August 13, 2020 – Significant, widespread warming will impact the Sacramento and San Joaquin Valleys late week and well into next week. This heatwave has the potential to pose high to very high heat risks for an extended period of time. A few isolated thunderstorms are possible over the mountains over the weekend. Well above normal temperatures are forecast to continue into next week.


Aside a few passing high clouds over far northern California, infrared satellite imagery reveals clear skies for the Sacramento Valley this early Thursday morning. Temperatures are generally running about 3 to 8 degrees warmer compared to 24 hours ago, following the warm-up that most communities experienced on Wednesday afternoon. As upper level heights and temperatures continue increasing and warming, most areas will again see a few-to-several degrees of warming this afternoon with highs in the upper 90s to low 100s across the Sacramento Valley and northern San Joaquin Valley.

Main story over the next several days and into next week will be the heatwave. The main driver this upcoming, high-impact heatwave will be a building upper level ridge that will set up residence for an extended period of time over the Desert Southwest and the Great Basin. Temperatures at 850 millibars (approx a mile above ground level) will warm from about 21 deg C now to around 27 deg C this weekend…with additional warming next week. Consequently, this synoptic setup will lead to a prolonged, high-impact, and potentially dangerous heatwave this weekend and well into next week. Triple digit heat will be realized in nearly every Valley location by Friday with many communities expected to approach and even surpass 105 deg F through the weekend. Overnight low temperatures will further compound the scope of this heatwave; many communities in the Sacramento Valley, northern San Joaquin Valley, and adjacent foothills will remain in the 70s or even low 80s for several consecutive nights. This will be particularly impactful for those without air conditioning. An Excessive Heat Watch is presently in effect from mid-day Friday through Sunday.

That said – there is some uncertainty in regard to high temperatures as the clockwise flow around the ridge will allow for some subtropical moisture in the mid/upper levels of the atmosphere to advect northward to California. Any mid/high clouds would prevent maximum afternoon potential, and could also bring a slight chance of thunderstorms to the some of the higher elevations. On the other hand, increased cloud cover during the overnight hours would potentially keep overnight lows even warmer than presently forecast.

In terms of heat risk, many communities in the Sacramento Valley and northern San Joaquin Valley will be subjected to high to very high heat risk when taking into consideration the very hot afternoon highs, warm overnight lows, and duration of the heatwave – which has the potential to go through at least Tuesday next week (more details in the Extended Section of the discussion). Very high heat risks occur during heat events that are rare, and pose risk to the entire population. Heat is the number one weather-related killer in the United States. Please take the necessary precautions to stay hydrated, cool, and safe during this prolonged heatwave.

Extended Discussion (Monday through Thursday)

An anomalous upper ridge will promote the continuation of dangerous heat into next week as it remains anchored over the West. Forecast 850 mb temperatures up to 26-30 degrees C could support widespread triple digits in the 100-110 degree range across the Valley, foothills and portions of the Delta. Moreover, little to no overnight relief is anticipated as low temperatures stay in the upper 60s to 70s, locally around 80 degrees in some northern foothill locations. There is some uncertainty regarding Monday highs as cloud cover could cool forecast temperatures. Peak heating may occur on Tuesday, when forecast highs could potentially soar up to 110 degrees in the Valley. High to very high heat risk is possible, particularly over the central/southern Sacramento Valley down into the northern San Joaquin Valley. This very dangerous heat event could impact the general population. Please learn the symptoms of excessive heat exposure, drink plenty of water, stay in an air-conditioned room, and never leave pets and children unattended in vehicles.

Thursday might trend a few degrees cooler, but moderate heat risk is still possible given the potential for widespread triple digits. In addition, morning lows will trend slightly cooler by Thursday morning, with mid 60s to low 70s.

Some shower and thunderstorm activity is possible over the Sierra crest during the afternoon through early next week, mainly south of Highway 50.