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June 3, 2020 – Hot weather with high temperatures 10 to 15 degrees above average resulting in moderate to high heat risk through Thursday. Slight chance of late-day showers and thunderstorms over the Sierra Nevada south of Lake Tahoe through Thursday. Cooling with an increased chance of showers and thunderstorms by the weekend.
Clear skies cover the region early this morning as the high clouds associated with the cutoff low off the SoCal coast remain to our south. Winds are generally light and current temperatures are quite mild, ranging from the mid 40s in the colder mountain valleys to the mid 60s to mid 70s across most of the Central Valley and foothill and northern Sierra thermal belts.
The big cutoff low will remain near 30N/125W the next couple of days. The COL (saddle), between the low and the westerlies to our north across the PacNW, and its axis of dilatation, will continue to keep NorCal relatively cloud-free today and Thursday as it erodes clouds moving north around the low. The pattern will also maintain hot weather across the region as the marine layer is suppressed. Thunderstorm chances will likely remain to our south and east as well.
Pretty good agreement continues in the model ensembles indicating the large trough over the Gulf of Alaska will approach the West Coast late this week kicking the cutoff low inland to our south on Friday. This will initiate a cooldown on Friday which will accelerate over the weekend, as onshore flow returns and synoptic cooling increases with the approach of the trough. It will also lead to breezy to windy conditions across the region. Best chance of showers and thunderstorms appears at this point to be on Saturday with perhaps some light snowfall accumulation across the higher elevations.
Extended Discussion (Sunday through Wednesday)
The upper level trough axis will be pushing east on Sunday. Shower chances will remain in the forecast though mainly for higher elevations and the far northern Sac Valley. We do see some instability build in during the afternoon and a few isolated thunderstorms will be possible. Snow levels will remain on the low side for this time of year and some light snow will be possible above 6000 feet. Short wave ridging builds in for Monday with dry weather expected. Flow will become more zonal after Tuesday as an upper level low in the Gulf of Alaska pushes a cold front into WA and OR. Highs will be quite cool on Sunday running 10-20 degrees below average in the valley and as much as 25 below average in the mountains. Warming trend then takes place early next week with highs returning to near average by Tuesday.