August 14, 2020 – Dangerous and prolonged heatwave will impact the region through the middle of next week. Daytime temperatures will be 10 to 20 degrees above normal with limited overnight relief. Slight chance of afternoon thunderstorms over the mountains this weekend into Monday.


The dangerous and prolonged heatwave arrives today. Excessive Heat Warning goes into effect for the entire Central Valley beginning this afternoon, and continues through through 9 PM PDT next week Wednesday.

A strong and broad upper level ridge residing over the Desert Southwest and Great Basin is responsible for this extended period of excessive heat. Temperatures at 850 mb will warm to the 27 to 30 deg C range over the weekend and into the first half of next week, and will result in maximum surface temperatures on the order of 100 to 110 deg F Valley-wide. Ensemble means from the ECMWF and GFS even suggest a remarkable 110+ for the Sacramento area around Monday/Tuesday. It’s quite conceivable that Sacramento could match/exceed 105 for six consecutive afternoons, and perhaps 10 days at/above 100. These highs equate to about 10 to 20 deg F above middle August climatology. Overnight lows will be oppressive as temperatures hold in the 70s to low 80s well into next week.

Subtropical moisture will continue to entrain in the clockwise flow around this upper level ridge, and will bring periodic mid/high clouds to the region through the weekend. This moisture could help initiate some isolated mountain thunderstorms near the Sierra crest through Monday afternoon. Furthermore, these clouds could help dampen peak temperatures through Sunday, but conversely, would also hold overnight minimum temperatures higher than officially forecast. Regardless, it will be hot – extremely and dangerously hot for an extended period of time, even for Central Valley summer standards. The heat risk will be high to very high for much of the region through at least Wednesday of next week.

This will not be a short-lived heatwave. The Excessive Heat Warning is in effect for six days and five nights through next week Wednesday 9 PM PDT. A combination of the extended duration, relentless afternoon heat, and poor overnight cooling is what will compound the severity of this heatwave. Heat is the number one weather-related killer in the United States. Take the necessary precautions to protect yourself from this extreme heat:

  • Stay hydrated and avoid caffeinated and alcoholic beverages.
  • Use air conditioning.
  • Take frequent breaks in the shade if you must be outdoors.
  • Wear light-colored and lightweight clothing.
  • Recognize the signs and symptoms of heat exhaustion and heat stroke. A heat stroke is a medical emergency; dial 9-1-1!
  • Check up on neighbors and relatives.
  • Ensure pets and livestock have shelter and water.
  • Never leave a person or pet unattended in a parked vehicle: look before you lock! Wear a life jacket in the water.

Please be safe, and beat the heat!

Extended Discussion (Tuesday through Friday)

Minimal changes to the extended forecast as models continue to depict high pressure dominating the forecast. Unfortunately, this means we’ll see triple digit heat for the Valley and foothills through the extended period. Guidance does show a broad trough slowly digging into western BC; however, we’ll still be experiencing the influence of high pressure. The “good” news is that this will drop high temperatures by a few degrees, with most areas being in the 100 to 105 range on Friday. Overnight lows during this time frame are expected to be miserable as well, with most areas ranging from the 70s to near 80s.

With that being said, it’s important to re-iterate the danger of this heat event. Know the signs of heat illness, drink plenty of water, stay in air-conditioned rooms, check on your neighbors, the elderly, children and pets. If you’re traveling with children and/or pets, look before you lock ever leave so that they’re never left unattended in vehicles.

This very dangerous heat event will have impacts on the general population. Please learn the symptoms of excessive heat exposure, drink plenty of water, stay in an air-conditioned rooms, and never leave pets and children unattended in vehicles.

Some moisture will remain in place over the area on Tuesday. This will lead to some instability over the higher elevations. There is no forcing but the terrain could be enough to kick off an isolated storm or two in the afternoon near the Sierra crest. No thunderstorms are expected after Tuesday.