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August 15, 2020 – 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 showers/thunderstorms over the mountains this weekend into Monday. Fire Weather Watch from Sunday morning through Sunday evening, Excessive Heat Warning until 9 PM PDT Wednesday.

Discussion

Water vapor imagery this morning shows fairly dry conditions of northern CA as we remain trapped between high pressure over the desert southwest and a closed low over the Pacific. Guidance continues the same trend noted by the evening shift, where heights/thicknesses for tomorrow have increased by 1 to 2 degrees C at the 850mb level. Opted to start with yesterday’s temperatures and then nudged them up by a few degrees. Most climate sites have the chance to either tie or break records if the forecast pans out. Highs for the Valley are forecast to range from 105 to 112 degrees while the foothills range from 95 to 105. Not much more relief will be found in the higher elevations either as temps range from 85 to 95 degrees. In addition to the heat tomorrow, models continue to show elevated instability which could lead to a slight chance of afternoon showers or thunderstorms over the higher elevations of the Coastal Range and Sierra Nevada.

Saturday night and into Sunday, the offshore closed low will move a touch closer to the north coast. Models are hinting at height falls and short wave trough sliding coast. Temperatures will likely reflect Fridays forecast, but theres some concern about potential cloud cover. Guidance from the CAMs suggest the chance for showers and thunderstorms; spreading from south to north Sunday morning and into the afternoon. PWATs are around an 1 inch, but the moisture is found in the upper levels. Should convection occur, isolated dry thunderstorms are not out the question for portions of interior NorCal. As a result, have issued a Fire Weather Watch for this threat as fires could rapidly grow given the heat, primed fuels, and unstable nature of the atmosphere.

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Beyond Sunday, there is a slight chance for thunderstorms over the Sierra and southern Cascades. If storms form they should stay confined to the terrain. Heights are forecast to rise Monday and Tuesday as high pressure builds over the Great Basin area. High temperatures will generally be in the 105 to 110 degree range for the Valley. Unfortunately, we won`t see much in terms of overnight relief as highs will span the 70s and 80s until midweek. As a reminder this is a significant and dangerous heat event. Heat related illness to humans and animals is likely with extended outdoor exposure. Heat is the number one weather-related killer in the U.S. Take necessary precautions to stay hydrated, cool, and safe during this prolonged heatwave.

Extended Discussion (Wednesday through Saturday)

Ensembles in general agreement weakening the center of strong high pressure, and shifting it eastward from southern Nevada back toward the Four Corners region later next week as a trough approaches the PacNW. The result is expected to be a gradual easing of temperatures across interior NorCal, back to only around 5 degrees above average by next weekend. Deeper moisture and instability is forecast to be east of the region.