August 5, 2020 – Below normal temperatures through Thursday. Warmer weather returns for the weekend. Breezy onshore winds today will keep temperatures anywhere from 7 to 15 degrees below normal across the region. These winds will be accompanied by increased relative humidity values, which will minimize fire weather concerns. As high pressure rebuilds late week and into the weekend, temperatures will warm to about 5 to 10 degrees above normal with moderate heat risk, especially in the northern Sacramento Valley where the hottest temperatures are forecast. Additionally, there will be an isolated chance of mountain thunderstorms in the Sierra from during the afternoon and evening hours from Friday to Sunday.
Discussion
Upper trough pushing through NorCal has initiated unseasonably strong onshore flow with southwest wind gusts 20 to 35 mph. This will bring below normal temperatures Today and Thursday with high temperatures running 5 to 15 degrees below normal. A few locations over the Southern Sacramento Valley, Northern San Joaquin Valley and Delta will struggle to reach 80 degrees today. Some early morning status will develop, but should remain in the scattered category, although could see a bank of broken coverage along the eastern Sacramento valley against the foothills for a few hours.
IR Sat is showing band of alto cu rotating north over the northern Sierra in front of upper low. Radar also has shown a few isolated showers near the crest around Quincy. Have introduced isolated shower chances for the morning mainly near the crest. Expect activity to transition north out of the area into Siskiyou and Lassen counties by afternoon, although can’t rule out an isolated thunderstorm over extreme northwest Shasta county. The chances should remain west of Sims.
Upper trough flattens Friday and Saturday with rising heights and diminishing onshore flow. This will allow temperatures to warm to near normal levels Friday then slightly above normal Saturday. Highs Saturday will be 95 to 105, warmest Northern Sacramento Valley.
Extended Discussion (Sunday through Wednesday)
Ensembles continue to depict broad troughing breaking down into early next week, which means slightly above normal temperatures. Models disagree on trough deepening again around mid-week with GFS deeper than Euro. GFS is also more aggressive in developing thunderstorms over the Sierra crest and Southern Cascades. Will maintain dry forecast in line with ensembles and NBM at this time until there is better model agreement.