Potential rain accumulation through Saturday. Weak showers will push across Central California from Thursday to early Saturday. Light accumulation is expected in places across the Central Valley, with higher totals in the Sierra.
The Sacramento Bee
As summer draws to a close, the unending sunshine and heat will soon begin to fade. That’s not to say there aren’t sweltering hot and sunny days ahead this year — there will be plenty still over the next month — but these days are numbered.
Central California is set for an early peek of what can be expected in the coming months and into winter: rain showers.
Moisture will overspread Central California late Thursday through Friday. Early rains may evaporate in the Central Valley before reaching the ground as virga. Better chances for accumulating rains in the Sierra and Central Valley are on Friday. SEAN MACADAY sean.macaday@sacbee.com
As of midday Thursday, rain showers were beginning to envelop the Central Coast from San Luis Obispo north through Big Sur, potentially offering some relief to the raging Big Timber Fire.
These showers will continue to push north and inland into Friday, with light showers expected across the Bay Area and in the Sierra.
For the Central Valley, while the radar might say rain as soon as early Thursday afternoon, conditions remain too dry for raindrops to reach the ground intact. The result will be virga — streaks of evaporating raindrops — across the valley from Sacramento south to Fresno Thursday afternoon that will cap high temps in the mid- to upper 90s.
While there is an off-chance for accumulating rain Thursday in the Central Valley, any such measurable rain is far more likely Friday as the atmosphere continues to moisten.
Most showers will be weak, with a few sparse and weaker thunderstorms possible at elevation in the Sierra. Thanks to the widespread nature of these showers, the wildfire threat remains modest. Rainfall accumulations in the mountains could reach up to a quarter of an inch in portions of the Sierra, with isolated spots pushing up to a tenth of an inch in the Central Valley. Most areas between Sacramento and Fresno can expect less.
Moisture of former Tropical Storm Iselle responsible 
Moisture from what is left of Tropical Storm Iselle is pushing north into California this weekend, bringing showers and increased humidity as far north as the Sacramento area. The Sacramento Bee
All of these late summer showers are in part brought on by a pool of tropical moisture leftover from the remains of Tropical Storm Iselle — which died Wednesday after absorbing the even smaller Tropical Storm Julio over the eastern Pacific Ocean earlier this week.
As lower pressure sets up across the Pacific Northwest this weekend, the resulting motion of the atmosphere across California is out of the south-southwest, conveying all of this tropical moisture straight towards the Golden State.
The result is a steady stream of weak and widespread showers reaching well inland through Friday and the early morning hours of Saturday. In a preview of the winter to come, this moisture does have some similarities with the more powerful atmospheric rivers that will be possible in the coming months thanks to El Niño — although the weekend’s weather is far weaker and more tame.
Rain ushers in cooler temps for Sacramento Valley
These showers will work to cool the atmosphere, but the majority of the cooler temps arrive thanks to lower atmospheric pressure to the north between Oregon and Washington.
On Friday, high temperatures will quickly fall into the 80s for Sacramento and the Delta region south to Merced, with the low 90s possible for Fresno and areas further south.
These temperatures will continue to tumble through the weekend as skies clear, with highs in the low 80s for Sacramento, Stockton and Modesto. Some spots between the Delta region and inland Bay Area will only rise into the 70s Sunday despite a sunny forecast.
The Sacramento Bee 
The Sacramento Bee
The Sacramento Bee
Sean Macaday covers the weather for McClatchy Media in California. Originally from Chicago, Sean worked 6 years covering the weather on local television in Minnesota. He holds a bachelor’s degree in meteorology and marine science from the University of Miami.