Update: State water officials: 'Big Melt' has been a 'Slow Melt' so far in snowbound Sierra

PHILLIPS STATION -- With a new storm front bearing down on the Sierra Monday, state water officials were measuring the massive snowpack that will linger for months in the upper elevations.

Officials said the May 1st measurement -- usually the last of the season -- found just over 5 feet of snow on the ground. The last time there was measurable snow at the Phillips snow course on May 1 was 2020, when only 1.5 inches of snow and .5 inches of snow water equivalent was measured.

"The statewide snowpack overall melted at a slower pace than average over the month of April due to below average temperatures early in the month and increased cloud cover," officials said in a news release. "An average of 12 inches of the snowpack's snow water equivalent has melted in the past month and it now contains an average of 49.2 inches."

The runoff already has triggered  flooding in the San Joaquin Valley and was rapidly filling up reservoirs. On Monday,  Oroville was at 91 percent of capacity, Lake Shasta 97 percent and Folsom at 82 percent -- all well over their historical levels on May 1st.

UCLA Climate expert Daniel Swain said the flooding threat will continue into June.

"This snowpack is still enormous, and perhaps 20 to 25% of it has now melted," he said. "But that means that in all likelihood, at least 75% of the snow water is still up there. So in other words, the significant majority of what was up there at its peak is still there, even as we head into May, even in the wake of major heat waves."

"So this melt really is still just getting started. I know that's hard to believe, but we're getting into May and the peak is probably yet to come. "

"Peak is definitely yet to come," Swain continued. "The majority of the water is still up there. There's still a lot of there's still a lot of water frozen into that higher elevation snowpack. And we may not see peak outflows in some watersheds for another month."

"No matter how you look at the data, only a handful of years in the historical record compare to this year's results," said Sean de Guzman, manager of DWR's Snow Surveys and Water Supply Forecasting Unit. 

According to historical records, only the April 1 measurements from the years 1952, 1969, 1983 and this year were above 200 percent, although it is difficult to directly compare individual years across the decades due to changes in the number of snow courses measured over time.

Meanwhile, a storm front rolling across Northern California will bring rain to the Bay Area and snow flurries in the upper elevations in the Sierra.

"The main upper low center will rotate energy and moisture over the region through mid-week with periods of mountain snow and valley rain showers," forecasters at the National Weather Service in Reno predicted.

The storm front will arrive in two waves. The first on Monday night and then a second on Tuesday though Wednesday.

"This second wave will bring a stronger surge of moisture with a better chance -- 30-50% -- of a few inches of impactful snowfall across Sierra passes Tuesday afternoon into Wednesday morning," forecasters said. "Travel impacts are not out of the question, so be sure to check with CalTrans."

In the Bay Area, the storm front dump about half of an inch of rain along the coast and lesser amounts inland.

"Rain showers will increase in coverage Monday night  into Tuesday along with increasing potential for isolated thunderstorms through Tuesday evening," forecasters said. "Temperatures will also remain unseasonably cool through midweek. Unsettled conditions prevail through Thursday, yet precipitation will diminish in coverage and shift southward. Drier conditions return by Friday."