Spring was fun while it lasted.

After ushering in spring just over four weeks ago with above average rain and below average highs, the National Weather Service expects the high to hit 91 degrees on Thursday.

That’s 18 degrees above the average high for temperatures for April in the South County.

The National Weather Service indicated, on average, the first day of 90 degree plus temperatures in Manteca, Ripon and Lathrop isn’t until May 4. The earliest 90-day record since temperatures have been kept back to the 1850s was on April 7, 1989.

And while the 91 degrees expected on Thursday is nothing for the Northern San Joaquin Vallely that often has stretches of 100-egree plus days, valley temperatures surpassing 90 degrees in late April with a record Sierra snowpack is not what the operators of the state’s reservoirs want to see happen.

The surge in valley heat, though, is not expected to warm up the higher elevations too much at least over the next week.

New Melones Reservoir on the Stanislaus River watershed was 99 percent of normal on Monday.

That reflects 62 percent of the 2.4 million acre-foot reservoir being full.

On the Tuolumne River watershed, Don Pedro Reservoir storage was at 102 percent of average for April 21. That is 77 percent of the 2,030,000 acre foot capacity.

Outflow at Don Pedro on Monday was 11,319 cubic feet per second  versus an inflow of 8,632 cfs with a massive snowpack yet to begin melting in earnest.

The cfs measurement represents the volume of one basketball filled with water passing a point at a given second. That means the equivalent of 11,319 basketballs of water are now being released into the Tuolumne River from Don Pedro.

The release at New Melones was smaller than then inflow — 2,078 versus 4,739.

That reflects the fact New Melones has more room to handle a sudden inflow of snowmelt caused by rapidly waring temperatures if and when they occur.

Water from both rivers dump into the San Joaquin River south of Manteca just before it reaches the Airport Way bridge. It is up four tenths of a foot from Friday

The water at that point 210 miles south of Manteca was at 27.59 feet on Monday at 11 p.m. and two feet from its peak of 29.59 feet on March 26.

The biggest concern with levees protecting rural areas south of Manteca and east of Tracy isn’t a sudden string of blistering hot days in the higher elevation that create a torrential snow melt that in turn increases reservoir releases.

The main issue is the ability of the levees and surrounding ground to resist seepage and boils from the consistent high flow of water that is expected to last for several months.

To contact Dennis Wyatt, email dwyatt@mantecabulletin.com