higher elevations.

Many of the mountaintops could still see some more snowfall. But as Cody Moser, a hydrologist with the Colorado Basin River Forecast Center in Salt Lake City, looks ahead to predicting how the spring will go, he doesn’t foresee any significant change in weather patterns. Now he’s expecting peak snowmelt flows to occur earlier than ever recorded in many locations, he said this week.

“I think it’s highly likely we’ve seen peak snowpack,” Mr. Moser said.

Even after a winter that was the warmest on record for Arizona, New Mexico, Colorado, Utah, Wyoming and Oregon, the heat that set in across much of the West this past week was extreme. Meteorologists said they were expecting to set record highs for the month of March in many locations, and the earliest arrivals of 100-degree temperatures in records that go back more than a century.

Across the Colorado River Basin, even at elevations as high as 10,000 feet, temperatures were forecast to surge into the 50s and 60s Fahrenheit on Friday and Saturday, Mr. Moser said, some 15 to 20 degrees warmer than average.

Relatively light winds and dry air over the region could limit snowmelt to some degree, he said, but the warmth and sunshine may prevent some moisture from ever reaching stream beds, said John Fleck, a water policy expert at the University of New Mexico.

“A lot of it is going to evaporate off before it even has a chance to hit the stream,” Mr. Fleck said.

This heat wave is so extreme that it would only be expected to occur once about every 500 years in the current climate, according to World Weather Attribution, a group of scientists who study links between extreme weather events and climate change.

“These temperatures are completely off the scale for March, and our data shows that they would be virtually impossible in a world without human-caused climate change,” said Ben Clarke, a research associate in extreme weather and climate change at Imperial College London.

In places like the Colorado Front Range, home to the majority of that state’s population, snowpack serves as the largest source of water. For the utility Denver Water, snowpack usually contains significantly more water than its largest surface reservoir, said Taylor Winchell, the agency’s climate adaptation program lead.

Denver Water has enough supply to handle a low-water year, but the snowpack conditions are creating “very high levels of concern,” Mr. Winchell said. The Denver Water Board is poised to officially declare Stage One drought restrictions, asking residents to significantly reduce their outdoor watering. If the snow drought were to repeat for multiple years, the problem could compound and worsen, he said.

The snow drought occurs at a critical time for the larger Colorado River Basin. An agreement among the basin’s seven states over how to divide its water expired at the end of last year, and negotiations to develop a new water plan fell apart last month. (The states are also obligated to share a small portion of the water with Mexico.)

The snow drought is complicating that work. Snowpack from the river’s Upper Basin, across mountains of Colorado, Utah, New Mexico and Wyoming, accounts for a majority of the river’s natural flow each year. Declining spring precipitation and rising temperatures have caused the Colorado’s flow to decrease by nearly 20 percent over the past quarter century.

Recent forecasts estimated that inflows to Lake Powell, a key reservoir that straddles the Utah-Arizona border, will be the third-smallest on record. The lake’s surface could drop to a critical level for hydroelectric power production by the end of this year, affecting a power grid that serves seven states.

Officials at the Bureau of Reclamation, the federal agency that oversees the Colorado River and its reservoirs, declined to be interviewed but said in a statement they were monitoring hydrologic conditions to guide decisions about how to manage the Colorado River system.

Mr. Fleck said a crisis without precedent could be brewing. While a drought that hit the basin in 2002 was worse, it was relatively more manageable than what the West now faces: “We’re having one of the worst years in many decades, but with no cushion of reservoir storage to fall back on to bail us out.”