FXUS66 KSTO 020851 AFDSTO Area Forecast Discussion National Weather Service Sacramento CA 151 AM PDT Sun Jun 2 2024 .SYNOPSIS... Slight cooldown through Monday before a significant warmup to widespread Major HeatRisk and triple digits in the Valley by mid- week. An Excessive Heat Watch remains in effect for the lower elevations from Tuesday morning until Thursday evening. Peak heat expected Wednesday. Slight chance of showers for the Coastal Range and northern mountains on Monday. Slight chance for late-day shower/thunderstorm over the Sierra Nevada Thursday-Sunday. && .DISCUSSION... Discussion at a Glance - Cooler through Monday with periods of increased onshore flow and Delta Breeze. - An Excessive Heat Watch remains in effect from Tuesday morning through Thursday evening across the Valley and adjacent foothills. - Areas of Major HeatRisk Tuesday for the Valley, becoming widespread Wednesday & Thursday for the Valley & Foothills with widespread triple digit high temperatures. Prepare now for elevated daytime highs and overnight lows and reconsider outdoor activities during the hottest part of the day (3:00-7:00 PM). - Moderate HeatRisk next Friday, with a few isolated spots of Major HeatRisk in the Valley; Triple digits in the Northern Sacramento Valley and adjacent Valley foothills with low to mid 90s in the Delta and Sacramento metro area Short-Term Discussion (Today THROUGH Wednesday)... Similar weather today as Saturday with warm afternoon temperatures and onshore flow moves across the Southern Sacramento and Northern San Joaquin Valleys. West-southwesterly winds continue across the Delta into the Northern San Joaquin with periods of 15-30 MPH gusts until Tuesday morning. Today's highs will be in the upper 80s and low 90s for the Valley with 70s an 60s across the foothills and mountains. Monday looks to be several degrees cooler with Valley highs in the mid to upper 80s. A weather system is impacting the Pacific NW, resulting in slightly cooler temperatures with chances for showers across the mountains on Monday for Northern CA. There is a 20-40% chance of precipitation over the Coastal Range and Southern Cascades, highest chances late Monday morning. Forecast still indicate this will be a marginal rain event if anything does develop. Latest WPC totals show 0.01-0.25" on Monday with highest totals forecasted for the Southern Cascades near Lassen National Park. NBM probabilities indicate a 20-40% chance of seeing 0.10" or greater so no impacts are anticipated from this brief weather passage. There will be increased cloudiness as a result which can help with relief from hot temperatures. The focus of our forecast will be the Tuesday into the late week period as upper level ridging develops over Northern CA with the aforementioned weather system exiting the Pacific NW. Major HeatRisk develops in the Valley, primarily from the Central Sacramento Valley to the Northern San Joaquin on Tuesday. Also, north winds pick up late Tuesday morning with gusts of 15 to 25 MPH before tapering off overnight. Conditions peak on Wednesday with widespread Major HeatRisk across much of the Central Valley and adjacent foothills. Afternoon highs of 95-108 degrees are possible Tuesday and Wednesday with little overnight relief with low temperatures in the 60s to mid 70s. Area waterways will continue to run cold and fast, creating dangerous conditions for those seeking relief in rivers and lakes. As a result an Excessive Heat Watch continues today from Tuesday morning into Thursday evening. NBM probabilities of exceeding 100 degrees on Tuesday is 45-95% and on Wednesday, 75-100%. This level of hear affects anyone without effective cooling and/or adequate hydration, particularly those working outdoors or apart of heat-sensitive groups. Stay tuned for updates as we get closer and plan ahead now to practice heat and cold water safety! To view the latest HeatRisk, please visit www.wpc.ncep.noaa.gov/heatrisk/ To view current water temperatures, please visit www.cnrfc.noaa.gov && .EXTENDED DISCUSSION (Thursday THROUGH Sunday)... Ensemble guidance indicates that upper level ridging continues to build mid-week over the Western US, bringing along the potential for dangerously hot temperatures through at least Thursday. Widespread Major HeatRisk is currently expected to impact the Valley and foothills on Thursday, with triple digit temperatures in the Valley and warm overnight low temperatures. The Excessive Heat Watch remains in effect through Thursday evening to address the significant heat. The National Blend of Models (NBM) shows a 50 to 95 percent probability of 100 degree highs or greater in the Valley on Thursday, with a 20 to 65 percent probability in the lower foothills over the same timeframe. The Delta Breeze should help cool temperatures across the Delta and Southern Sacramento Valley Thursday night to begin seeing relief from the stretch of warm temperatures. By Friday, slightly cooler but still hot temperatures are in the forecast with Moderate HeatRisk, as ensemble guidance indicates the ridge shifting east. However, there is still some uncertainty with the details in the cluster analysis that is resulting in a larger spread of potential high temperatures for Friday and into next weekend. Late-day mountain shower and thunderstorm chances return to the Sierra Nevada as well, with generally a 10 to 20 percent probability over the High Sierra mainly south of Interstate 80 from Thursday through Sunday, in the afternoon and evening hours. Additionally, area waterways continue to run fast and cold from snowmelt, creating dangerous conditions for those seeking relief in rivers and lakes next week. && .AVIATION... VFR conditions expected over the next 24 hours. Sustained surface winds generally under 12 kts except southwest to west wind gusts 15 to 25 kts in the Delta. Local northwesterly wind gusts up 20 kts in the northern San Joaquin Valley after 20Z Sunday. && .STO WATCHES/WARNINGS/ADVISORIES... Excessive Heat Watch from Tuesday morning through Thursday evening for Carquinez Strait and Delta-Central Sacramento Valley- Motherlode-Mountains Southwestern Shasta County to Western Colusa County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Shasta Lake Area / Northern Shasta County-Southern Sacramento Valley. && $$