May 9, 2020 – Dry weather with above average temperatures continues through the weekend. Much cooler weather along with widespread precipitation returns early next week.
Another hot day is in store for interior Northern California today. Upper level ridging continues over the West, though the ridge axis will shift slightly east over Nevada and Arizona today. This will allow for the return of onshore flow and associated Delta Breeze, which will result in high temperatures around 5 degrees cooler than yesterday southern Sacramento Valley and northern San Joaquin Valley and around 15 degrees cooler in the Delta region. Elsewhere, temperatures will be nearly the same as yesterday. Overall, temperatures will still be well above normal for this time of year with Valley highs peaking in the low to upper 90s. Heat risk will remain in the moderate category for the northern Sacramento Valley and surrounding foothills. Sensitive groups should prepare for hot conditions and stay hydrated.
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The upper level ridge continues to move east Sunday as a trough approaches the West Coast. A significant cooling trend is expected Sunday into midweek as the upper trough will bring stronger southwest flow and precipitation to the forecast area. High temperatures Monday across interior NorCal will be 15 to 25 degrees cooler than today, dropping another 5 to 10 degrees by Tuesday. Periods of breezy south to west winds are also possible Sunday through Tuesday as the trough moves through the area. At times, could see gusts to 20 to 35 mph, up to 30 to 35 mph in the Delta and higher terrain.
Multiple rounds of precipitation are also expected Monday through late week. The first round of precipitation is expected to slowly move north to south across NorCal Monday into Monday night as a shortwave and associated surface front moves through. Another shortwave will produce more precipitation Tuesday, with a chance of thunderstorms also possible in the northern Sacramento Valley and associated foothills. A few inches of high elevation snow is possible Tuesday as temperatures continue to cool and snow levels fall to around 6500 to 7000 feet. The ensembles are still a little uncertain about how much precipitation is possible Monday into Tuesday. The latest NBM is quite a bit lower, especially in the southern Sacramento Valley and Northern Sierra. However, if convection occurs Tuesday, QPF could be higher in stronger cells. Generally, up to 0.2 inch is expected for much of the Valley, except up to 0.5 to 0.75 inch in the far northern Sacramento Valley. Mountains and foothills could see 0.75 to 1 inch, with 1 to 2 inches possible over northwest Shasta County mountains.
Extended Discussion (Wednesday through Saturday)
Anomalous late-season wet pattern expected to continue shower chances across the northern half of the forecast area on Wednesday and Thursday with generally dry conditions south of I-80. Dry weather briefly forecast for late next week before another system potentially brings more precipitation to the region early the following week. Temperatures will be around 5-10 degrees below average at mid-week, warming to around average by next weekend.