Juneteenth weekend forecast: Why Central Valley is cooling and when it warms again

Last weekend, many of us in the Central Valley were sweating with widespread temps above 100 degrees and several extreme heat warnings. It was as if summer was officially here a week early.

This weekend, as summer officially arrives Sunday at 1:24 a.m., temperatures are much more manageable if not downright pleasant.

This is all thanks to an upper level trough — an area of lower air pressure further up in the atmosphere — that has been spinning off the north coast of California the past few days. Folks along the coast have felt this as a deep, persistent, cloudy and breezy marine layer that has kept some spots on the west side of San Francisco into the 50s these past few days. This trough has been slowly eroding the ridge of higher pressure that brought us our scorching temps last weekend — it is why the week has been feeling progressively nicer for folks in the valley.

This trough hits its stride this weekend as it moves over land Friday and Saturday. In doing so, it fully “eats” that high-pressure ridge. What we are left with is ideal Juneteenth weekend conditions — highs in the low to mid 80s and plenty of sun to go around for the valley between Sacramento and Modesto and the low 90s for Fresno and the southern San Joaquin Valley. Whether you are planning an outdoor watch party as the United States takes on Australia in the World Cup on Friday, or planning a nice Napa Valley holiday weekend getaway, the skies will not be a problem.

However, the trough will cause problems Friday in and near the Sierra Nevada’s high peaks and portions of Northern California closer to Shasta County. Be sure to watch for growing cumulus clouds if you have any plans in the mountains and be mindful that a few small but intense thunderstorms could kick up fast.

Because of the winds and potential thunderstorms, the National Weather Service has issued a red flag warning from noon to 9 p.m. Friday for parts of the Eastern Sierra in Lassen, Alpine and Mono counties.

By Sunday, as the summer sun rises for the first time, our trough waves goodbye and we can expect temps to return to around 90 degrees.

The weekend’s forecast

Sacramento: Friday morning begins with our usual start in the upper 50s, but don’t be surprised to see occasional bouts of clouds out the door as our upper level trough keeps skies just a bit cooler through the rest of the day. With high temps in the low 80s, it’s the coolest day since June 8. And with the sun out in force by the afternoon, it’s a perfect holiday to spend outside however you can. As our lower pressure breaks down, high temps start to creep back up into the mid 80s by Saturday and back towards 90 degrees Sunday afternoon. Plenty of sun to go around through Sunday.

Fresno: The cooldown is not quite as pronounced for the valley around Fresno and south. While the passing of the upper level trough does bring in lower pressure, it also brings air a bit out of the north higher up, helping to cool temps a smidge further. For the southern half of the valley where the low sits more to the north, this will have the opposite effect — dragging up some elevated air from the south. Thanks to this, highs for Fresno and the south valley will hold nicely in the low 90s through the weekend. Just remember that it could always be worse (just look back to last weekend).

Modesto: Modesto, Merced, Stockton and most of the Central Valley, like Sacramento, will also benefit from the nice cooldown this weekend. High temps in 80s both Friday and Saturday with a return to the low 90s on Sunday.

The mountains: A few small but intense thunderstorms kicked up on the Sierra crest Thursday afternoon as that pesky trough was kicking up the southerly breeze. This will repeat on Friday as that trough continues to move over the mainland. Two points worth noting: First, these storms will still be quite isolated and brief, so don’t expect any long term delays or impacts to your outdoor plans on Friday even if you do encounter one of them. Second, there is a wildfire concern across the northern Sierra and much of Northern California from lightning thanks to lots of dry fuels available to burn and a gusty breeze at 25-35 mph above the treeline.

Heat returns next week

The weekend is not only wonderful for getting outside just because the forecast is nice, but also when looking at what is set to return — the heat. Highs could soar back to around 100 degrees by Thursday-Friday next week in the San Joaquin Valley as higher pressure returns, and closer to the upper 90s for Sacramento.

Sean Macaday

The Sacramento Bee