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August 16, 2020 – Extreme heat impacts are expected to continue through at least Thursday as the longest stretch of hot weather of the season is forecast. High temperatures will be around 10 to 20 degrees above normal most days. Today may be a few degrees cooler than the rest of the week, though it will still be very hot. There will be little to no overnight relief. With this long duration extreme heat event, significant heat impacts are expected for the general public, especially for those sensitive to the heat, pets, and livestock. There is a chance of isolated thunderstorms with little to no rainfall over much of NorCal today through tomorrow. These dry thunderstorms could produce fire starts, so a Red Flag Warning has been issued.  Isolated afternoon mountain thunderstorms are also possible over the Sierra Crest through Tuesday.

Discussion

This morning we’ve been watching a cluster of storms travel up the coast of CA and it is now entering into the Bay area. The forecast looks to remain mostly on track, with some changes occurring in the short term. We trimmed the Red Flag Warning back, cutting out the northern San Joaquin Valley. The Southern Sacramento Valley was trimmed as well, but we kept areas west of I-5 included. Based on the current convection, that seems to be working out. If storms do hold together and make it into our area, we may see a few sprinkles or virga. PWATs are favorable, but with the lower levels of the atmosphere being dry, we expect don’t expect to see much, if any rain from thunderstorms.

Along with the chance of thunderstorms today, the next challenging aspect of the forecast will be cloud cover and temperatures. Satellite imagery this morning depicts scattered to widespread upper level clouds due to the short wave riding up the coast. With renewed chances of convection throughout the day, partly to mostly cloudy skies seem to be reasonable for the forecast. This, in conjunction with slightly lowered heights, should keep temperatures in the 103 to 108 range for the Valley. The higher end of the spectrum is possible if areas are able to get some clearing.

Later tonight and into Monday morning, CAMs are suggesting another round of nocturnal storms. The track of this disturbance looks to be a touch farther west and should mostly impact the coastals and perhaps even the southern Cascades in Shasta County. Because of this, we’ve extended the Red Flag Warning through 18Z Monday. Heights will be rising again, which means we’ll also see high temperatures bump up a few more degrees. Triple digit heat will be widespread across the Valley and foothills, with some locations getting close to 110 degrees. Tuesday will be another scorcher and may actually be closer to what we saw yesterday (Saturday). Will need to keep an eye on 850 temps and overall heights and adjust the forecast accordingly.

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Wednesday and into Thursday, the high weakens a bit which will knock high temperatures down a bit. Triple digit heat is still expected, which combined with oppressive overnight lows, means little to overnight relief. The “good” news is that Wednesday night into Thursday, we could see our first reasonable night of temperatures. Latest model guidance shows temperatures in the upper 60s to upper 70s instead of the low 70s to mid 80s we’ve been seeing. This doesn’t mean that we’ll be out of the woods though, as our dog days of summer look to continue into the extended forecast.

As a reminder this is a significant and dangerous heat event. Heat is the number one weather-related killer in the U.S. Heat related illness to humans and animals is likely with extended outdoor exposure. Take necessary precautions to stay hydrated, cool, and safe during this prolonged heatwave.

Extended Discussion (Thursday through Sunday)

Ensembles weaken the ridge slightly later this week as the high center retreats from southern Nevada into Arizona, but there are indications it may extend northward again next weekend. While not as hot as it is currently, high temperatures are still forecast to be around 5-10 degrees above average with very mild overnight lows for most of the region. Models indicate potential for another tropical system off the coast of Baja late in the week and next weekend, so we’ll be alert for signs of another surge of elevated moisture into NorCal from the south.